By the end of this month, astronomers expect to have found 500 planets beyond our solar system. The current count is 494, including one that may be inhabitable by humans.
Less than 20 years after first finding a planet beyond our solar system, astronomers are poised to hit a big milestone — the discovery of alien world No. 500.
As of Tuesday, the confirmed tally stands at 494 extrasolar planets, with more than 70 discovered so far in 2010 alone. At that rate, No. 500 could be announced before October is out — just a month or so after another watershed moment, the discovery of the first potentially habitable alien planet.
"Where we are, I'd expect that by the end of October, we'll be at 500 if things keep going the way they're going," said Jon Jenkins of the SETI (Search for Extraterrestrial Intelligence) Institute. Jenkins is the analysis lead for NASA's planet-hunting Kepler mission.
And the 1,000th world could be discovered surprisingly soon, as the space-based Kepler mission has already offered up hundreds of planet candidates that await further observation and confirmation.
Alien worlds piling up
Most of these extrasolar planets, or exoplanets, have been found using two different strategies.
Astronomers can scrutinize the movements of faraway stars, watching for the telltale gravitational tug of orbiting planets. Or they look for tiny dips in a star's brightness — evidence that a planet is transiting, or passing in front of, the star from our perspective.
Astronomers first definitively found an alien world in 1992, when researchers led by Alex Wolszczan of Penn State detected two planets orbiting a rotating neutron star, or pulsar, about 1,000 light-years from Earth. Confirmation of a planet circling a "normal" main-sequence star did not come until 1995.
Since then, however, the alien planet finds have been rolling in, accelerating in recent years as planet hunters honed their techniques and instruments became more powerful. [Gallery: Strangest Alien Planets]
That trend will likely continue, Jenkins said, as data keeps pouring in from Kepler — which was launched in March 2009 to search for Earth-like alien planets — and other telescopes.
The Kepler mission, for example, has confirmed and announced seven new alien worlds to date. But Kepler, which hunts by looking for transiting planets, has already identified more than 700 "candidates" — stars that may harbor alien planets.
Researchers are following up on these promising leads, trying to rule out any false alarms. They're checking out the candidates with ground-based instruments as well as orbiting assets like the Hubble Space Telescope and the Spitzer Space Telescope.
When such work is done, the number of known alien worlds could increase by several hundred — and it could happen soon.
"One could reasonably expect many of these to be vetted within the next year or so," Jenkins told SPACE.com.
The number of confirmed exoplanets could rise even more dramatically if some of the candidates host multiple planets.
Recent discoveries suggest this is not an unreasonable expectation. Five planets are known to orbit the star HD 10180, for example, and at least six — including the potentially habitable planet Gliese 581g — circle the star Gliese 581 just over 20 light-years from Earth.
"We have a sense now that our solar system isn't such a weirdo," Jenkins said. "It may be that multi-planet systems are quite common."
Reaching other milestones
Other important milestones could follow closely on the heels of exoplanet 500, not all of them numerical. One big moment that should happen relatively soon, Jenkins said, is the discovery of a potentially habitable alien world that's transiting its star, a setup that would allow for more detailed observations.
The rocky, roughly Earth-sized Gliese 581g apparently does not transit its star as seen from our point of view, and as a result astronomers will have trouble searching for signs of life on that alien world, at least for a while.
When a planet transits its parent star, starlight passes through its atmosphere (if the planet has one). Astronomers can scrutinize this light for compelling evidence of life in the planet's air, such as significant quantities of both methane and oxygen. As one example, a gas giant planet called HD 209458b has been found to contain oxygen, carbon and water.
So the discovery of a transiting Earth-like world in its parent star's habitable zone — that range of distances that supports the existence of liquid water — would occasion some celebration in planet-hunting circles, as well as a frenzy of follow-up research. And it could be just around the corner.
"I wouldn't be surprised if that happened next year," Jenkins said.