Astronomy passed a milestone last month(October 2013) when the tally of known exoplanets passed the 1,000 mark. In fact, science has found 1,028 confirmed planets in 782 planetary systems to date, including 170 multiple planetary systems. That’s a lot of progress, considering that the first extrasolar planet was confirmed in only 1992.
While we may wonder at everything the universe holds – stars, nebulae, dark matter – our lives are spent on the surface of a planet. Looking for planets, therefore, is unique in astronomy because each announced discovery comes tagged with the unvoiced question: is this the one with life? We haven’t found life beyond Earth yet. We can’t be sure that it exists or what it might be like. But we have moved into a universe that’s much more interesting and diverse than anyone knew just a few decades ago.
The search tools
Exoplanets – even the biggest ones – are found indirectly. Planets don’t emit light of their own so scientists find them by sensing their effects on the stars they orbit. Star brightness dims, for example, when a planet transits in front of it, and a star’s path will “wobble” from the gravity of the planet circling around it. Planets can even give away their presence by altering the gravitational lensing effect of their parent stars on other stars behind them.
Specially designed terrestrial telescopes are productive as planetary search tools but they’re augmented by the increased capabilities of space-based telescopes. To date, the Kepler Space Telescope has been the “star” of exoplanet searches. Launched in 2009, Kepler detected thousands of candidate planets, including 262 that may be habitable planets, before its functions were lost earlier this year following a critical gyroscope failure.
So, what have these search efforts taught us about other worlds?
All planets aren’t equal
Early programs were most successful finding big planets. Large mass bodies have large effects on their parent stars and are therefore easier to detect. Better sensors and better computer processing steadily revealed smaller planets, and a better picture of planetary distribution in the galaxy. These search programs now show that a substantial fraction of stars have planets near them and that small planets (i.e., earth sized or slightly larger) are more common than big ones. And the variety of these discoveries is remarkable:
· The farthest planet found is about 22,000 light years away, while the closest is just over 4 light years distant
· Some planets are so massive they’re comparable to small stars, while the least massive planet is comparable to our Moon
· The largest planet found has a radius about twice that of Jupiter, while the smallest is Moon-sized
· There are exoplanets made of lava, of water, and even of diamonds
Based on the limited sectors of the sky that have been surveyed so far, astronomers have even been able to make projections about the kinds of planets that the entire galaxy contains.
There are approximately 100 billion stars in our Milky Way galaxy. Searches have so far turned up an average of 1.6 planets per star, which means that our galaxy could be home to 160 billion worlds.
Although estimates from different sources vary – different organizations use different standards to confirm a new planet discovery – all of the projections are similarly large.
How many of these might be habitable? That’s a debated topic among scientists but, assuming a starting requirement that a planet should be about the size of earth, then astronomers have calculated that 17 percent of known planets fit the bill. That’s 17 billion worlds.
Not all planets orbit stars, either. Rogue planets are interstellar wanderers that have broken out of a star’s orbit or were never in one. They’re found using the same methods as other planetary searches – scientists just have to wait until one comes near or in front of a star to see the necessary effects. Based on present results, some scientists think that rogue planets could outnumber orbiting planets by 50 percent. A rogue planet was found near our own solar system in 2012.
These wanderers may have a lot to teach astronomers about the origin of planets: are they created only in the proximity of stars or can they be born on their own?
The search continues until . . .
The planetary search programs aren’t slowing down. NASA is planning on launching the Transiting Exoplanet Survey Satellite (TESS) in 2017, and there’s still a lot of data from past searches that needs to be analyzed.
In fact, some of the best discoveries are being made with amateur help; One of seven planets found orbiting a dwarf star this year was discovered by volunteers using the Planet Hunters website, where users can sift through much of the Kepler data and contribute to planet discovery.
The burning question for most of us, of course, will always be whether we’ll discover life. There’s no answer to that one yet but, if we’re going to ever find life, we first have to find the right planets.