Planets of the Milky Way

A new study estimates that there may be as many as 160 billion planets in our Milky Way Galaxy.  That's about 1.6 planets per star, many more than were thought to exist just a few years ago.  Most of these worlds are thought to be rocky type planets, as opposed to gas giants.  Just how many of these planets could support life, however, remains to be seen.

Interest in planets outside of our solar system, known as exoplanets, has increased dramatically in the last few years.  The amazing success of NASA's Kepler mission has fueled much of the interest.  To date, the orbiting Kepler telescope has identified nearly 3000 exoplanets.  Kepler does this by measuring the slight decreases in the amount of light a star appears to emit when a planet passes between the star and the telescope.  This is called the 'transit method' of exoplanet detection.  After a candidate exoplanet is identified this way, scientists employ ground based telescopes to back up Kepler's findings.  It is only after this that an exoplanet can be confirmed to exist.  Around 700 of Kepler's candidate exoplanets have been confirmed, and it is estimated that up to 80% of all candidates will indeed turn out to be exoplanets once further observations are carried out.

Artist's depiction of the Kepler telescope (NASA).

After Kepler identifies a candidate planet, observing that world from ground based telescopes can be done in one of a few ways.  Scientists can measure small wobbles in a star's movement caused by the gravitational pull of planets orbiting the star.  This is called the 'radial velocity' method.  Both this and the transit method are biased toward large planets that are close to their parent stars, however.  Another method that doesn't have the same bias is called 'gravitational microlensing.'  This is where a star is observed just as a closer body, such as another star, passes in front of it.  The gravity of the closer body bends and magnifies the light of the distant star, similar to a lens.  This can help resolve the relatively tiny exoplanets that may be orbiting the star.

The big question now becomes: Are these worlds habitable?  The Kepler telescope, by itself, cannot tell us this.  However, we can infer several things from Kepler's findings.  For life, as we know it, to exist, a planet must harbor liquid water.   For that to happen, a planet must be in its star's 'habitable zone'; a distance, particular to each star, where it is neither so cold that water freezes, nor so hot that water boils away.  Earth, for example, is right in our Sun's habitable zone.  Venus, with its 480 degree Celsius surface temperature and runaway greenhouse effect, is just a little too close to the Sun to be in the habitable zone.  Kepler has already identified exoplanets that are in their habitable zones, and is sure to observe more in the coming years.  It will be up to other telescopes, like the as yet to be launched James Webb Space Telescope (JWST), to probe deeper into these potentially life harboring exoplanets.  JWST will have the ability to measure the atmospheres of exoplanets, and depending on the composition, that could reveal whether or not life exists on those planets in any great quantity.

Artist's depiction of the James Webb Space Telescope (NASA).

The Kepler telescope is confirming what scientists have only been able to speculate up to now; that our galaxy is filled with stars that have planets orbiting them, just like our own solar system.  The wealth of knowledge Kepler is bringing to humanity is astounding.  Hopefully, these discoveries will drive home the point that space based astronomy is an important and vital resource in unlocking the mysteries of our universe.