Photo marks milestone in planet search

A familiar yet different planetary system
The picture of GQ Lupi and its planet is exciting to astronomers because the system resembles in some respects our own solar system in its formation years.

The planet is about 3,140 degrees Fahrenheit (2,000 degrees Kelvin) — not the sort of place that would be expected to support life. Neuhaeuser's team has also detected water in the planet's atmosphere. The world is expected to be gaseous, like Jupiter. It is about twice the diameter of Jupiter. The mass estimate — one to two times that of Jupiter — is "somewhat uncertain," Neuhaeuser said.

The planet is three times farther from GQ Lupi than Neptune is from our sun. "We should expect that the planet orbits around the star, but at its large separation one orbital period [a year] is roughly 1,200 years, so that orbital motion is not yet detected."

It's not known why it is so far out.

"It is unlikely, but not impossible, that the planet formed at that large separation, because circumstellar disks around other stars often are that large or even larger," Neuhaeuser said.

Or perhaps the planet had a close brush with another developing world. The interaction could have thrown the newly discovered planet outward while tossing the other one, which has not been detected, in toward the star. It's also possible that the newfound planet has a highly elliptical orbit and is currently near its outer bounds.

The star GQ Lupi is part of a star-forming region about 400 light-years away. At 70 percent the mass of the sun, it is "quite similar to our sun," Neuhaeuser said. But GQ Lupi is only about 1 million years old. The sun is middle-aged, at 4.6 billion years old.

Nagging doubt
"What's most exciting about this discovery is that it raises a plethora of new questions regarding the origin of a planet so far out from its parent star," Jayawardhana, who is an expert on the disks around young stars from which planets form, said in a email interview.

Jayawardhana wonders whether it formed in a protoplanetary disk much closer in, roughly where Jupiter is in our solar system, and then got flung out. Or perhaps it was born almost at the same time as its star, fragmenting out of a contracting protostellar cloud.

"One way or another, this object must have formed pretty quickly," given the star's age, he said.

Knots of gas and dust have been detected around other young stars in setups that astronomers believe are solar systems in the making. Theorists believe our solar system formed when the sun's leftovers developed into a thin disk of orbiting material. Rocky planets like Earth formed when chunks stuck together. Astronomers do not agree, however, on how gas giants are born.

Alan Boss, a planet formation theorist at Carnegie Institution of Washington, called the image "really exciting." But he said there is "one little nagging doubt" in that the object's mass is only an estimate.

Christophe Dumas, who worked on the European team that announced a possible photo of an extrasolar planet last year, said of the new image: "There is still a large uncertainty on what the mass of this object is."

Weighing it precisely would involve noting the gravitational wobble the apparent planet induces on the star, but this object is too far from the star to produce a meaningful wobble. Yet even if the object is four times the mass of Jupiter it would still be considered a planet, Boss said in a telephone interview.

"I think there's a really good chance that this is an historic photo," Boss said.

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