sky telescope magazine

8/18/2019 Sky Telescope Magazine

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T H E E S S E N T I A L G U I D E T O A S T R O N O M Y

Riding the Winged Horse: GALAXY GROUPS IN PEGASUS p. 32

The Next Blue Dot: HOW

WE’LL IMAGE ALIEN EARTHS p. 16Going DeDELPHINU

MakingMassive

Stars p. 24

ShootingNightscapes p. 66

The Moon’s Southern Imbrium p. 52

A Triad ofSky Myths p. 45

Visit SkyandTelescope.com Download Our Free SkyW

Test Report: VixenGrab&Go Ref lector p. 60

OCTOBER

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0 3 M D c 0 O D A 4 M D I y M D c 2 k Q = =

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UC

Display until September 28, 20


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OBSERVING OCTOBER

41 In This Section

42 October’s Sky at a Glance

43 Binocular HighlightBy Gary Seronik

44 Planetary Almanac

45 Northern Hemisphere’s SkyBy Fred Schaaf

46 Sun, Moon & PlanetsBy Fred Schaaf

48 Celestial Calendar By Alan MacRobert

52 Exploring the MoonBy Charles Wood

54 Deep-Sky Wonders By Sue French

57 Going Deep By Ken Hewitt-White

S&T TEST REPORT

60 Vixen R130Sf Reflector &Porta II Mount Package

By Gary Seronik

ALSO IN THIS ISSUE

4 Spectrum By Peter Tyson

6 Letters

, & Years Ago By Roger W. Sinnott

10 News Notes

65 Book Review By Susan N. Johnson-Roehr

72 Telescope Workshop By Gary Seronik

74 Gallery

84 Focal Point By Gary Seronik

October 2015 VOL. , NO. On the cover:

The Orion Nebula

is the nearest star-

forming region to

Earth and contains

thousands of stars.

PHOTO: NASA / ESA / M.ROBBERTO / HUBBLE SPACETELESCOPE ORION TREASURYPROJECT TEAM

COVERSTORY

FEATURES

16 The Next Blue DotAstronomers are working to directlyimage alien Earths, with several pro-mising space missions in development.By Ruslan Belikov & Eduardo Bendek

24 Making Massive StarsResearchers are refining the recipefor some of the brightest stars in thenight sky. By Monica Young

32 A Few of My Favorite ThingsA sense of wonder and a good imagina-

tion will aid you in this tour of Pegasusgalaxy groups. By Ted Forte

66 Secrets of NightscapePhotography

Here are some essential tipsfor shooting the night sky abovepicturesque landscapes. By Alan Dyer

2 October 2015

16

E S O / P E T R H O R Á L E K


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October 2015

October 2015 Digital Extra

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feature article on exo-planets? Read the latestexoplanet news here.

BONUSWEB CONTENT

• Pegasus Galaxy GroupsObservers, create your ownobserving challenge!

• Going Deep:Delphinus

Peruse additional imagesto help you navigate thearea around NGC .

• More onMassive Stars Dive into sites of massivestar formation in yournight sky, and watch theprocess in action.

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ONLINE PHOTO GALLERY

Astrophotographer Slawomir captured this close-up ofthe Carina Nebula. The brightest star, Eta Carinae, is oneof the most luminous and most massive stars known.

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4 October 2015

Peter TysonSpectrum

Hed.GillSans.30

Founded in 1941by Charles A. Federer, Jr.and Helen Spence Federer

The Essential Guide to Astronomy

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Editor in Chief

a group of astronomers released a report championinga single bold idea. It’s a proposal that, in my opinion, ranks up there withPresident Kennedy’s challenge to put a man on the Moon “before thedecade is out.”

The notion is to build a space telescope so big, so sensitive, and so tech-nologically advanced that it might enable astronomers to answer some ofthe grandest questions we’ve ever asked: Are we alone? Do Earth-like plan-ets with breathable air exist elsewhere? Are they, in fact, common? How didlife emerge in the cosmos in the first place?

The group that wrote the report knows whereof it speaks. It is the Asso-ciation of Universities for Research in Astronomy, or AURA. A consortiumof U.S. institutions and four international affi liates, AURA, among

many other things, conducts scienceoperations for the Hubble SpaceTelescope (HST) and for the upcom-ing James Webb Space Telescope.

In short, AURA’s study calls fora High Definition Space Telescope,a sort of “super Hubble.” Withits -meter (-foot) segmentedprimary mirror, the HDST wouldbe times more sensitive to faintlight than HST is and provide viewsfive times as sharp. You can down-load the AURA report here: hdstvi-

sion.org/report.Parked a million miles from Earth

in a gravitationally stable location, the HDST would also have devices — acoronagraph and perhaps also a star shade — that would so effectively blocka star’s blinding glare as to allow astronomers to directly image exoplanetsorbiting that star. (See page for our story on where such direct imaging ofextrasolar planets stands today.)

More importantly, such a telescope could, in principle, characterize theatmospheres of nearby exoplanets. Detecting an abundance of water vapor,oxygen, methane, and other organic compounds sheathing an exoplanetmight very well signal an active biosphere on its surface.

I think the HDST idea merits serious consideration. The hurdles are

many and huge: For this project to reach fruition would take decades ofpainstaking effort, the development or perfection of entirely new technolo-gies, sustained political will, and many billions of dollars.

But just imagine knowing, with a high degree of certainty, that anotherEarth exists out there, perhaps with beings as or more intelligent than weare. What would that understanding do to our worldview? To our sense ofself as human beings? To our hubris? ✦

I J,

A Super Hubble

How the primary mirror of the HDST stacks

up against those of Hubble and James Webb

Hubble JWST HDST .m .m .m

C. GODFREY (STSCI)


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6 October 2015

Letters

The Herschel SprintAfter some attempts over years, I’vehad my fill of Messier Marathons. Thanksto Mark Bratton (S&T: Apr. , p. ),

the Herschel Sprint will be my replace-ment. Ideally I’ll tackle it on April thnext year, to match the lunar conditionsHerschel faced in . As Bratton pointsout, the transit-instrument constraintseverely restricts the time I can spend oneach object, but it might help recreate theexcitement that the Herschel siblings felton that historic night.

Paul DawsonOlancha, California

Moon Over StonehengeLet’s be honest: most astronomy-clublogos are dull and unimaginative. When’sthe last time you saw one fit to grace thebumper of a lime green AMC Gremlin?So when I set out to help a group of thgraders in San Anselmo, California,design their club’s insignia, I was deter-mined that it would be thrilling. It wouldinspire wonder and a sense of adventure.

Inspiration seemed close at hand.Only a few blocks away, in the townsquare, stands a statue not of a Civil War

hero but of San Anselmo’s most famousson, Yoda. What says “astronomy” betterthan a -foot-tall green space alien withpointy ears? Alas, the idea was a falsestart. Copyright issues aside, the powerof Yoda’s personality overwhelmed theinsignia — making it look more like anadvertisement for a comic-book conven-tion than the emblem for a grade-schoolastronomy club.

Just when all seemed lost, I happenedacross an image that fired my imagina-

tion. While preparing for the Apollo mission in , astronaut-geologistHarrison “Jack” Schmitt asked renownedspace artist Robert McCall to design a

mission patch depicting the Moon overStonehenge. McCall’s design was ulti-mately rejected, but Stonehenge symbol-izes mystery and our quest to understandthe universe. What could be better? Withthe help of a graphic artist, the design

above soon emerged, and it was all I hadhoped for. So, Dr. Schmitt, my th-gradestudents and I thank you for your idea!

Gordon Reade Palo Alto, California

Jack Schmitt replies: Actually, I think that

the San Anselmo th graders’ design for

their astronomy-club insignia is much better

than Bob McCall’s. It captures even more ofthe significance of Apollo and of Apollo ,

the most recent mission of human explora-

tion to the Moon.

Taking the Measureof Light PollutionJan Hattenbach’s “Surveying Skyglow”(S&T: May , p. ) advocates that wemeasure light pollution. Bravo! That’s goodold empirical science. Surveys will helpmap our world’s light at night and prepareus for the next step: to match those surveyswith the incidence of human disease. Lightpollution — the misuse of light at night— is not just about sea turtles, migrating

birds, or better astro photos. It’s about us.Time is of the essence.Bob Guzauskas West Palm Beach, Florida

I belong to a group called Dark Skies, Inc.,in the Wet Mountain Valley of south-central Colorado. We have a -year historyof public education about nightscape pres-ervation and have completed many projects

that retrofit unshieldedlighting fixtures with shielded ones. IFebruary , we began taking quart

sky-quality measurements (SQMs) at locations within the boundaries of ouadjoining two towns, Westcliffe and Sver Cliff. We knew our efforts had to bpaying off but were very surprised whour annual average SQM reading camat magnitude . per square arcsecon— not far from the . that corresponto a naturally dark starry sky with a liing visual magnitude of .!

But the real value came last summOne of our members noted that the

International Dark-Sky Association ha program for certifying towns as anInternational Dark Sky Community. Awe’d already accomplished most of thcriteria, except for getting Westcliffe aSilver Cliff to pass lighting ordinanceand collecting letters of support. Ourongoing education efforts really paid for both situations: the town councilsunanimously passed outdoor-lightingordinances, citing the need to protectthe dark-sky heritage and to stimulateastro-tourism. And our call for letters

support produced more than letteemails, and Facebook “likes” from oucombined population of just ,.

In just three months, the IDA’s bounanimously approved our applicatioand we became the ninth InternationDark Sky Community. Since then we’been contacted by other groups wantinknow how we did it. So we’ve developewebsite (http://bit.ly/FormDarkSkyGrouthat details not only our story but alsothe hard-learned lessons on changing

mindsets, dealing with elected offi ciaeducating the public, and more.Ed Stewart Westcliffe, Colorado

Naming Names on the MoonI very much enjoyed Andrew Livingstarticle on Giambattista Riccioli and thnaming of lunar features (S&T: May p. ). However, his point that astrono

Write to Letters to the Editor, Sky & Telescope,

Sherman St., Cambridge, MA -,

or send e-mail to let [email protected].

Please limit your comments to words.

N A S A

G O R D

O N R E A

D E


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Letters

75, 50 & 25 Years Ago Roger W. Sinnott

September-October Dark Matter “[A very interesting star] is Har-vard Variable , located in one of the darkpatches of nebulosity that abound in Sagit-tarius. [It is] a Cepheid-type variable [with] anabsolute magnitude of –.5. Since the observedmedian apparent magnitude is ., the star

should be 5, light years from the sun. . . .“When measured, however, H. V. was

found to have the unusually large color index ofabout +m., which is comparable to the colorsof the very red N-type stars. . . . The extremeredness of the star indicates that it lies in aregion of the Milky Way that is heavily obscuredby fine dust particles. . . . When the obscura-tion is allowed for, the star is found to be only light years away.

“This investigation is but one phase of alarger program to determine the distribution ofdark matter in space, and thus to aid in solvingthe problem of the size and structure of the

Milky Way system.”What gives Henrietta

Swope’s report such a

modern ring is her refer-

ence to “dark matter.”

Today this term has a

far grander meaning in

cosmology. So much

dark matter is needed

to explain, gravitation-

when warping harnesses were used in figurin

the mirror segments for the Keck telescopes,

scientists credited Leonard with pioneering t

warping technique.

October Close Encounter “The Whirlpool galaxy,

M5, and its unusual companion, NGC 5apparently brushed each other only milyears ago. Computer simulations by SethaHoward (Georgia State University) and GeG. Byrd (University of Alabama) show thatcompanion actually skimmed the edge of tlarger system’s disk. . . .

“The companion currently orbits M5 wperiod of about 5 million years and an innation to the plane of the disk of 5° or sosmaller galaxy is spiraling in toward the larone; the two should merge in less than thrmore orbits. . . .”

Recent modeling efforts do favor this sce

(that is, several enc

ters with the smalle

galaxy in an eccent

orbit). These also sh

that a single close p

of two stray galaxie

under the right con

tions, can create m

the features seen in

M1 system.

ally, the observed structure and distribution of

galaxies in the universe that theorists believe it

must consist of exotic subatomic particles that

permeate everything.

October Warping Harness “Arthur S. Leonard . . .

well-known satellite observer and opticaldesigner from Davis, California, [exhibited hisunobstructed] ‘Yolo’ reflector (named afterthe county in which he lives). What a strange-looking telescope! . . .

“At the eyepiece end of the main body isan -inch spherical mirror of inches focallength. Tilted ° ´ from being perpendicularto the incoming starlight, this primary reflectsthe image to a -inch mirror of the same focallength. Mounted in a harness to warp its figure,the secondary reflects light back through themain body and out of the adapter tube. . . .”

Art Leonard’s novelty at the 1 convention

of the Western Amateur

Astronomer s had far-

reaching consequences.

A mechanical engineer,

he’d worked out the

math for how a mirror

deforms under stress —

and then designed a har-

ness to do so precisely.

Twenty-five years later,

cal considerations played hardly any roleat all in the naming of three craters alongMare Nectaris for Christian figures ismistaken. It was almost inevitable thatRiccioli, a th-century Jesuit, wouldname a crater “Catharina.” Saint Cathe-rine is one of the traditional patron saintsof scientists in general and astronomers

in particular. In Orthodox icons, she isfrequently depicted with an armillarysphere, a quadrant, or some other piece ofastronomical equipment.

Bishop Theophilus, contrary to thearticle, is not a Catholic saint, and indeedhe’s remembered as the persecutor ofthe much-loved Saint John Chrysostom.So it is unlikely that naming a crater forhim would earn many “points with theChurch.” Instead, Theophilus and hiscanonized nephew, Saint Cyrillus, are on

the Moon for an impeccably astronomi-

cal reason. In the late s, Theophiluspublished a century-long list of accurateEaster dates based on the -year-longMetonic cycle; in the early s, Cyrilluspublished a -year-long list. This Eastercomputus remained an active topic ofastronomical research into the s, andconsequently the revered “giants” of this

field — Theophilus, Cyrillus, DionysiusExiguus, and Beda — were all assignedcraters by Riccioli.

Norman Hugh RedingtonCambridge, Massashusetts

Andrew Livingston replies: I’m very gratefulto Mr. Redington for pointing out the Easter

connection, and I’d be delighted if he or

anyone else can identify the man with themany eyes on the left of page . But there’s

no denying the “S.” in Riccioli’s label “S.

Theophil.” The Coptic Church considers him

a saint, and he still appears in some lists

current Roman Catholic saints, even thhistorian Edward Gibbon described him

far from saintly: “a bold, bad man, whos

hands were alternately polluted with gol

and with blood.” And Riccioli couldn’t hbeen too impressed either with Theophil

and Cyrillus’s wholesale destruction of c

sical learning — but, as elsewhere, he prdently left things open to interpretation.

for Saint Catherine, usually shown with

wheel of her martyrdom, she is the patro

saint of a long list of professions from law

to wheelwrights. But astronomy? Here toffi cial patron is Saint Dominic, for who

there’s no crater on the Moon.

For the Record✹ July issue, p. : The biblical citation

Nick Kanas’s article on celestial f rontisp

should be Isaiah :.

8 October 2015


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10 October 2015

News Notes

LUNAR I Moon’s Mysterious Twilight Clouds from Comets

To get astronomy news as it breaks,

visit skypub.com/newsblog.

On July th, NASA’s New Horizonsspacecraft flew through the Pluto sys-tem, culminating a -billion-kilometer,½-year-long journey. The piano-size

spacecraft came as close as , km(, miles) from the dwarf planet’s sur-face at : Universal Time, zipping pastat . km (. miles) per second.

Flight controllers at Johns HopkinsUniversity’s Applied Physics Labora-tory in Laurel, Maryland, report that thespacecraft performed flawlessly. Resultsfrom its seven instruments were not sentall at once but instead will trickle back incompressed form through mid-November.Then the entire data set will be sent again

without compression throughout .Pluto’s diameter is close to , km(, miles), slightly larger than previousestimates and also just a bit more thanEris’s , ± km. Spacecraft imagesresolved Pluto’s mottled landscape into awonderland of bright and dark regions.

Nitrogen and methane ices cap thenorth polar region, while a concentra-tion of frozen carbon monoxide exists inthe western half of a large, heart-shapedregion that’s been provisionally namedTombaugh Regio. Dark regions elsewhere

might be dominated by carbon-richcompounds. The dwarf planet’s tenu-

MISSIONS I New Horizons Reaches Pluto

Using measurements by NASA’s LunarAtmosphere and Dust EnvironmentExplorer (LADEE) spacecraft, MihályHorányi (University of Colorado, Boul-der) and colleagues have revealed thedistribution, origin, and size of clouds of

dust particles that float above the Moon’ssurface at dawn and dusk.These dust clouds were discovered

in , when NASA’s last robotic lunarlander, Surveyor , took images of astrange twilight glow along the lunarhorizon. The glow comes from light scat-tering off fine dust particles.

LADEE’s measurements reveal thatthe dust cloud is much closer to the

Moon’s surface and less dense than previ-ously thought. They also confirmed thedust’s origin: comets. LADEE recordedan asymmetric dust cloud instead of acircular one along the Moon’s day-nightline. If the dust is coming from aster-

oids, as was originally thought, then theresulting impacts would produce a muchweaker and more symmetric cloud, dueto the particles’ near-circular orbits asthey move inward toward the Sun. Inaddition, the team also found that thecloud increases in density during annualmeteor showers, which generally comefrom comet debris, the team reports inthe June th Nature.

These observations rekindle the deabout what causes the dust to levitate hang about the terminator line in theplace. The most generally accepted thstatic levitation, points to changes incharge: shaded areas become negative

charged when the solar wind bombarthe surface with electrons, whereas suareas become positively charged by thSun’s photons displacing these electrOnce an area becomes charged, the duparticles begin to leap about, attemptto get away from their like-charged nebors. The Moon’s terminator would thbe whipped into a chaotic frenzy.■ ANNE MCGOVERN

ous nitrogen atmosphere extends at least, km (, miles) from its surface.

A frozen plain (dubbed SputnikPlanum) shows no craters at all, so thissurface can’t be more than millionyears old — and it’s likely much younger— having been resurfaced by someunknown process. How such a smallworld has remained geologically active for½ billion years remains a mystery.

Meanwhile, ,-km-wide Charonhas a pronounced, reddish-black stain at

its northern pole that might be a veneof methane molecules captured fromPluto’s escaping atmosphere and con-verted by radiation over time into comhydrocarbons. Craters, fractures, andtively smooth plains appear elsewhereOne gash crosses the mid-northern latudes and appears to be longer and mdeeper than Earth’s Grand Canyon.

Next month’s issue includes detail

coverage of New Horizons’ historic fly■ J. KELLY BEATTY

Left: New Horizons took this view of Pluto on July th. The bright, prominent, heart-shaped

area has been provisionally named Tombaugh Regio, to honor Pluto’s discoverer. Right: A viewCharon, taken the same day, shows unexpected geologic diversity and a dark-stained polar ca


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MISSIONS I Lander Philae Phones Home . . .After seven months of electronic hiber-nation, Philae awakened on the surfaceof Comet P/Churyumov-Gerasimenkoin mid-June and resumed contact with itshandlers on Earth.

When last heard from, early on

November , , the European SpaceAgency’s washing-machine-size landerhad survived an unexpectedly rough-and-tumble arrival on the comet’s surface(S&T: Feb. , p. ). Its transmissionsended prematurely, after only hours,because Philae had wedged itself in aheavily shadowed location that offeredlittle direct sunlight to recharge itsonboard batteries.

But the comet’s changing solargeometry eventually provided much moresunlight, enough to revive Philae’s basicfunctions. About seconds of telemetry,relayed via the lander’s comet-orbitingmother ship, Rosetta, reached Earth on

June th at : Universal Time. Thenthe lander inexplicably went silent again,despite multiple attempts to restorecontact, until a -minute-long commu-nication session on July th. This secondcontact confirmed that Philae is receivingcommands, one of which was to transmitmeasurements from its radar sounder.

Philae appears to be in good shapedespite its extended shutdown. More than

, packets of data are stored in thecraft’s mass memory, which, when finrelayed to Earth, should reveal detailsabout the comet’s activity during the fdays prior to when the lander last phohome. Philae can only transmit these

when Rosetta is within its line of sighThe mission team also hopes the

renewed transmissions will help themangulate the landing site’s exact locatiwhich remains unknown. More impotantly, Philae woke up in time to perhadd important ground-zero measuremof the nucleus’ activity surge as ComeP neared perihelion on August th■ J. KELLY BEATTY

. . . and Rosetta Spots Sinkholes on its CometThe spacecraft orbiting Comet P/Churyumov-Gerasimenko has found holes in the nucleus. (And no, missionplanners didn’t think to pack golf clubs.)

ESA’s Rosetta spacecraft has beenorbiting Comet P since August (S&T: Aug. , p. ), gathering obser-vations of the funny-looking nucleus,which is shaped rather like a dog’s head.The observing campaign has turnedup the new pits. They’re not the first

holes seen on comet nuclei, but they arethe first that look like this. The pits tendto cluster together in small groups, andthey range from to meters ( to, feet) wide. Some pits are cylindricaland deep; others are shallow. The deepones seem to be “active,” with dusty jetsspewing from their walls or floors. Thedeepest one reaches more than mbelow the surface.

The holes can’t be from erosion,because erosion wouldn’t create such

nicely circular holes. And outbursts fromthe nucleus exhume only a thousandth asmuch material as a typical large, active pitwould have expelled.

Instead, Jean-Baptiste Vincent(Max Planck Institute for Solar SystemResearch, Germany) and colleaguesthink the pits are sinkholes. Somehow,the team writes in the July nd Nature,cavities form beneath the comet’s surface.

Once the cavity’s ceiling becomes too thinto support its own weight, it will collapse,creating deep, circular pits like thoseobserved. The collapse would exposefresh material in the pit’s sides, whichwould then partially sublimate awayand fill the pit with debris. That wouldexplain both why deep, cylindrical pitsseem to be active and why quiescent pitshave had their sides eaten away and theirbottoms filled with rubble. But why the

cavities form remains unclear.■ CAMILLE M. CARLISLE

BLACK HOLES I ToBig for its BritchesThe supermassive black hole CID is at least times too massivefor its host galaxy, raising questionsabout how closely galaxies and blacholes actually coevolve.

On average, a supermassive blachole has a mass / to / that of itshost galaxy. This consistent relationship has led astronomers to suspec

that a black hole’s growth and that oits host galaxy are intertwined.

But shining at us from a mere billion years after the Big Bang,CID- has / its galaxy’s mass. Tolook like black hole–galaxy systemswe see today, the galaxy would haveto grow by maybe another factor of without the black hole growing at aBenny Trakhtenbrot (ETH Zurich,Switzerland) and colleagues reportJuly th in Science. If so, that woul

suggest that black holes and galaxiedon’t grow in lockstep; instead, blacholes evolve quickly and their hostgalaxies follow. Other observationshave also hinted at this independenevolution, but uncertainties in allthe results make it diffi cult to knowfor sure. Read the behind-the-scenediscovery story at http://is.gd/cid■ SHANNON HALL

Seth_ is the most active of pits

discovered on Comet P. It is

meters wide and meters deep. V I N C E N T E T A L . / N A T U R E P U B L I S H I N G G R O U P

SkyandTelescope.com October 2015


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News Notes

GALAXIES I Dark Galaxies Suffuse Coma . . .

EXOPLANETS I Planet with a TailAstronomers have confirmed thathe exoplanet Gliese b is trailinggigantic, coma-like cloud behind itsin its orbit. Gliese b has about thmass and size of Neptune but orbitsits red dwarf star in just . days. In, astronomers watched Gliese pass in front of its host star in ultrav

let (UV) light and found that the stastayed dim long after the planet hadostensibly finished transiting.

David Ehrenreich (Geneva Obsevatory, Switzerland) and colleagueshave now confirmed that observatioThe team studied the planet’s transin a specific UV spectral line calledLyman alpha, which is associatedwith hydrogen. The astronomers thfound that the planet blocks morethan % of the host star’s UV ligh

whereas it blocks less than % of itsvisible light. The result suggests ththe planet is trailing a gigantic, comlike tail of hydrogen behind itselfthat continues to block starlight aftthe main transit, the team writes inthe June th Nature. Read more anwatch a video of the system at http:is.gd/btail.■ JOHN BOCHANSKI

Late last year, astronomers reported aweird find: “dark” galaxies minglingwith the other denizens of the ComaCluster (S&T: Mar. , p. ). Nowanother team has discovered a whopping of these faint, fluffy, and hard-to-

explain objects in the same cluster.Even ordinary galaxies contain a lot of

dark matter, which makes up about %of the universe’s mass. But the dark gal-axies discovered within the Coma Clustercontain even more of the exotic and invis-ible stuff, up to % of their total mass.

The galaxies are incredibly dim, mak-ing them diffi cult to pick out even withadvanced instruments. Yet despite their

lack of stars, most still span roughly thesize of the Milky Way. Only dark mattercould hold this diffuse collection of starstogether in the collision-prone environ-ment of a crowded galaxy cluster.

Jin Koda (Stony Brook University)

and colleagues found the dark galaxiesby poring over archival images from the.-meter Subaru Telescope. Astronomershaven’t seen so many of these objectselsewhere, suggesting that the crowdedcluster environment strips star-forminggas out of these galaxies, leaving onlydark matter behind, the team reports inthe July st Astrophysical Journal Letters.■ MONICA YOUNG

This color composite of Subaru images sho

some of the newly discovered “dark” galax

circled in yellow. Blue circles highlight two

axies discovered late last year. The image a

covers less than % of the studied area.

. . . and Mystery of Dust-Poor Early GalaxiesNew submillimeter observations reveallow levels of dust in nine early galaxies.The result is in line with previous predic-tions, but it does highlight a problem withsome observers’ calculations.

Peter Capak (Caltech) and colleaguesused the ALMA array, with other opticaland infrared data, to look at nine galaxiesshining at us from about a billion yearsafter the Big Bang. The astronomers

detected dust emission from only four ofthe nine galaxies, but detected a form ofionized carbon known as [CII] in all nine.

As the authors explain in the Juneth Nature, the presence of all thisionized carbon suggests a low level ofdust. Carbon likes linking up with otherelements to form molecules, so it doesn’thang around by itself for long. But withfew heavy elements to bond with, andwith minimal dust around to protect thecarbon atoms from the ionizing influ-

ence of ultraviolet radiation — which ispouring out from the young stars in thesenascent galaxies — the normally rare[CII] has become fairly concentrated.

The implication is that these galax-ies have amounts of dust similar to thatin the Small Magellanic Cloud (SMC).That’s unsurprising: dwarf galaxies likelyundergo star formation in on-and-off fits,so they’ll take longer to build up dust.

Interestingly, two of the team’s galax-ies have similar amounts of dust toA-zD, the remarkably dusty galaxythat raised eyebrows earlier this year(S&T: June , p. ). But A-zDexisted million years before thesegalaxies. What counts for a “moderate”amount of dust a billion years afterthe Big Bang is “challenging” only million years after the Big Bang, says

Veronique Buat (Astrophysics Laboratoryof Marseille, France). Perhaps in generaldust buildup was slow, but some galaxiesjumped the gun and got dusty fast.

Dust warmed by starlight is usuallythe dominant source of a galaxy’s infra-red emission, a fact astronomers exploitto estimate the rate of starbirth in distantgalaxies. If there’s less dust in earlygalaxies, then the particular correlationused to calculate the starbirth rate in anindividual galaxy will give an estimated

rate that’s too high. But the assump-tion probably has much less of an effecton estimates of star formation acrosscosmic time, because most stars form inless luminous galaxies that we alreadyassume have very little dust. And studiesthat assume a galaxy’s dust properties aresimilar to the SMC’s (such as S&T: Feb., p. ) appear okay as well.■ CAMILLE M. CARLISLE

12 October 2015


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News Notes

N A S A / J P L - C A L T E C H / J H U A P L / U N I V E R S I T Y O

F A R I Z O N A

Scientists have detected glass in sev-eral Martian craters, created by the fierceheat of impacts that melted these parts ofthe Red Planet’s surface.

Mars is pockmarked with craters,but thus far planetary scientists haven’tdefinitively found impact glass there. Theyhave seen glass in general, though: a hugedeposit lies in the north polar sand sea andacross the northern plains, covering some million square kilometers ( million

square miles). But that glass likely camefrom ancient, explosive volcanism.

Kevin Cannon and John Mustard (bothat Brown University) combined lab workand spacecraft observations to find the

newly identified impact glass. Research-ers can differentiate between glass andminerals based on how the materialabsorbs certain wavelengths of light. SoCannon whipped up a rock-powder recipesimilar to Martian dirt , then fired it tocreate glass. With the pseudo-Martianglass in hand, he determined the mate-rial’s spectral pattern.

Then the team looked for this signal inseveral impact craters observed by NASA’s

Mars Reconnaissance Orbiter. Theirhomemade computer algorithm success-fully teased glass’s signature out of thespectroscopic mess of many of the craters.The glass also matches up well with other

features interpreted as impact-spurresometimes even following the sharp mgins of melts (such as in Alga Crater) staying confined to a melt’s thin drapacross the surface (as in Ritchey Crate

the team reports June th in Geology.“This is pretty cool,” says Briony

Horgan (Purdue University), whocodiscovered the northern glass depoDetecting glass’s spectral signal is amajor challenge: glass is partially tranlucent, so it doesn’t absorb light well. the other hand, iron-bearing minerallike olivine and pyroxene — which alsappear in the craters — absorb a lot mlight, producing absorption bands thaare much deeper and easier to recogn

“Because they absorb so much morelight than glass, their signature tendsswamp out the glass signature,” she s

Glass is an intriguing find becausesince it forms by very rapid cooling, itgood at entombing biosignatures, Cannon explains. “If the impact melt coolslowly, it would cook any kind of organmatter trapped inside,” he says. Plus, is friendlier to microbes, with weakerchemical bonds than rock, making iteasier for microbes to tunnel inside.

Less exotically, scientists could use

this type of spectral analysis to find mother kinds of minerals on Mars, or oother bodies, and to better understanthe range of materials impacts produc■ CAMILLE M. CARLISLE

MARS I The Glint of Martian Glass

Scientists have found deposits of impact glass (green) preserved in several Martian craters,

including Alga (crater’s central peak shown above). Also detected are the minerals pyroxene

(purple) and olivine (red). The color-coded composition information from NASA’s Mars Recon-

naissance Orbiter is shown over a terrain model based on observations, but with the vertical

dimension exaggerated by a factor of two.

IN BRIEFExoplanet or Illusion? An analysis of stellaractivity casts doubt on whether Kapteyn b,

a supposed super-Earth circling in its star’s

habitable zone, is real. Kapteyn b is one oftwo exoplanet candidates around Kapteyn’s

star, an old, cool red dwarf (S&T: Sept. ,

p. ). Astronomers detected it using the

radial velocity method, which looks at the

small blue- and redshifts in starlight created

as the star and planet move around their

common center of mass. But starspots and

other activity can mimic this effect. Paul

Robertson (Penn State) and colleagues found

Kapteyn b’s orbit is “worryingly close” to an

integer fraction of the star’s rotation period.

They conclude in the June st Astrophysical

Journal Letters that the exoplanet isn’t real.

But other astronomers are unconvinced, and

the planet’s existence remains disputed.■ EMILY POORE

Hot Jupiter Stratospheres Explained? When present in an atmosphere, radiation-

absorbing molecules create an inversion

layer, with the temperature first decreasing,

then increasing, with altitude. This hap-

pens because certain molecules at the top

of the atmosphere absorb radiation, keeping

the middle parts cool. In the solar system

compounds like ozone and methane do th

job, but they are too flimsy to withstand t

heat in a hot Jupiter’s stratosphere. Using

the Hubble Space Telescope, Korey Hayn

(NASA Goddard) and colleagues found talizing traces of the heavy-duty absorber ti

nium oxide on the hot Jupiter WASP-b,

reported in June th’s Astrophysical Jou

Astronomers had hoped this molecule co

explain the inversions seen on hot Jupiter

(S&T: May , p. ), but until now no o

had detected it. If confirmed, this detecti

would be the first definitive correlation. ✦

■ ANNE MCGOVERN

14 October 2015


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16 October 2015

Imaging Exoplanets


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In , at the request of Carl Sagan, NASA engineersturned Voyager toward the inner solar system and com-manded its telephoto camera to take a picture of Earthfrom a distance of ½ billion kilometers ( billion miles).This produced the famous “pale blue dot” image of ourhome planet, which Sagan likened to “a mote of dustsuspended in a sunbeam.”

With many billions of Sun-like stars in our Milky

Way, it’s natural to ask whether there are other “bluedots” like ours — stable, hospitable, and teeming withlife. The search for Earth-like planets and extraterrestriallife is one of the most fundamental and noble pursuitsin all of science. Such a discovery would prove to bea major milestone of our civilization, on par with theApollo landings on the Moon.

We have taken the first steps in that quest. NASA’sKepler mission and ground-based searches have alreadydetected several roughly Earth-size planets in the tem-perate “habitable zones” of their respective stars. But wehave not yet been able to directly image them or searchtheir spectra for chemical signs of life (biomarkers) like

oxygen, methane, and liquid water.Many exoplanet astronomers and instrumentalists

are working hard to accomplish this goal. Standing onthe shoulders of the previous planet-detection efforts, weare now gearing up to image potentially habitable worldsdirectly. Depending on how close the nearest Earth-like planet exists, we might be able to capture its imagebetween and years from now.

Observational LimitationsAs of this writing, close to , confirmed planets areknown — more than , if all of Kepler’s planet candi-

dates are included. Almost all of them have been foundby one of three indirect methods: radial-velocity (Dop-pler) detections, transit photometry, and microlensing.A handful of these discoveries are the right size and dis-tance from their stars to be potentially habitable worlds.

Next Blue DotAstronomers are working to directly image alien Earths,

with several promising space missions in development.Ruslan Belikov & Eduardo Bendek

To be suitable for life as we know it, such a planet wouldhave at least three observable characteristics:

• A diameter roughly . to . times that of Earth, toenable a rocky surface with an atmosphere.

• An orbit in the star’s habitable zone, to enable liquidwater to exist on the surface; for Sun-like stars, this cor-responds to an orbital radius of about . to . astro-nomical units.

• The presence of multiple biomarkers in the atmo-spheric spectrum; for example, an atmosphere contain-ing oxygen, methane, and water vapor could not be easilyexplained without life.

Planets satisfying the first two requirements arecommonly referred to as “potentially habitable,” whilethe third would establish a “likely inhabited” planet.Thanks to Kepler’s rich trove of discoveries, a statisticalpicture is starting to emerge about how often stars hostpotentially habitable planets. Most of astronomers’ latestestimates range from about % to as high as %. Thisimplies the existence of tens of billions of potentiallyhabitable planets in our galaxy alone — every person

A PLETHORA OF WORLDS By some estimates, the MilkyWay is home to as many planets as its hundreds of billions of

stars. Astronomers are trying to determine how many of these

might be like Earth.

PAST AND FUTURE Left: When Voyager looked back toward Earth in from ½ billion km ( billion miles) away, our world barely registered as a slig

bluish blip less than a pixel wide. Right: During the next two decades, astron

mers hope to master the technology to reveal other “pale blue dots,” like the

in this simulated image of planets orbiting a distant star (hidden by mask).

Voyager’s “Pale Blue Dot” Future “Pale Blue Dot”

ESO / PETR HORÁLEK

N A S A / J P L - C A L T E C H

The


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18 October 2015

Imaging Exoplanets

alive on Earth can have at least one to call his or her own!However, despite the phenomenal success of Kepler

and other searches to date, planet hunters have beenstymied by two key limitations. First, we don’t yet have away to obtain the spectra of small exoplanets (though inthe coming decades we expect that to change). Second,current methods are not particularly good at detectingpotentially habitable planets around the nearest stars.

For example, the transit method (which Kepler uses)misses out of every Earth-like planets around

Sun-like stars because a planet’s orbit must be inclinedvery little, almost precisely edge-on to our line of sight,to be detected. As a result, since detections are rare, allof Kepler’s planets lie hundreds or thousands of light-years away.

FIRST FINDS During the past decade astronomers have successfully imaged several exoplanets orbiting their host stars. Most are young

planets still hot and glowing from their formation. In each of these images, the central star itself has been masked out.

SCANNING THE GALAXY Virtually all the thousands ofknown exoplanets were found using one of three methods:

radial-velocity measurements (dots nearest Sun), microlensing

observations, and transits recorded by NASA’s Kepler spacecraft.

Meanwhile, the radial-velocity method can anddoes probe the nearest stars, but it doesn’t yet have th

sensitivity needed to detect potentially habitable planets around Sun-like stars, and it’s blind to planets wiface-on orbits. Consequently, we’ve only detected plaaround a small fraction of stars in our immediategalactic neighborhood — even though we expect mothose stars to have planets.

Direct Imaging of ExoplanetsSince about , a new planet-detection method hasbeen gaining prominence: direct imaging. Astronomeuse one of several starlight-suppression techniques toblock a star’s bright light in order to directly image thplanets around it as small dots. While other detection

methods will continue to improve and play key roles inthe future of exoplanet science, direct imaging is the oconceivable means to perform a complete survey of annearby star’s potentially habitable planets and to obtaitheir spectra. If we want to find the nearest planet wit

D A T A : N A S A ; M I L K Y W A Y M A P : E S O

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Keplerdiscoveries, light-years

N R C - H I A / C H R I S T I A N M A R O I S / K E C K

O B S E R V A T O R Y

C H R I S T I A N M A R O I S / G E M I N I O B S E R V A T O R Y

N O A J / N A S A G S F C

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Venus

Mars

Earth

Mars

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Rayleighscattering

(blue sky)

CO

CO

CO

O

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HOHO

HO

. . . . . ..

.

.

.

.

.

.

life on it, direct imaging is arguably the only way to do it.At least nine planets have been directly imaged to

date, and dozens of other detections are possible planets.In every case thus far, and for the foreseeable future,any direct image of an exoplanet will be a spatiallyunresolved dot — just as images of stars are unresolved— because the planet’s tiny disk is orders of magnitudesmaller than the diffraction (resolution) limit of even thelargest telescopes.

Yet even though we have no hope of resolving conti-nents, oceans, or polar caps on an Earth-like exoplanetwith foreseeable technology, the amount of science wecan infer from that unresolved dot is remarkable. Forexample, we could record the planet at different timesand then fit a Keplerian orbit to its observed positions

around the host star. This, together with the star’scharacteristics, establishes whether the planet is in thehabitable zone.

Although we can’t use direct imaging to estimate theplanet’s size (as the transit method does) or its mass (asthe radial-velocity method does), both can be inferredwith some confidence from the planet’s brightness andspectrum — or from just its color. Imagine the dis-covery of a pale blue dot in the habitable zone of somenearby star. It could be either a small “Earth” or a larger“Neptune.” However, if the planet also appears billiontimes dimmer than its star, then it would have to have

an impossibly low albedo (reflectivity) to be the size ofNeptune. Thus, by elimination, only a small rocky worldwould fit all the observations.

We can attempt several other observations with directimaging. Watching a planet’s brightness and polariza-tion vary as it moves through different phases (crescent,gibbous, and so on) during each orbit can reveal thepresence of clouds or oceans. Short-term periodic bright-ness variations might disclose the length of the planet’s“day,” while chaotic and annual brightness variations give

SPECTRAL CLUES These simulated spectra reveal that

“biomarkers” (such as atmospheric oxygen and water vapor)are prominent on Earth but missing on Venus and Mars. The

detailed spectra from large space observatories will be able to

resolve these signatures. Smaller spacecraft should still be able

to differentiate between an “Earth” and a “Venus.”

JUST RIGHT Astronomers hope to find Earth-likeplanets in the habitable zones of nearby stars, temperate

regions where water can exist in liquid form. The hotter

the star, the more distant this zone lies from the star.

information about weather and seasons, respectively.Arguably the most exciting and powerful charac-

terization that direct imaging enables is recording thespectrum of the planet’s atmosphere. It’s now possibleto perform spectroscopy indirectly, by observing a planetat different wavelengths during transits or eclipsesinvolving its star (S&T: May , p. ). However, these

methods only work with a small fraction of transitingplanets, and they’re not very sensitive to Earth-like plan-ets around Sun-like stars, in which case direct-imagingspectroscopy might be the only viable option.

Key to the compositional characterization of Earth-like exoplanets is the detector’s spectroscopic resolution— that is, how finely the detector can subdivide theobserved wavelengths. Space-based missions proposedin the next decade or so might be limited to only spectral channels for potentially habitable worlds beforesignal noise becomes prohibitive. But even three-chan-nel color imaging is suffi cient to differentiate a “Venus”

or a “Mars” from an “Earth.” A white planet would indi-cate either an opaque cloud cover (like that surroundingVenus) or a snowball planet. An orange hue would sug-gest photochemical atmospheric haze similar to Titan’s.With spectral channels, we can detect oxygen andwater in the atmosphere of an Earth-like exoplanet and aplethora of other features.

Furthermore, if a planet has a relatively cloud-freeatmosphere, its atmospheric pressure can be deducedby noting how blue the planet appears, because deeper,

Hotterstars

Sun-likestars

Coolerstars

Habitable(temperate)

zone

N A S A A M E S

S & T : L E A H T I S C I O N E / S O U R C E : T Y R O B I N S O N &

V I K K I M E A D O W S


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20 October 2015

Imaging Exoplanets

denser atmospheres will exhibit more Rayleigh scatteand cause a bluer color. This density can, in turn, be to infer whether the planet is capable of sustaining liqwater on its surface. There will, of course, be ambiguties, but even low-resolution spectroscopy is good enoto establish the diversity of exoplanet atmospheres anto pave the way for the higher-resolution spectroscopyrequired to look for more definitive biomarkers.

Direct-Imaging TechnologyTwo key challenges complicate any attempt to directlimage a potentially habitable planet: contrast (how fathe planet appears compared to its star) and angular ration (how close it is to the star). As shown in the plat left, the brightness contrast is roughly – to – Earth-like planets around M dwarf stars such as ProCentauri or Barnard’s star; about – for those arouSun-like stars such as Alpha Centauri, Tau Ceti, or Elon Eridani; and about – for those around intensebright, early-type stars such as Procyon and Altair.

Meanwhile, the angular separation of the habitablzone ranges from about . to arcsecond for the neaest few dozen Sun-like stars — but the separation isonly about a tenth of that for the nearest few dozen Mdwarfs. It’s like trying to detect a firefly buzzing aroua searchlight from many miles away.

Generally speaking, direct imaging of potentiallyitable worlds requires contrasts of a billion or better except for a few of the nearest stars, a diffraction limclose to what modern telescopes can reach. Remarkainstrument concepts exist that achieve these levels operformance, with laboratory demonstrations very clto success and getting better every year. The two me

ods likely to attain these high-contrast thresholds inv“internal coronagraphs” and “external starshades.”

Internal coronagraphs utilize specially designedoptics and masks to suppress a star’s inherent brightness. They are essentially much more advanced versiof the coronagraph that Bernard Lyot invented in in order to see the Sun’s corona. The basic principle the same: a mask, placed at one of the telescope’s focplanes, blocks the star but not its planets.

However, complications arise because of the wavenature of light: diffraction causes concentric Airy ringthe image of the star, which are not blocked by the ma

and are still millions of times brighter than the planeadditional optics and masks must be used to suppressof the starlight, including the Airy rings. Another comcation is that slight optical imperfections cause starligleak through the coronagraph, obscuring planets. Mo

2030s: ≥-mspace telescope

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space telescope30- to -cm

space telescope

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2020s:Ground-based

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coronagraphs

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Star-planet separation angle (arcseconds). .

BIGPICTURE PERSPECTIVE The capabilities of present and future high-con-

trast instruments determine the types of exoplanets they can image. A samplingof the worlds imaged to date occupies the upper-right corner. Circles at lower leftindicate Earth “twins” in the habitable zones of every nearby star out to light-years. The colored regions show how the detection ability of current ground-basedtechnology compares with that of ever-better space-based imagers in the future.

BEATING THE GLARE To detect faint exoplanets, futurespace observatories will utilize coronagraphs that block the

star’s light with a combination of adaptive optics and specia

masks to suppress diffraction from the star.

R . B E L I K O V / E . B E N D E K / O . G U Y O N

N A S A / J P L - C A L T E C H ( 2 )

Normal starlight

Wavefront errors

Flat mirror

Deformablemirror

Coronagraph

Residual starlight

Planet


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coronagraphs use adaptive optics and deformable mirrorsto remove these slight aberrations.The starshade alternative suppresses starlight by

blocking the star before its light ever reaches the tele-scope. A specially designed mask placed far in front ofa space telescope — perhaps , km away! — wouldblock the star’s light. For this to work, the telescope muststay precisely positioned within the starshade’s shadow, alevel of formation flying that will be challenging but notimpossible to achieve. Even then, starlight will diffractaround the edges of the starshade, creating a bright spotright in the center of the shadow — where the telescopeis. Specially shaping the edge of the starshade can sup-

press this diffraction effect, known as “Poisson spot”or “Arago spot,” and this challenge has sparked a lot ofinnovation among mission designers.

Since there’s no practical way to keep a starshadethat’s thousands of kilometers away in space preciselyaligned with a specific point on Earth, ground-basedtelescopes can only use internal coronagraphs. However,starshades are, in principle, compatible with any space-based telescope positioned beyond Earth orbit.

Both methods have advantages and disadvantages,and conceivably both will be used in future efforts.Either way, an important tool in high-contrast imaging

is “post-processing” of the images, which can typicallyboost their contrast by a factor of or more. Sometimesimage processing used alone, without a coronagraph,can achieve good results.

Direct Imaging Instruments and MissionsAs the contrast performance of direct-imaging technolo-gies improves, so does the capability of ground- andspace-based telescopes. High-contrast coronagraphson ground-based telescopes have already recorded

direct images of numerous young, hot exoplanets farfrom their stars (well outside their habitable zones).The next generation of these instruments, collectivelyknown as “extreme adaptive optics,” is currently push-ing the performance envelope. Examples includeProject , Subaru Coronagraphic Extreme AdaptiveOptics (SCExAO), Gemini Planet Imager (GPI), andSpectro-Polarimetric High-contrast Exoplanet Research(SPHERE). Some of these are capable of directly imagingmature giant planets such as a “Jupiter” in a star’s habit-able zone, but they can’t quite pick out smaller, poten-tially habitable worlds.

The next big leap in ground-based, high-contrast

imaging will come from powerful coronagraphs attachedto extremely large telescopes (ELTs). Terrestrial observa-tories are fundamentally limited by Earth’s atmosphereto detecting

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