The Planets of Other Stars. The Astronomy Diagnostic Test (ADT): The Sequel On the first day of class, the University requested that everyone fill out.

The Planets of Other The Planets of Other StarsStars

The Astronomy Diagnostic Test (ADT): The Sequel

On the first day of class, the University requested that everyone fill out an online questionnaire through Angel. Now they want you to do it again.

As before, the survey won’t be graded, but participation will count towards your homework grade. Just answer the questions the best you can. (Taking the test may even help you study for the final!)

The test is 30 minutes long, and the link will disappear after Thursday.

Detecting Other Planetary SystemsFor 50 years, astronomers have been looking for planets around other stars. Since planets are at least 100,000,000 times fainter than stars (because all they do is reflect a bit of the star’s light), direct detection is not (yet) possible. But planets can be found in 5 different ways:

• Astrometrically (via a positional “wobble”)

• Spectroscopically (via blueshifts and redshifts of absorption lines)

• Photometrically (via transits)

• Timing phenomenon

• Gravitational lensing

Astrometric (Wobble) DetectionsBecause our Sun (and other stars) are moving through space, the positions of stars on the sky change ever so slightly each year. This is called proper motion.

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Astrometric (Wobble) DetectionsIf a star’s proper motion wobbles with time, it could be due to an unseen companion. Only Jupiter-mass planets have enough mass to be detected in this way.

Astrometric (Wobble) DetectionsThe greater the mass of the unseen companion, or the closer the separation, the greater the wobble. Detecting binary stars in this way is tedious, but do-able. Detecting planets astrometrically is extremely difficult.

0.001 arcsec

If one were to observe the Sun from 10 pc away, its wobble (due principally to Jupiter and Saturn) would be less than 0.002 over 30 years. (Recall that the atmosphere blurs things out by 1, and, with our best measurements, we can measure parallaxes to 0.003)

Spectroscopic DetectionsPlanets can be detected via measurements of the Doppler effect. The planet won’t be detected, but the reflex motion of the star might. (Thus, the star is like a single-line spectroscopic binary, with an extremely low-mass companion.)

The Sun’s reflex motion due to Jupiter is about 13 meters/sec. For reference, the absorbing gas in the Sun’s atmosphere moves about 13 km/sec, due to thermal effects alone.

Only Jupiter-mass planets can be detected in this way.



Transit DetectionsIf a planet moves in front of its star, the light from the star will decreases very slightly, (less than 1%), depending on the size of the planet. Only Jupiter-sized planets are big enough to be detected.

Mercury transit

Venus transit

Jupiter transit (artist conception)

Timing DetectionsIf a star system contains a very accurate clock, you can tell when the star is closer to you (or further away) by timing when the clock’s signal arrives. (In practice, the only objects that this can be applied to is millisecond pulsars.)

The faster the pulsar, the more accurate the timing. In theory, objects as small as Mercury could be detected around a millisecond pulsar by the gravitational force it exerts on its parent star.

Gravitational Lens Detections

If a star/planet moves exactly in front of a background star, the brightness of the background star can be greatly magnified by the gravitational lens effect.

Gravitational Lens Detections

In principle, the gravitational lens technique can detect planets of any mass. However, once the event is over, the planet is lost forever (since we are only seeing the background source). It is impossible to learn anything more about the system.

History of Extra-Solar Planets

• 1960’s – 1990’s: Numerous claims (and retractions) of planet detections via astrometry and spectroscopy

• 1991: First extra-solar planetary system (accidentally) found by timing a millisecond pulsar (PSR B1257+12)

PSR B1257+12

Orbiting the 6.2 millisec pulsar are (at least) 4 small planets, with masses of 0.02, 4.3, 3.9, and 0.0004 M. These objects were mostly likely formed after the supernova, and after the pulsar evaporated its companion star. The orbits of planets B and C are in a 3:2 resonance.

• 1995: First planet found around a “normal” star (51 Pegasi) using spectroscopy


• 1960’s – 1990’s: Numerous claims (and retractions of planet detections via astrometry and spectroscopy


51 Pegasi

51 Pegasi is a G5 main sequence star 15 pc from the Sun, whose Doppler motion changes by 53 meters/sec over a period of 4.2 days. The data imply the presence of a planet with

a roughly circular orbit a semi-major axis of 0.052 A.U. (For comparison,

Mercury is 0.38 A.U. from the Sun.) a mass of 0.46 MJup! It’s like a “hot” Jupiter!!

• 1995 – present: Over 250 planets found -- mostly “hot” Jupiters





Hot Jupiter Systems

Some of the planets’ orbits are significantly elliptical!

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• 1999: First extra-solar planet seen transiting its star (HD 209428)





• 1995 – present: Over 250 “hot Jupiters” detected around 230 stars

HD 209428

Every 3.523 days, the G0 main sequence star HD 209428 dims by about 1.7%. This indicates that the planet is 60% larger than Jupiter.

The star’s Doppler measurements imply a mass of 0.63 MJup.

The density of the planet (0.27) is much less than water. The planet must be a gas giant that is “puffed up” by the heat from the star.



• 2003: First probable planet found (temporarily) via a gravitational lens





• 1995 – present: Over 130 “hot Jupiters” detected around ~120 stars

• 1999: First extra-solar planet seen transiting its star (HD 209428)

OGLE 2003-BLG-235/MOA 2003-BLG-53

The modeling of the gravitational lens event implies the existence of a planet in orbit about a 0.36 M lensing star. The planet has a mass of about 1.5 MJup and a distance of about 3 A.U. from its star.



Results from Extra-Solar Planet Studies

Reflex motion techniques work best when the planet is large and close to its star. Thus the data we have are biased. However, it is clear that many stars have Jupiter-mass planets in their inner solar system.

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The data also show that the more metals in a star, the more likely the star is to planets around it. This suggests that planets cannot form out of hydrogen and helium alone -- even gas giants need a solid core around which to form.

As of December 8, 2008, 267 planets are known outside our solar system around 228 stars (not counting four gravitational lens events).

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Metallicity (compared to Sun)


Many stars have hot Jupiters, and not all are in roughly circular orbits. But according to the solar nebula hypothesis, Jupiter-type planets cannot form close to a star, due to the star’s radiation pressure and stellar wind. How can this be?

Explaining the Hot Jupiters

Best Model: the hot Jupiters must have formed in the outer regions of their star systems, and then spiraled in due to friction in the protostellar disk. (But if they spiral in too much, they collide with the star.)

If this is the case, then smaller terrestrial planets in the inner part of the disk were destroyed when the Jupiter-mass planet passed by.

The Planets of Other Stars. The Astronomy Diagnostic Test (ADT): The Sequel On the first day of class, the University requested that everyone fill out.

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