Extrasolar planet is under construction

[rhoenix]

Nearly 900 extrasolar planets have been confirmed to date, but now for the first time astronomers think they are seeing compelling evidence for a planet under construction in an unlikely place, at a great distance from its diminutive red dwarf star.

The keen vision of NASA's Hubble Space Telescope has detected a mysterious gap in a vast protoplanetary disk of gas and dust swirling around the nearby star TW Hydrae, located 176 light-years away in the constellation Hydra (the Sea Serpent). The gap's presence is best explained as due to the effects of a growing, unseen planet that is gravitationally sweeping up material and carving out a lane in the disk, like a snow plow.

Researchers, led by John Debes of the Space Telescope Science Institute in Baltimore, Md., found the gap about 7.5 billion miles from the red dwarf star. If the putative planet orbited in our solar system, it would be roughly twice Pluto's distance from the Sun.

The suspected planet's wide orbit means that it is moving slowly around its host star. Finding the suspected planet in this orbit challenges current planet formation theories. The conventional planet-making recipe proposes that planets form over tens of millions of years from the slow but persistent buildup of dust, rocks, and gas as a budding planet picks up material from the surrounding disk. TW Hydrae, however, is only 8 million years old. There has not been enough time for a planet to grow through the slow accumulation of smaller debris. In fact, a planet at 7.5 billion miles from its star would take more than 200 times longer to form than Jupiter did at its distance from the Sun because of its much slower orbital speed and a deficiency of material in the disk.

An alternative planet-formation theory suggests that a piece of the disk becomes gravitationally unstable and collapses on itself. In this scenario, a planet could form more quickly, in just a few thousand years.

"If we can actually confirm that there's a planet there, we can connect its characteristics to measurements of the gap properties," Debes says. "That might add to planet formation theories as to how you can actually form a planet very far out. There's definitely a gap structure. We think it's probably a planet given the fact that the gap is sharp and circular."

What complicates the story is that the red dwarf star is only 55 percent the mass of our Sun. "It's so intriguing to see a system like this," Debes says. "This is the lowest-mass star for which we've observed a gap so far out."

The disk also lacks large dust grains in its outer regions. Observations from ALMA (the Atacama Large Millimeter Array) show that millimeter-sized (tenths-of-an-inch-sized) dust, roughly the size of a grain of sand, cuts off sharply at about 5.5 billion miles from the star, just short of the gap. The disk is 41 billion miles across.

"Typically, you need pebbles before you can have a planet. So, if there is a planet and there is no dust larger than a grain of sand farther out, that would be a huge challenge to traditional planet-formation models," Debes says.

The Hubble observations reveal that the gap, which is 1.9 billion miles wide, is not completely cleared out. The team suggests that if a planet exists, it is in the process of forming and not very massive. Based on the evidence, team member Hannah Jang-Condell at the University of Wyoming in Laramie estimates that the putative planet is 6 to 28 times more massive than Earth. Within this range lies a class of planets called super-Earths and ice giants. Such a small planet mass is also a challenge to direct-collapse planet-formation theories, which predict that clumps of material one to two times more massive than Jupiter can collapse to form a planet.

TW Hydrae has been a popular target with astronomers. The system is one of the closest examples of a face-on disk, giving astronomers an overhead view of the star's environment. Debes's team used Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to observe the star in near-infrared light. The team then re-analyzed archival Hubble data, using more NICMOS images as well as optical and spectroscopic observations from the Space Telescope Imaging Spectrograph (STIS). Armed with these observations, they composed the most comprehensive view of the system in scattered light over many wavelengths.

When Debes accounted for the rate at which the disk dims from reflected starlight, the gap was highlighted. It was a feature that two previous Hubble studies had suspected but could not definitively confirm. These earlier observations noted an uneven brightness in the disk but did not identify it as a gap.

"When I first saw the gap structure, it just popped out like that," Debes says. "The fact that we see the gap at every wavelength tells you that it's a structural feature rather than an instrumental artifact or a feature of how the dust scatters light.

This is a trip. So, this planet is forming way out from its star (a red dwarf at that), at a distance past where our star has the Kuiper Belt.

This is awesome, but I'm betting that planet won't be a warm and cozy place once it finishes baking.

[rhoenix]

A team of researchers has discovered evidence that an extrasolar planet may be forming quite far from its star—- about twice the distance Pluto is from our Sun. The planet lies inside a dusty, gaseous disk around a small red dwarf TW Hydrae, which is only about 55 percent of the mass of the Sun. The discovery adds to the ever-increasing variety of planetary systems in the Milky Way. The research is published in the Astrophysical Journal.

Astronomers using NASA's Hubble Space Telescope have found compelling evidence of a planet forming 7.5 billion miles away from its star, a finding that may challenge current theories about planet formation.

Of the almost 900 planets outside our solar system that have been confirmed to date, this is the first to be found at such a great distance from its star. The suspected planet is orbiting the diminutive red dwarf TW Hydrae, a popular astronomy target located 176 light-years away from Earth in the constellation Hydra the Sea Serpent.

Hubble's keen vision detected a mysterious gap in a vast protoplanetary disk of gas and dust swirling around TW Hydrae. The gap is 1.9 billion miles wide and the disk is 41 billion miles wide. The gap's presence likely was caused by a growing, unseen planet that is gravitationally sweeping up material and carving out a lane in the disk, like a snow plow.

The planet is estimated to be relatively small, at 6 to 28 times more massive than Earth. Its wide orbit means it is moving slowly around its host star. If the suspected planet were orbiting in our solar system, it would be roughly twice Pluto's distance from the sun.

Planets are thought to form over tens of millions of years. The buildup is slow, but persistent as a budding planet picks up dust, rocks, and gas from the protoplanetary disk. A planet 7.5 billion miles from its star should take more than 200 times longer to form than Jupiter did at its distance from the sun because of its much slower orbital speed and the deficiency of material in the disk. Jupiter is 500 million miles from the sun and it formed in about 10 million years.

TW Hydrae is only 8 million years old, making it an unlikely star to host a planet, according to this theory. There has not been enough time for a planet to grow through the slow accumulation of smaller debris. Complicating the story further is that TW Hydrae is only 55 percent as massive as our sun.

"It's so intriguing to see a system like this," said John Debes of the Space Telescope Science Institute in Baltimore, Md. Debes leads a research team that identified the gap. "This is the lowest-mass star for which we've observed a gap so far out."

An alternative planet-formation theory suggests that a piece of the disk becomes gravitationally unstable and collapses on itself. In this scenario, a planet could form more quickly, in just a few thousand years.

"If we can actually confirm that there's a planet there, we can connect its characteristics to measurements of the gap properties," Debes said. "That might add to planet formation theories as to how you can actually form a planet very far out."

The TW Hydrae disk also lacks large dust grains in its outer regions. Observations from the Atacama Large Millimeter Array in Chile show dust grains roughly the size of a grain of sand are not present beyond about 5.5 billion miles from the star, just short of the gap.

"Typically, you need pebbles before you can have a planet. So, if there is a planet and there is no dust larger than a grain of sand farther out, that would be a huge challenge to traditional planet formation models," Debes said.

The team used Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to observe the star in near-infrared light. The researchers then compared the NICMOS images with archival Hubble data and optical and spectroscopic observations from Hubble's Space Telescope Imaging Spectrograph (STIS). Debes said researchers see the gap at all wavelengths, which indicates it is a structural feature and not an illusion caused by the instruments or scattered light.

Another perspective from another journal, on the same item. This is making astronomers all kinds of curious, and I can't blame them one bit.