The Kepler observatory was launched into orbit in early 2009. Its mission: to search for planets in solar systems other than our own. Their recent results point to a staggering number of planets that share the galaxy with us, many of which orbit their sun in a habitable temperature zone: between 0 and 100 °C. This means that water-based life such as ourselves would neither freeze nor boil away, assuming that the planet has atmospheric pressure similar to Earth.
Normal, Earth-bound telescopes can detect light emitted from stars throughout the galaxy, but reflections and emissions from their orbiting planets are too weak to be detected that way. In order to “see” planets, Kepler actually measures a drop in the intensity of light we see when a planet passes directly between Kepler and a star. The astronomers then analyze that dip in signal–knowing the mass and temperature of the star, they can determine the distance and the average temperature of the planet. (You can help search the data for planets at the Planet Hunters website!)
From May to September of 2009, Kepler focused on a small portion of the sky where they could see over 156,000 stars. Here’s a breakdown of the findings so far: 1,235 planet candidates, 68 of which are Earth-sized, and 54 of which are in the habitable zone. They use the phrase “planet candidates” to point out that a drop in light intensity can also be caused by other phenomena (such as binary stars), and more statistical analysis is needed before the planet’s existence is confirmed.
Now I’m going to give you a few very large numbers, so I hope you can remain awestruck, despite the nearly inconceivable magnitude of the subject. Sit back for a moment and think about the size of the Earth. Now consider the size of the sun (about 1 million times greater volume). I just told you that Kepler’s field of view contains 156,000 stars, so if you can, imagine a field of many thousands of suns, each one of which could contain at least one million Earths.
Pretty big, right? I hope you’re sitting down. That enormous amount of space and mass is only about one millionth of the number of stars in the Milky Way. Based on the approximate value of 100 billion stars in our galaxy, scientists with Kepler estimate at least 50 billion planets (one out of every two stars is expected to have a planet). And 500 million or so of those planets are in the habitable temperature zone. If you’re like me, your mind is blown trying to imagine this immense number of unexplored worlds, orbiting stars that would take thousands of years to reach if we were traveling at light speed. Since I’m using to thinking of Earth as a pretty special place, I was floored to discover the number of planets in our galaxy.
By the way, did I mention there are over 100 billion galaxies in the universe? Whoa.
William J. Borucki, et al. (2011). Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data. The Astrophysical Journal, arXiv: 1102.0541v1
J. J. Lissauer, et al. (2011). A closely packed system of low-mass, low-density planets transiting Kepler-11. Nature, 470 (7332), 53-8 PMID: 21293371