Astronomy 1101 --- Planets to Cosmos


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, is an old one.

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.

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.

Radial Velocity 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.

The Doppler Effect

The observed wavelength of a wave will change when the source of the waves and the observer are moving either towards or away from each other.


The amount of the shift and its sign depends on

Doppler Effect in Sound

Two cats are sitting between a windup mouse toy that emits an electronic squeak. The mouse is moving towards the left towards the first cat and away from the second:
Doppler Effect in Sound
(Graphic by R. Pogge) The mouse emitted a squeak when it was at the location of each of the green dots. The sound wave ("squeak!") moves outward spherically from each point of emission. Because the mouse is moving, the sound waves have different emission centers. Those waves ahead of its motion (towards the left) are are scrunched together, while those behind are spread out.


Doppler Effect in Light

The Doppler Effect in light works the same way as it does for sound:

A Way to Measure Speeds

Observe the wavelength lobs of a light source with a known emitted wavelength lem.

The difference between the observed and emitted wavelengths is directly proportional to the speed of the source towards or away from you (v), given by the Doppler Formula:

Doppler Effect Formula (Graphic by R. Pogge)
Here c is the speed of light.

The Doppler Effect in Practice

The Doppler Effect in light is used by astronomers to measure the speeds of objects moving towards or away from the Earth.

But, we also use the Doppler Effect in light in everyday settings. Some examples:

Traffic Radar Guns:
Radar gun bounces a pulse of microwaves (or infrared laser light) of a known wavelength off a car or truck, measure the wavelength reflected back. The Doppler shift gives the vehicle's speed. Most traffic radar guns are of the portable microwave doppler radar type in this example. A problem of microwaves is that their excess emission can be detected by small receivers mounted in cars and used to warn drivers that a radar gun is in use on the road. Some types of police "radar" guns don't use microwaves but instead use laser ranging techniques called LIDAR. This works by time-of-flight calculation rather than the Doppler effect.

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

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 2014, all of these techniques have found more than thousands of planets or planet candidates. The first planets discovered were via the RV method. These found so-called "hot Jupiters" --- Jupiter-mass planets on very short orbits around their host star. Most of these systems have only one detectable planets. Searches via transits with the Kepler Satellite have revealed thousands of potential planets. These range from Hot Jupiters (which are rare) to Neptune-mass objects, and even down to near-Earth masses on near-year orbits.

Strange New Worlds

Although microlensing has revealed one system with a Jupiter-mass and a Saturn-mass planet at the right semi-major axis to be an analog for our solar system, most of the systems discovered so far are not like ours. Many of the multi-planet systems discovered by Kepler are very "compact" in the sense that the bodies have small semi-major axes --- but, this could be just a "selection effect" since we have no way yet to find systems that are like ours via transits.

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.

Updated: 2014, Todd A. Thompson
Copyright Richard W. Pogge, All Rights Reserved.