The number of planets that astronomers have discovered orbiting distant stars hovers right below 500. But confirming which remote flicker of light is the milestone alien world will be a tricky affair.
At NASA's last count, astronomers had confirmed the discovery of 494 planets around alien suns. There are signs of dozens more, if not hundreds, but it will take time to weed out which of the detections are actual worlds and which are merely false alarms.
Some astronomers now expect that official discovery of the 500th alien planet by January 2011.
In the meantime, scientists lean on telescopes and space observatories, as well as a tried-and-true bag of tricks, for identifying and confirming planets beyond our own solar system. [Gallery: Strangest Alien Planets]
How to spot alien planets
There are four primary techniques currently used to find exoplanets, each with its own pitfalls.
The radial velocity method looks for repeated wobbles in a star's movements that are signs of a planet's gravitational pull yanking it back and forth.
However, if a planet has very little mass, it hardly exerts much of a pull ? if an astronomer is trying to detect something like an Earth-size planet, the noise or static in the data can be mistaken for a planet. Overcoming this problem largely requires measuring the star over and over and over again, said astrobiologist Alan Boss at Carnegie Institution of Washington.
"That can take a lot of telescope time, which can be very, very expensive," said planetary scientist Sara Seager at the Massachusetts Institute of Technology. "One night of time at the Keck telescope can cost $50,000."
The transit method looks for dips in a star's brightness whenever a planet crosses in front of it. The problem is that if the star under observation is in mutual orbit with another star, it's that other star that could lead to regular dips and surges in brightness.
Another technique, called the microlensing method, looks for distortions in light resulting from the pull of gravity. The gravitational field of a planet can have a measurable effect on light that passes by it.
However, this occurs only when a star with a planet happens to line up with another star ? a brief event that never happens again, "like two ships passing in the night," explained astronomer Geoffrey Marcy at the University of California at Berkeley.
The difficulty in reproducing results can make microlensing hard to rely on, although there have been solid examples of microlensing that overcame any doubts.
Astronomers also may directly image the light from an exoplanet. "The down side there is, how do you know if that candidate is a planet or a faint star?" Marcy said. "Faint stars look a lot like glowing planets."
What makes a planet?
There is no exoplanet list formally sanctioned by the International Astronomical Union, the body that assigns official designations to celestial bodies.
Instead, there are only unofficial lists maintained by researchers in the field, such as astrobiologist Jean Schneider at the Paris-Meudon Observatory and astronomer Jason Wright of the University of California in Berkeley.
There are also no hard and fast rules as to whether a candidate should be declared an exoplanet; each researcher and group has its own preferences, Schneider said. To get others to accept their results, scientists often wait until the probability that their results are false alarms falls below 1 percent or so.
The standard way that the field confirms the report of a planet is through its acceptance by knowledgeable referees into a scientific journal. Still, as many as 50 to 100 exoplanets were revealed in talks, only to wait years before their appearance in a journal. The discoverers may simply have been too busy doing actual work to write up the papers, Schneider explained.
In addition, even after publication, a few exoplanets have been retracted as false alarms ? "five to 10 since 1989," Schneider estimated.
"My research group publishes data on an exoplanet when the false alarm probability drops below 1 percent, which means about 1 percent will be wrong," Marcy said. "There's always a chance to be wrong, and as scientists we try to calculate what that probability is and present it openly."
"There's always a chance there's a few errors in data to make something look like a planet," he added. "This can happen to anyone ? just one of those things that happens when you're pushing a frontier, pushing instruments to their bitter limits. This kind of astronomy is hard work, and there are lots of ways to make a mistake. A number might slip through, but they're generally corrected in a year or two."
Another possible point of confusion is the fuzzy boundary that separates a planet from a "brown dwarf" ? a large gaseous body, more than 13 times the mass of Jupiter, that failed to become a star. "Something 20 Jupiter masses and below is likely a planet, but there's ambiguity there," Schneider said.
All these concerns might give the impression of a list of published exoplanet being a bit of a mess, but overall, Schneider contended, only 1 or 2 percent of these discoveries are unclear so far.
"The real acid test in the field is getting two methods to detect an object ? for instance, a radial velocity signature plus a transit detection," Boss said. "There are about 100 of such absolutely, positively identified planets so far."
In the end, "there is no real honor roll of planets, no real way to say which the 500th planet will be," Boss said.
Still, while it has taken scientists roughly 15 years to confirm the detection of the nearly 500 planets known so far, the pace promises to grow rapidly.
NASA's Kepler mission, a space observatory surveying a large sample of stars as it orbits the sun, revealed in June that it had detected more than 750 possible exoplanets using the transit method within its first 43 days of operation.
"Kepler is beating us all by a million miles," Marcy said.
Many of the candidates Kepler discovered are now getting verified with radial velocity confirmations. "On Feb. 1, we'll announce all of them ? a huge avalanche of exoplanet candidates," Marcy said.
"The days of having to have perfect exoplanets are going away," Seager noted. "We're going to publish so many planets that we're not going to be able to validate all of them. Instead, we'll have so many we can start studying them statistically in groups."
Even without Kepler, there are roughly 100 exoplanet candidates that researchers are working hard to confirm, Marcy said.
"We could well hit 500 on Jean Schneider's list by January," Boss said.
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