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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 9:30

Lecture 46:
ExoPlanets: Planets Around Other Stars

Key Ideas:

Search for planets around other stars.

Current Search Techniques:

Extrasolar planetary systems:

Are we alone in the Universe?

The question of the existence of other planets beyond the solar system, and whether any can harbor life is an old one in astronomy.

Scientific Questions

In science, we prefer questions we can answer quantitatively to idle speculation.

The scientific problem has become one of searches for:

Searches for Extrasolar Planets

There are two basic search strategies:

Direct Detection:

Gravitational Detection:

Wobbling Stars

Recall Newton's form of Kepler's 1st Law:

Viewed from afar, the star will appear to "wobble" about the center of mass of the star-planet system.

There are two manifestations of this wobble: Astrometric Wobble and Doppler Wobble.

Astrometric Wobble

Parent star wobbles back & forth on the sky as seen relative to more distant "background" stars.


From 18 light years away, the astrometric wobble of the Sun is <0.001 arcseconds!

This method has been tried, but with limited success so far. Future high-precision astrometric satellites like SIM (Space Interferometer Mission) and GAIA will have the precision required to measure astrometric wobbles from planetary systems.

Doppler Wobble Method

Another way is to use the "Doppler Effect" to detect the orbital motions of the wobbling star.

Measuring the orbital motions provides an estimate of the unseen planet's mass via Newton's form of Kepler's third law.

Doppler Wobble Measurements

The greater mass of the star put it close to the center-of-mass of the star-planet system, and thus it has a very slow orbital speed.

Example: Orbital Speeds of the Sun & Jupiter

The challenge is to measure the Doppler shifts of the lines with extremely high precision.

To convey some idea of the scale of the problem, a person can walk at a speed of about a meter per second, while a car moving 65 MPH is moving at 29 meters/second.

51 Pegasi

Michel Mayor & Didier Queloz at Geneva Observatory observed a periodic wobble in the star 51 Pegasi in 1995.

This was the first planet found around a sun-like star using the Doppler wobble method.

It was quickly followed by other discoveries by teams in California (Geoff Marcy & Paul Butler) and Texas (Bill Cochran & Artie Hatzes).

Planetary Transits

If the orbital plane of an extrasolar planet is aligned with the line of sight: At present, there are about 5 cases of candidate transiting planets.

The Case of HD209458

A star with a Jupiter-sized planet found via the Doppler Wobble method:

Using an orbit prediction from the Doppler work, astronomers observed HD209458 and were able to see the planet transit its parent star.

A number of searches are under way for more transiting planets, and at least three more have been found so far. OSU is engaged in a couple of different searches for transiting planets, but no discoveries to date.

Gravitational Microlensing

If two stars line up, one near and the other far, the light from the background star passing around the foreground star will be bent by the foreground star's gravity.

If there is also a planet around the foreground lensing star, its gravity will also produce a brief, intense amplification if it passes close to the line of sight.

To date, two planets have been found by gravitational microlensing by the OSU team

Roster of New Planetary Systems

As of November 2006, various techniques have found more than 200 planets with masses of <18 Jupiter Masses around around other stars:

Some caveats are in order:

Strange New Worlds

None of the planetary systems found so far resembles our Solar System.

The biggest surprises:

What is going on?

Migration and Other Ideas

There are a couple of possible explanations:



The migration idea is currently the subject of intense research. For example, why didn't the 4 Jovian planets in our Solar System migrate inwards?

Part of the answer is that they have migrated at least a little, since we see asteroids swept into resonances (Kirkwood Gaps), and the Plutinos swept into 3:2 resonance with Neptune. For whatever reason, migration was nowhere near as extreme as implied in these other systems.

The theoretical and observational picture is still very confused, but we are making exciting progress in trying to understand what is going on.

The Future

There are continuing searches for other planetary systems.

Basic Goals:

Future Goals:

This will be one of the most important projects of the 21st Century.


Recalling the caveats mentioned above, the Doppler Wobble experiments are most sensitive to systems with large planets close to their stars, but cannot (yet?) tell us about planetary systems like our own. To do that, the experiment would need to achieve at least 1 meter/second precision, and be carried out over a much longer time. Recall that Jupiter is 5.2 AU from the Sun with an 11.2 year orbital period. To get an orbit, you need to observe the system for at least 1, and preferably 2 orbits, which would require at least 11 years of data, and better, 22 years of data. All of the Doppler Wobble experiments have been going on since 1988, and not really at full precision until the last decade. We still don't know if the systems we have detected are representative, and therefore do not know if our solar system is a rarity or commonplace. Understanding the "selection effects" in all of these studies in an on-going (and contentious!) undertaking.

For more information, and breaking news, try these websites:

California & Carnegie Planet Search

The Geneva Extrasolar Planet Search Programmes

The Extrasolar Planets Encyclopaedia (a multi-lingual site in France).

The Space Interferometry Mission

Kepler Mission search for Earth-size planets using the transit method from space. Currently scheduled for launch in October 2008.

COROT Mission mini-satellite to study stellar oscillations and search for exoplanets using the transit method, being prepared for a December 21, 2006 launch from Baikonour Space Center.

Planet Quest at JPL. A good source of information about NASA projects to look for Earth-like planets.

Extrasolar Visions is an informative and imaginative page with some cool (if highly speculative) artwork.

There are a number of consortia undertaking Gravitational Microlensing searches, including an active group led by OSU:

The MicroFUN Project Homepage home of a gravitational microlensing search consortium coordinated by OSU astronomers (including me). In summer 2005 we discovered our first planet by microlensing, with the help of two amateur astronomers in New Zealand.

Finally, a nice but fairly technical overview of changing theories of planetary systems is given on this page by Pawel Artymosicz of the Stockholm Observatory. The historical overview is the most accessible to Astronomy 161 students.

Readings in Universe: None. You can look at Section 7-9 if you like, but it is relatively out of date.
Return to [ Unit 6 Index | Astronomy 161 Main Page ]
Updated: 2006 November 28
Copyright Richard W. Pogge, All Rights Reserved.