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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 45:
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 First Law of Planetary Motion: 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 motion: Astrometric Wobble and Doppler Wobble.

Astrometric Wobble

Parent star wobbles back & forth on the sky as seen relative to 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 or Radial Velocity (RV) 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 of Planetary Motion.

RV 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, most people can walk at a speed of about 1 meter/sec, while a car moving 65 MPH is moving at a speed of 29 meters/sec.

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, Texas, and Europe. RV searches are now the primary way people search for exoplanets around nearby stars.

Advantages of the RV Method

So far, the RV method has been the most successful to date The RV method is very sensitive to massive planets around relatively nearby stars

Planetary Transits

If the orbital plane of an extrasolar planet is aligned with the line of sight: So far, 33 transiting planets are known, 5 of which were previously discovered using RV techniques. Large-scale searches are underway that are rapidly increasing this count.

The Case of HD209458

The first confirmed transiting planet. HD209458 is a star with a Jupiter-sized planet found originally via the RV method:

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

Advantages of Transits

Transits offer the only way we currently have to make a direct measurement of the radii of exoplanets The only way we have to probe the atmospheres of exoplanets The latest application of the Transit Method from space holds out the possibility of detecting Earth-mass planets. Recent missions are the European COROT satellite and the upcoming US KEPLER mission.

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, 8 planets have been found by gravitational microlensing, 6 by OSU's Microlensing Follow-Up Network (MicroFUN) collaboration.

Advantages of Microlensing

Microlensing is superbly sensitive to planetary systems like our own Solar System: It offers one of the few ways to find planets around more distant stars In principle, Microlensing may be the only way we currently have that could detect Earth-mass planets from the ground. This is much cheaper than expensive space missions, and can be done by networks of small amateur and professional telescopes.

OSU is a leading player in the Microlensing Planet Search effort, and has organized the largest amateur and professional observing network, the MicroFUN Collaboration.

Roster of New Planetary Systems

As of November 2007, all of these techniques have found more than 260 planets around more than 220 stars:

Strange New Worlds

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

The biggest surprises:

What is going on? This is a subject of much current research.

The Future

There are continuing searches for other planetary systems.

Basic Goals:

Future Goals:

This is considered one of the most important astronomical research programs of the 21st Century.


For more information on the search for exoplanets 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 planets, hopefully down to Earth-mass planets, using the transit method from space. Currently scheduled for launch in February 2009.

COROT Mission, a French (CNES) mini-satellite launched in December 2006 to study stellar oscillations and search for exoplanets using the transit method.

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 Collaboration, 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. Since then our group has made crucial contributions to a number of microlensing planet detections. This work is primarily funded by the NASA Origins Program.

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Updated: 2007 November 26
Copyright Richard W. Pogge, All Rights Reserved.