"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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