What ever happened to extrasolar planets?  They used to make the news.  The search for these distant worlds, however, is as fervent as ever.  As better technology and new techniques have been developed, finding them has become commonplace.  To date, the known number of exoplanets, as they are commonly known, has increased to over 350.  But what are exoplanets, exactly?  How are they detected? And why are we looking for them?

The term extrasolar planet refers to any planet orbiting a star other than our sun.  Though their presence had been predicted for hundreds of years, the first one wasn’t found until 1992.  We didn’t have the technology to detect them.  After all, compared to its parent star, even a Jupiter-sized planet is very small and dim.  Furthermore, most of these planets orbit close to their parent star, making it even harder to distinguish them.  Thus far most exoplanets found have been very large, multiple times larger than Jupiter.

Astronomers use several methods to search for and identify exoplanets.  Each method has its advantages and disadvantages.  Depending on the distance, the size, or the orientation of a planet’s orbital plane one method may be more effective than another.  Sometimes more that one method can be used on the same planet giving a more complete picture of what the planet may be like.

The most successful method for detecting exoplanets is the radial-velocity or Doppler shift method.  In this method, the presence of a planet is detected by measuring tiny changes in the frequency of the star’s light.  As a planet orbits a star it causes it to wobble very slightly about the system’s center of mass (see image below).  As the star is pulled away from us its spectrum is shifted towards the red end, and as it is pulled towards us it is shifted to the blue end.  This method only works if the planet’s orbital plane is aligned parallel to the Earth’s orbit.  It is not possible to determine the size of these planets using this method.

Click here to see an animation of the wobble.

The first planets were detected using pulsar timing.  Pulsars are neutron stars that rotate very quickly.  As they rotate, they emit flashes of radio waves at very regular intervals like a light-house.  These flashes can be detected and timed.  A planet orbiting a pulsar will cause very slight variations in the timing of these flashes which can be used to detect it.

When a planet’s orbital plane is perpendicular to Earth’s, another method known as astrometry works well to detect the star’s tiny wobble.   In this method the star’s position is precisely measured against the background stars.  Tiny shifts in its position indicate the tug of a planet orbiting it.  Astronomers are hopeful that this method will lead to the detection of smaller Earth-sized planets.

In transit photometry the dimming of a star is detected as a planet crosses in front of it.  Using this method, astronomers can measure the size of a planet.  Even more intriguing is that astronomers can sometimes determine the absorption spectrum of a planet’s atmosphere as the star’s light passes through it.  This allows them to determine the composition of the planet’s atmosphere.

For the average person the most exciting method of observation is direct imaging. Unfortunately this requires a rare set of conditions.  The method works best when the planet’s orbital plane is perpendicular to Earth’s, the planet is bright and its star dim, and the star is relatively close to Earth.  So far only a few planets have been found using this method.

In November of 2008 the Hubble telescope imaged a planet orbiting the star Fomalhaut.  The planet is estimated to be about 2 times the size of Jupiter and is extraordinarily bright.  Since then several others have been seen.

A trio of planets (faint dots indicated with arrows) orbits the young, massive star HR 8799, some 130 light-years from Earth. Discovered using the Hawaii Keck telescope. Credit: Marois, National Research Council/Canada, Keck

In recent months, astronomers have been able to identify planets thought to be more Earth-like than the gas giants they have been finding so far.  These large “super-Earths” lack the dense atmosphere of the gas giants and have a dense rocky composition.  So far around 30 such planets have been found, but scientists believe that they probably far outnumber the gas giants.

There are some exciting implications of these recent finds.  If a rocky planet orbits a main sequence star like our sun in the so-called “Goldilocks zone, it is possible that it could support life. In the near future, Astronomers hope to analyze the atmospheres of these super-Earths using new telescopes such as the James Webb Telescope, scheduled for launch in 2013.  If they can find signs of carbon dioxide and water, it could mean that the planet may support life.  On the other hand, if they find oxygen and methane, it may indicate that life already exists there!