Largest Known Planet Found, Has Density of Cork

The biggest alien planet found so far is baffling scientists with properties that defy current scientific explanation.

By all rights, TrES-4, a gas giant recently discovered about 1,400 light-years away in the constellation Hercules, shouldn't exist.

The planet's size is much larger than predicted for its mass, said Georgi Mandushev of Lowell Observatory, lead author of a new study on the exoplanet.

Though 70 percent bigger than Jupiter, TrES-4 contains only three-quarters of the red giant's mass. (Related: "First Proof of Wet 'Hot Jupiter' Outside Solar System" [July 11, 2007].)

That means the alien planet is about as dense as balsa wood or cork, said Mandushev, who is part of a planet-hunting team known as the Trans-Atlantic Exoplanet Survey.

"At this point the most valuable thing about this study is it presents a challenge to our theoretical models," Mandushev said. "Most advances in science come from confrontations just like this one."

The findings will be published in an upcoming issue of Astrophysical Journal Letters.

Hot Find

Peter McCullough leads another exoplanet discovery team, called the XO Project, from his position at the Baltimore, Maryland-based Space Telescope Science Institute.

Both he and Mandushev suspect that the unusually large size of TrES-4 has something to do with extreme heat.

The planet is only 4.5 million miles from its parent star, and it orbits in about three days, compared to Jupiter's 12-year revolution.

That means TrES-4 is superheated to about 2,300 degrees Fahrenheit (1,260 degrees Celsius).

"There must be more heating than we have anticipated for the planet's size to be larger than we anticipated," McCullough said.

Hydrogen and other elements could be trapping heat in a similar way to the gases that contribute to atmospheric warming on Earth, study author Mandushev speculated.

Teasing out the exact mechanism that would allow a planet to get so large is now work for theoreticians, the astronomers say.

Seeing is Believing

TrES-4 may be the largest planet known, but it's not the most massive.

That honor is reserved for HAT-P-2, a planet eight times Jupiter's mass located about 440 light-years away, also in the constellation Hercules.

The discovery of that exoplanet, which is only slightly larger than Jupiter, was announced in early May by the Harvard-Smithsonian Center for Astrophysics. (Read the story: "'Weird' New Planet Weighs as Much as 2,500 Earths" [May 3, 2007].)

Both HAT-P-2 and TrES-4 are among the 20 or so planets found using the "transit method," where scientists spot a world when it crosses between Earth and the parent star.

Planetary scientists first began using the method about five years ago, and success has crept along at a snail's pace.

Meanwhile, more than 200 exoplanets have been discovered by the alternative wobble method, which relies on detecting a planet's gravitational pull on its host star. (Related: "New Planet 'Bonanza' Discovered at Center of Milky Way" [October 4, 2006].)

But "at this point, I think that the [transit] discovery process is accelerating," Mandushev said.

The Trans-Atlantic Exoplanet Survey-which uses telescopes in California, Arizona, and the Canary Islands-found its first transiting world in 2003, and another was unveiled last fall.

The next two finds, TrES-3 and TrES-4, were discovered in May, but TrES-3-which is about twice the mass of Jupiter-was confirmed more quickly and announced earlier this year.

Mandushev prefers the transit method because it yields more information about a planet, including its mass, size, orbit, and even its chemical composition. The wobble method only reveals a planet's mass.

The fledgling method is soon likely to reveal even more scientific puzzlers.

The Space Telescope Science Institute's McCullough said his team is working to confirm an exoplanet they're calling XO-3b.

They believe that planet will dwarf all others, at 13 times Jupiter's mass and twice its size.

It's like "the troll in the fairy tale of Billy Goat Gruff," McCullough said.

"TrES-4 is a very big planet," he said. "But if you wait, there will be even bigger planets in the future."

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