Published June 19, 2007
Alien worlds, once hidden from knowledge, are now being discovered in droves, stunning astronomers with their unique features and sheer numbers.
The discoveries are so common that more and more don't even get reported outside scientific circles.
Take the announcement at the end of May of a massive planet, dubbed TrES-3, that zips around its star in an amazingly rapid 31 hours, giving the planet a 1.3-day year.
Astronomers issued a press release, but you might not have heard about it because the discovery was so overshadowed by other planet announcements and barely received news coverage.
"It's pretty routine now," said Alan Boss, a planet formation theorist at the Carnegie Institution of Washington. "Most planets that are found are not deemed worthy of a press release because they are sort of becoming 'one more planet.'"
The total is now more than 200 extrasolar planets confirmed. And this is the tip of the iceberg in planet finds. Astronomers have more tools than ever, and technology is so advanced that planet discovery has become almost mundane.
The regularity of planet finds, luckily, is buffered by the wild variety in the discoveries themselves, including the following contrasts: nascent worlds of just a million years versus those that are billions of years old; hot gas giants and icy Neptune-like orbs; planets that whip around their parent stars with cosmic speed and others that seem to creep at a slug's pace; and planets orbiting double stars, red dwarfs and even so-called failed stars.
Astronomers spotted TrES-3 as part of the Trans-atlantic Exoplanet Survey while looking for transiting planets, or those that pass directly in front of their home star with respect to Earth. It was detected with a network of telescopes in Arizona, California and the Canary Islands.
When TrES-3 coasted in front of its home star, the telescopes picked up a slight dimming of the star's light, by about 2.5 percent. The scientists used the dimming to estimate the planet's mass, size and other properties.
It is located 800 light-years away in the constellation Hercules about 10 degrees west of Vega, one of the brightest stars in the summer skies of the northern hemisphere.
"It is also a very massive planet — about twice the mass of the solar system's biggest planet, Jupiter — and is one of the planets with the shortest known periods," said a co-discoverer of TrES-3 Georgi Mandushev of the Lowell Observatory in Arizona.
The giant orb orbits so close to its parent star, about 50 times closer than Earth is to the Sun, the astronomers estimate its temperature soars to about 1,500 degrees Kelvin, or about 2,240 degrees Fahrenheit.
While the "transit method" provides astronomers with the best indirect information about an exoplanet, so far only about 20 transiting planets have been spotted.
That's why the most successful teams (based on the number of planet finds) have relied on the so-called wobble method, or radial-velocity technique.
"The radial-velocity teams are the most successful," Boss told SPACE.com. "They are a victim of their own success. They are able to get more and more telescope time, because they can prove to the assignment committees that give out the time that 'If you give us so many more nights we can probably find you so many more planets,'" Boss said.
He added, "The key bottleneck for finding more planets is simply more time on a telescope."
The firsts and superlatives
In addition to finding new worlds, the burgeoning field has achieved many firsts.
In 2001, a team led by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics used the Hubble Space Telescope to detect for the first time the atmosphere of an extrasolar hot Jupiter called HD 209458b.
Another hot Jupiter, Upsilon Andromeda b, revealed for the first time an exoplanet with a temperature variation across its surface. One side has temperatures rivaling those found deep in a volcano while the other face could plunge below freezing.
Superlatives abound as well, with discoveries gaining fame as the windiest, tiniest, most massive and fastest orbiter.
— Shortest orbital period in catalog: HD 41004Bb completes a full orbit in 1.328 days.
— Longest orbit: HD 154345b takes 13,100 days to orbit its parent star.
— Lightest planet: Gliese 581c weighs just five Earth masses.
In an effort to keep track of the rapidly increasing list of exoplanets, a group of astronomers published a catalog of nearby exoplanets within 652 light-years of Earth in a 2006 issue of the Astrophysical Journal, though they realize updates will be a must on a routine basis.
"Without question, the catalog presented here will become out of date before it is printed," the researchers say in the published report of the catalog.
But with such a huge sample of relatively nearby planets, theorists now have the chance to test out their theories in the "real world."
"This whole business of extrasolar planets has been a real boon for theorists, because so far they had only one planetary system to study — and that was ours," Mandushev said in a telephone interview.
For instance, when does an object stop being a planet and become a star, a threshold that theory places at 10 to 15 Jupiter masses and beyond which an object can ignite hydrogen fusion to power a stellar glow?
The real goal
The ultimate goal, say many planet hunters, is to find Earth-like planets, or those with masses, orbits and rocky compositions similar to Earth. And beyond finding the physical Earth-like attributes would be to find life.
So far no "Earths" have been identified, though observatories are coming online with the sensitivity to detect small objects that orbit far from their host stars, as our planet does.
"The hunt is still on for rocky, Earth-like planets," said Jason Wright, an astronomer at the University of California, Berkeley, who was part of the team compiling the exoplanet catalog.
Astronomers have identified the first Earth-like planet that could support liquid water and harbor life. The "super Earth," Gliese 581c, weighs about five Earth masses and is either a rocky planet or one covered entirely by oceans, astronomers speculate.
Multi-planet systems are also a goal. So far about 25 multi-planet systems have been identified, with two such systems supporting four planets.
"We haven't found a clone of the solar system yet," Boss said. "But that's only ruling out maybe 10 percent of the stars. The other 90 percent could have exact solar system analogs and we wouldn't know it because we haven't been able to take data for long enough to actually find their planetary systems."
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