extrasolar planetsextrasolar planets - meteor mcampbell/a9601/chapter 12 -...

Extrasolar planetsExtrasolar planets

Astronomy 9601

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Topics to be coveredTopics to be covered

12 1 Physics and sizes12.1 Physics and sizes 12.2 Detecting extrasolar planets

12 3 Ob ti f l t 12.3 Observations of exoplanets 12.4 Exoplanet statistics 12.5 Planets and Life

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What is a planet? What is a star?What is a planet? What is a star?

The composition of Jupiter closelyThe composition of Jupiter closely resembles that of the Sun: whos to say that Jupiter is not simply a failed star p p yrather than a planet?

The discovery of low-mass binary stars y ywould be interesting, but (perhaps) not as exciting as discovering new true planets.

Is there a natural boundary between planets and stars?

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Planets and brown dwarfsPlanets and brown dwarfs A star of mass less than 8%

of the Sun (80x Jupiters mass) will never grow hot

Luminosity bump due to short-lived deuterium burning mass) will never grow hot

enough in its core to fuse hydrogen

This is used as the boundary between true stars and very

Steady luminosity due to H burning

between true stars and very large gas planets

Objects below this mass are called brown dwarfs

The boundary between BD and planet is more controversial some argue it should be g

based on formation other choose 0.013 solar

masses=13 Mj as the boundary, as objects below this mass will never reach even deuterium fusion

Nelson et al., 1986, AJ, 311, 2264

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Pulsar planetsPulsarplanets In1992,WolszczanandFrail

announcedthediscoveryofamultiplanetplanetarysystemaroundthemillisecondpulsarPSR1257+12(anearlierannouncementhadbeen

Artists conception of the planet orbiting pulsar PSR B1257+12

retracted). Thesewerethefirsttwoextrasolar

l t fi d t b di dplanetsconfirmedtobediscovered,andthusthefirstmultiplanetextrasolarplanetarysystemdiscovered,andthefirstpulsarplanetsdiscovered

However these objects are not in However,theseobjectsarenotinplanetarysystemsasweusuallythinkofthem 6

Worlds Beyond Our SunWorlds Beyond Our Sun In 1995 a team of Swiss

astronomers disco eredastronomers discovered the first planet (in a non-pulsar system) outside

l t bitiour solar system, orbiting a sun-like star called 51 Pegasi.

Further discoveries bring the grand total of known extrasolar planets to 861 (as of March 2013) and counting.

Artist's rendition of the star 51 Pegasi and its planetary companion 51 Pegasi B.

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Unseen CompanionsUnseen Companions

Curiously enough,Curiously enough, most extrasolar planets remain unseen

They are usually detected by indirect means, though their effects on their parenteffects on their parent star.This artist's concept shows the Neptune-sized extrasolar planet

circling the star Gliese 436. g

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Obstacles to Direct Detection Direct detection is the only way to tell what these planets

are made of and whether there's water or oxygen in their t hatmospheres.

But most known exoplanets are impossible to see with current technology gy

Two reasons why: known exoplanets are too dim

Jupiter for example is more than a billion times fainter than Jupiter, for example, is more than a billion times fainter than the Sun. However it could easily be seen at large distances except for

known exoplanets orbit too close to their parent stars known exoplanets orbit too close to their parent stars most known exoplanets have orbits smaller than that of

Mercury

"It's like trying to see a firefly next to a searchlight from across town."9

The first confirmed image of an exoplanet:

GQ Lupi & Planetary Companion

21 Mj, 100 AU orbit. Imaged by ESOs VLT, then HST and Subaru confirmed (early Apr 2005) 10

Detection methods: Astrometryy oldest method,

used since 1943used since 1943 the wobble

induced in the plane of skyplane-of-sky motion of the star by planets is measured bymeasured by accurately observing its position over timeposition over time

1 detection

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Astrometry STEPS (Stellar Planet Survey)

detected periodic proper motion of VB 10 a nearby brown dwarfof VB 10, a nearby brown dwarf.

VB 10b is approximately 6 Jupiter masses, with a period of 9

hmonths. No sign of planet when examined

with other techniques: busted! q

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Astrometry:DifficultiesyExample: The Sun

bbl b b t itwobbles by about its diameter, mostly due to Jupiter. At 30 light-years, this would produce an apparent motion of lessapparent motion of less than 1 milliarcsecond. Typical good ground-based observingbased observing conditions produce positions with accuracies

A t ti f S f 30 lbelow but around 1 arc-second.

Apparent motion of Sun from 30 ly

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Detection methods: Pulsar planetsDetection methods: Pulsar planets

Pulsar planets are planets that are found orbiting pulsarsp p g p Pulsars are rapidly rotating neutron stars.

Pulsar planets are discovered through radio pulsar timing measurements to detect anomalies in thetiming measurements, to detect anomalies in the pulsation period. Any bodies orbiting the pulsar will cause regular changes in its pulsation. Since pulsars normally rotate at near-constant speed any changes cannormally rotate at near constant speed, any changes can easily be detected with the help of precise timing measurements.

The first ever planets discovered around another star The first ever planets discovered around another star, were discovered around a pulsar in 1992 by Wolszczan and Frail around PSR 1257+12. Some uncertainty initially surrounded this due to an earlier retraction of ainitially surrounded this due to an earlier retraction of a planet detection around PSR 1829-10

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PSR 1257+12 Pulsar located 2630 light years away These were the first extrasolar planets ever discovered These were the first extrasolar planets ever discovered Pulsar mass 0.3 Msun, rotational period 0.0062 seconds

Mass(ME) a(AU) Period(days) e

Firstplanet 0.020 0.19 25.26 0.0

Second planet 4.3 0.36 66.54 0.02

possible small fourth object has an upper mass limit of 0 2 M

Secondplanet 4.3 0.36 66.54 0.02

Thirdplanet 3.9 0.46 98.21 0.025

possible small fourth object has an upper mass limit of 0.2 MPlutoand an upper size of R < 1000km.

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5 of the 12 known pulsar planet systemsP l l t M O bit di t O bit i dPulsar planet Mass Orbit distance Orbit period

PSR B1620-26 c 2.5 Jupiters 23 AU 100yr

V391 Peg b 3.2 Jupiters 1.7 AU 1170 days

PSR 1257+12 a 0.02 Earths 0.19 AU 25 days

b 4.3 Earths 0.36 AU 66 days

c 3.9 Earths 0.46 AU 98 daysc 3.9 Earths 0.46 AU 98 days

d 0.0004 Earths 2.7 AU 3.5 years

QS Vir b 6.4 Jupiters 4.2 AU 7.9 years

HW Vi b 19 2 J it 16HW Vir b 19.2 Jupiters 16 years

c 8.5 Jupiters 332 days

Since neutron stars are formed after the violent death of massive starsSince neutron stars are formed after the violent death of massive stars (supernovae), it was not expected that planets could survive in such a system.Its now thought that the planets are either the remnant cores of giantIts now thought that the planets are either the remnant cores of giant planets that were able to weather the supernova, or later accretion products of supernova debris. 16

Detection methods: TransitsDetection methods: Transits

Planets observed at inclinations (measured with respect to the plane of the sky) near 90o will pass in front of (transit) their host stars dimming the light of the star( transit ) their host stars, dimming the light of the star. This may be detectable by high-precision photometry.

Note that the planet is invisible, being unresolved, only p , g , ythe brightness variation in the star is seen.

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The Observational ChallengeThe Observational ChallengeThe fraction of stars expected to have transits is:

f f f ff = fs fMS fCEGP ptfs = fraction of stars that are single = 0.5fMS = fraction of those on the main sequence = 0.5f = fraction of those that have a close in planet = 0 01fCEGP = fraction of those that have a close-in planet = 0.01pt = fraction of those with an inclination to transit = 0.1

Need to look at 4000 stars to find 1 that transitsNeed to look at 4000 stars to find 1 that transits. Need to sample often compared to transit duration. Need 1% accuracy for a 3s detection of a 2 hour transit. Need to look on sky for at least 1 orbital period.y p

Requires 1,000,000 15-minute samples with 1% accuracy to detect one transit.with 1% accuracy to detect one transit.

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TransitsTransits

Assuming The whole planet passes in front of the star

A d i i li b d k i f th t li ibl And ignoring limb darkening of the star as negligible Then the depth of the eclipse is simply the ratio

of the planetary and stellar disk areas:of the planetary and stellar disk areas:2

2

2

==

RR

RR

ff pp f = light flux

We measure the change in brightness, and estimate the stellar radius from the spectral type

*2

**

RRf

estimate the stellar radius from the spectral type 19

TransitsTransits

Ad t Advantages Easy. Can be done with small, cheap telescopes

WASP, STARE, numerous others Possible to detect low mass planets, including Earths,

especially from space (Kepler mission, launched Mar 2009)

Disadvantages Probability of seeing a transit is low

Need to observe many stars simultaneouslyy y Easy to confuse with binary/triple systems Nee