January 9, 2011 by Conway

While growing up, I dreamed of the possibility of discovering planets outside our Solar System.  I especially wanted to know if any of these potential planets could be habitable by humans.  We already knew that our Sun was not very unique among stars in our galaxy.  I wondered how unique the planet Earth might be.  I wanted to know if conditions around other stars could allow Earth-like planets to exist.

Now, I want to know more about the planetary systems that we’ve actually found beyond our Solar System.  And I still want to know about the possibility of finding another Earth-like planet.  Here is a quick survey of our progress so far.

http://planetquest.jpl.nasa.gov keeps track of the number of exoplanets that we’ve discovered, currently just over 500, and the number of Earth-like exoplanets that we’ve discovered, currently ZERO.  Most of these discoveries are very recent.

Before 1990, only eight planets were known to exist, given that poor Pluto has been demoted to the status of Dwarf Planet.  In 1992, the first two exoplanets were found.  By 2000, about 50 exoplanets had been discovered.  This number jumped to over 100 in 2003, over 200 in 2006, and over 300 in 2008.

To make these discoveries, we have used a variety of techniques.  Each large, massive planet tugs at the star it orbits.  This causes the star to “wobble” as the planet goes around.  We can measure this motion by observing the Doppler shift seen in the star’s spectrum over time.  As the star approaches us, we see a slight blue-shift, and as the star recedes from us, we see a slight red-shift.  We can also measure a star’s slight displacement directly using interferometry.  Another approach that works for large planets is to observe a dip in the brightness of a star due to a planet blocking some of its light as it gets between the star and us.  We can also notice a change in the apparent position of a star due to gravitational microlensing, when a massive planet literally bends the light from the star it orbits.

The problem with these approaches is that they work best for large planets with a high orbital speed.  If a planet is too small, the signs we look for are too insignificant to detect.  If a planet takes over a hundred years to orbit a star (like Neptune), it’s not easy to notice the slight changes we could detect over time.

Two exciting missions are planned to improve our chances of detecting Earth-like exoplanets.  Gaia, developed by the European Space Agency, is scheduled to launch this year.  The James Webb Space Telescope is scheduled to launch in 2013.  Both have advanced instrumentation that can help us find exoplanets, and both are going to be much farther from Earth than the Hubble.

I look forward to the day when we detect an Earth-like exoplanet!