Science 3210 001 : Introduction to Astronomy

Lecture 7 : Extrasolar Planets and Star Formation

Robert Fisher

Items

Reading/Homework set 7 has been posted to the website.

Lunar eclipse… (mostly) clouded-out.

March 23 is week of spring break -- no class!

Midterm Review.

Lunar Eclipse, March 3 2007

The lunar eclipse was almost completely clouded-out from Chicago, but provided spectacular images from elsewhere.

Lunar Eclipse, March 3 2007

The view from Austria was amazing :

Midterm Grading and Review

Review of Last Week

The Outer Planets

Today -- Extrasolar Planets and Star Formation

Planetary “Discoveries” Worthy of The Onion

Properties Hot Jupiters Planet Migration Extrasolar Planetary Atmospheres

Methods of detection and observation Direct Imaging Doppler (spectroscopic) method Transits (photometric) method

Star Formation -- The Big Picture

Planets Outside Our Solar System?

For centuries people have speculated that planets exist around other stars.

In 1584, Giordano Bruno wrote “There are countless suns and countless earths all rotating around

their sun exactly the same way as the seven planets of our system… The countless worlds in the universe are no worse and no less inhabited than our Earth.”

This idea was not always as popular as it is today -- Bruno was burned at the stake in 1600, partially because he held this view -- among many other radical views for the 16th century.

Statue of Bruno in Campo de’ Fiori, Firenze

Worlds Galore?

The basic ideas for the detection and scientific study of planets around other stars has existed for at least fifty years, but it has only become technologically feasible in the last few years.

Among the key questions…

How common are extrasolar planets?

How common are Earthlike planets?

Is is possible to detect the existence of life outside our solar system?

How do these lessons inform our understanding of our own solar system?

Early Ideas for Planet Detection

The primary method of detection and study of extrasolar planets used today was first suggested by the Russian-American astronomer Otto Struve in the early 1950s.

Other Worlds Around Other Stars…

Science fiction images of other worlds around other stars are pervasive in our culture, yet prior to 1990, no such planets were known.

Pulsar Planets

The first evidence for planets outside our own solar system came from an unusual place -- around a rapidly rotating dead star, known as a pulsar.

Detailed timing studies of the radio signals emitted from pulsars can be made to extraordinary precision.

Extraordinary precision of pulsar measurements have made pulsars unique timeclocks which allow for high-precision physics measurements… and also permitted the first discovery of a planet outside the solar system.

Pulsar Structure

The magnetic field surrounding a dead, extremely-dense, rapidly-spinning neutron star sends a beamed radio pulse at a regular interval of time.

Crab Pulsar

The Chandra Space Telescope produced a spectacular image of the Crab Pulsar in X-rays, showing the high-energy gaseous nebular powered by the emission from the pulsar.

Pulsar Signal

100 pulses from the first pulsar to be discovered, PSR 1919+21 were used for the cover of the Joy Division album Unknown Pleasures.

A Planet Around Pulsar PSR1829-10?

In 1991, a British team announced the first-ever detection of the first extrasolar planet in the prestigious journal Nature.

Oops … Not!

Less than a year later, Lyne and colleagues retracted their announcement.

Pulsar PSR 1257+12

Given the controversy surrounding PSR 1829-10, it is incredibly remarkable that within a year, the first actual discovery of an extrasolar planet was announced -- around another pulsar!

In 1992, while studying an isolated millisecond pulsar PSR 1257+12, Alex Wolzczan of Penn State University made a stunning discovery of planets surrounding the pulsar.

Eventually three planets surrounding PSR 1257+12 were discovered and confirmed, including a possible minor body -- an asteroid, comet, or Kuiper belt-like object.

Artist’s Rendition of the PSR 1257+12 System

PSR 1257+12 Planetary System

Three planets around PSR 1257+12 have been confirmed :

PSR B1257+12A Orbit at .19 AU, Period 25 Days, Mass about Twice Earth’s Moon

PSR B1257+12B Orbit at .36 AU, Period 66 Days, Mass about that of Earth

PSR B1257+12C Orbit at .46 AU, Period 98 Days, Mass about 4 times that of Earth

In addition, a tiny body (asteroid? comet? Kuiper-belt-like object?) may exist at about 2.6 AU, with a mass less than Pluto

Origin of PSR 1257+12 System?

The unusual nature of the pulsar planetary system, which must have formed after the explosion of the central star in a supernova, has fueled intense speculation.

One possibility is that these planets are left over from a solar system that existed before the star exploded.

More likely, however, is that these planets formed after the supernova, from a nebula of material blown off from a companion star, which then reformed into a new solar system.

Because pulsars systems are relatively rare, this system is an exceptional case which may be dissimilar from solar systems around sunlike stars.

Direct Detection

The most obvious method of detecting a planet around another star is to directly image the planet, just as we image stars.

Unlike stars, planets are both much smaller and observable only in reflected light, and so are far fainter. The glare of their parent star makes it incredibly difficult to see them.

Question

If you had to photograph a planet next to a star, and the star were literally billions of times dimmer than the star, how would you go about doing it?

Coronagraph

An astronomical instrument known as a coronagraph is designed to artificially block out the light from the sun in much the same way that the moon does during a total solar eclipse.

Using this instrument it is possible to see much fainter details around the sun. For instance, the detection of a comet near the sun --

Direct Detections

To date, however, direct detection methods have been almost entirely unsuccessful.

The only direct detection to date is the planet orbit brown dwarf 2M1207 and its companion planet 2M1207b.

2M1207

2M1207b

51 Peg -- The First Extrasolar Planet Around a Sunlike Star

The next major detection technique is the Doppler, or spectroscopic technique.

The first planetary system discovered with the Doppler technique was 51 Pegasus or 51 Peg.

This was the first planet detected around a normal star like our sun.

The system was first announced in the October 6, 1995 isue of Nature by Swiss astronomers Michael Mayor and Didier Queloz of the University of Geneva using the ELODIE spectrograph at an observatory in Provence, France.

Echelle Spectrograph

The modern spectrographic instrument used in planet searches is the echelle spectrograph, which is made from a special kind of diffraction grating called an echelle grating.

These spectrographs are capable of extraordinarily high resolution, equivalent to tens of thousands of pixels across the spectrum -- essential for measuring the small motions of the central star in a planetary search.

ELODIE echellespectrograph

ELODIE Sample Spectrum

The spectrum is broken up into a number of bands and imaged with a CCD detector, which is the same kind of detector in a digital camera.

The Discovery of 51 Peg

The discovery of 51 Peg was confirmed immediately by Geoff Marcy and Paul Butler (both then at UCSF) on October 12, 1995.

Marcy and Butler and their team went on to discover the vast majority of the extrasolar planets known to date (70 of first 100!).

Doppler Shift

We are all familiar with the shift in pitch of a moving source of sound either towards or away from us.

The shift is due to a “bunching up” of the emitted soundwaves when moving towards us, and a “stretching out” of soundwaves when moving away from us

Optical Spectrum Shifts

Visible light also undergoes a Doppler shift when the source is moving.

When the source is moving away from us, we see the spectrum shift “to the red” -- a redshift. Similarly, when moving towards us, we see the spectrum shift “to the blue” -- a blueshift.

The shift is only detectable when the spectrum has definite lines.

Doppler Planet Detection

In a stellar system with a planet, both the star and the planet revolve around the center-of-mass.

Even if the planet is not directly visible, the star can be observed to rotate around the center-of-mass of the combined system.

Doppler Detections

The motion of the central star can be detected by a periodic repeating Doppler shift in its spectrum -- through red, blue and back again.

Measurements reveal the period of the planet as well as the mass of the planet.

Curve for 51 Peg

High-precision measurements of the spectrum of 51 Peg revealed that the star was oscillating back and forth, with a period of 4 days.

51b Peg -- The Planetary Companion to 51 Peg

The properties of the planetary companion to 51 Peg turned out to be highly unusual.

It orbits 51 Peg with a period of 4.2 days, placing it at just .05 AU away -- far closer than even Mercury is in our solar system (.40 AU).

Much stranger still, its mass is much larger than any of the inner planets in our solar system, with nearly half the mass of Jupiter -- qualifying it as a giant planet.

Its extreme proximity to 51 Peg implies a surface temperature of nearly 1200 (!!) degrees Celsius.

Hot Jupiters

Since the discovery of 51 Peg, however, many more such “hot Jupiters” have been detected -- over 200 planets have been detected to date.

Some extrasolar planets the size of Saturn and even Neptune have been detected, though none are terrestrial planets like the Earth.

While the surveys are biased (they are currently unable to detect Earthlike planets, even if they do exist), this implies that our own solar system may be atypical in some ways.

The million dollar question was -- how could such a strange hot Jupiter system could form?

Giant Planet Shipping Express -- Disk Gaps and Disk Migration

Fundamental work done on protostellar accretion disks by Doug Lin of University of California Santa Cruz and colleagues demonstrated that as it is forming, a protoplanet exchanges mass and angular momentum with the protostellar accretion disk.

Giant planets are able to open up a gap in the disk, at which point they are expected to migrate inwards.

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Artist’s Conception of 51 Peg

The proximity of “hot Jupiter”-like planets like 51b Peg to their parent star suggests that they are slowly leaking their atmospheres through evaporation.

51 Peg in Popular Culture

The definite sign that a discovery has hit the big time…

Transit Detections

Both Mercury and Venus occasionally pass directly between the line-of-sight joining the Sun and the Earth, resulting in a spectacular “transit” of the planetary disk across the solar surface.

2004 Transit of Venus

Transit Detections

In our own solar system, transits were very important historically in establishing the actual physical distance from the Sun to the Earth.

Transit of Mercury in 2003

The Hypothetical Planet Vulcan

Even more interestingly, a number of amateur and professional astronomers have observed unexplained transits, beginning with the physician and amateur astronomer Edmond Lescarbault in 1859, who wrote Urbain Leverrier of his discovery of a “intra-Mercurial” planet.

Leverrier named the hypothetical planet Vulcan, and predicted an orbit and future transits for it, though the “planet” itself was never seen.

Interest in the hypothetical planet Vulcan peaked by around 1900, when Mercury was shown to deviate substantially from its predicted orbit.

The Hypothetical Planet Vulcan

In 1915, Albert Einstein published his General Theory of Relativity, which explained the deviations in Mercury’s orbit.

Since then, the general consensus is that the transits of bodies other than Mercury and Venus are likely a population of asteroids interior to the orbit of the Earth.

Work continues to this day in detecting a possible family of vulcanoid asteroids, which probably do exist, but are incredibly difficult to observe if they do exist.

Vulcan

Interest in the planet Vulcan continued in science fiction well after 1915…

Extrasolar Transit

If a planet transits over the disk of the star, by carefully measuring the amount of light emitted from a star over time, one can see a dip in the light emitted, even if the planet itself is too faint to see directly.

HD209458 Transit

The first detection of an extrasolar planet transit was made in 1999 in the HD209458 system by David Charboneau, then a graduate student at Harvard.

While an amazing achievement, it was made by observing a system already known to have an extrasolar planet from a Doppler survey.

See an Extrasolar Planet in Your Backyard!

The shift of about 1% in the brightness of the central star is observable even in larger amateur-sized telescopes.

The first amateur detection was made less than a year after Charboneau’s announcement at an observatory in Finland.

Upsilon Andromeda -- The First Extrasolar System of Planets Around a Sunlike Star

In 1999, three extrasolar planets were detected around Upsilon Andromeda A -- itself orbiting its companion star Upsilon Andromeda B, a much less massive red dwarf at 750 AU separation.

This was the first detection of an extrasolar system of planets around a sunlike star, as well as the first system around a binary star system.

Upsilon Andromeda b -- Mass .7 Jupiter, 4.6 Day Period, .06 AU Upsilon Andromeda c -- Mass 2 Jupiter, 240 Day Period, .83 AU Upsilon Andromeda d -- Mass 3.9 Jupiter, 1290 Day Period, 2.5 AU

Upsilon Andromeda

A comparison of the Upsilon Andromeda system with our Solar System.

Extrasolar Planetary Atmosphere Detection

First detection of an atmosphere around an extrasolar planet was made by David Charboneau, using Hubble Space Telescope observations of the transit system HD209458 we discussed earlier.

By carefully inspecting the spectrum of the star, Charboneau and colleagues were able to detect sodium absorption lines during transits of HD209458b -- a clear indication that they were detecting the atmosphere surrounding an extrasolar planet for the first time.

Later observations of HD209458b also revealed absorption lines of carbon and oxygen, which are thought to have originated from the strong wind being driven from deep interior to HD209458b by its star.

Extrasolar Planet Atmospheres

These observations suggest that extrasolar planets like HD209458b are losing their atmospheres over time.

Some extrasolar planets may have such a high evaporation rate they eventually lose their giant atmospheres altogether -- leaving behind a massive rocky core.

Spitzer Space Telescope

NASA had a grand vision of four “Great Observatories” to be launched into space, each covering a different portion of the spectrum.

In December 2003, Spitzer Space Telescope, covering the infrared portion of the spectrum became fully operational.

Lyman Spitzer, Jr. (1914 - 1997)

The Spitzer space telescope was the last of the four Great Observatories.

It was appropriately named after Lyman Spitzer, Jr. -- the man who first proposed the concept of space telescopes in the 1940s.

Spitzer also contributed fundamental advances to our knowledge of the interstellar medium and plasma physics.

Spectrum of an Extrasolar Planet

Just two weeks ago, the first-ever direct measurement of the spectrum of an extrasolar planet was announced!

The Spitzer Space Telescope was used to measure the spectrum of the now-familiar transiting planet HD204958b.

Looking to the Future

What stops the inward migration of giant planets, if anything?

If nothing stops the inward migration, then is the formation of a solar system analogous to a game of musical chairs?

Will we be able to infer the existence of life on other worlds?

What will direct images of extrasolar planets reveal?

Today’s Weather Forecast for HD204958b -- Hot and Dry

Formation of Stars and Planetary Systems

Astronomers have long known about cold, dark regions on the sky which obscure starlight.

These regions consist of very cold, dense regions of dusty molecular gas which will in most cases form stars.

M51 “Whirlpool Galaxy” in Optical

Collision of M51 with NGC 5195

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Atomic and Molecular Gas

Single Atom (eg, H, C, O) or Ion (H II, O VI)

Two or more Atoms (eg, CO, NH3)

Trace warm (~ 1000 - 104 K) gas in visible range of spectrum

Atomic or Ionized

Trace cold (~ 10 - 100 K) gasin radio - infrared range of spectrum

Molecular

Whirlpool Galaxy M51 in Optical and Submm

HST + OVRO (Scoville et al, 2004)

Overlay of Optical and CO Emission in M51

Traffic Jam

Traffic Jam (or “Spiral Density Wave”) Theory of Formation of Giant Molecular Clouds

<-- Atomic

Lin, C.C. & Shu, F. (1962)

Molecular -->

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Distance across spiral arm

Next Week -- Star Formation

How does a tenuous cloud of cold gas collapse under its own weight and form a star which ignites at millions of degrees?

How do binary and multiple star systems form?

How did our solar system originate?