A University of Wyoming professor co-wrote a paper that purports a planet may be nestled in a gap in the disk around the star known as TW Hydrae, which is located 176 light years away.
Hannah Jang-Condell, a UW assistant professor of physics and astronomy, is a co-author of the paper, titled “The 0.5-2.22um Scattered Light Spectrum of the Disk Around TW Hydrae: Detection of a Partially Filled Disk Gap at 80 AU.” The paper was published in The Astrophysical Journal today. The research publication is devoted to recent developments, discoveries and theories in astronomy and astrophysics.
According to the paper, the origin of the gap -- which is about 1.9 billion miles wide -- is unclear, but it could arise from a transition in the nature of the disk’s dust composition or could be the presence of a planetary companion.
“The finding of interest here is that we discovered a gap in the disk around the star known as TW Hydrae, and our calculations indicate that the gap may be harboring a planet of 6 to 28 times the mass of the Earth,” says Jang-Condell. “That’s a small planet. It’s not a ‘gas giant.’ Jupiter is 300 times the mass of the Earth. If it (gap) is a planet, it’s more like the size of Neptune.”
Because the possible planet is located so far (approximately 7.5 billion miles) from TW Hydrae and its heat source, the planet is probably cold and similar to Neptune’s temperature, she says. TW Hydrae is 80 AU, meaning it is located 80 times as far from the sun as Earth.
Discovering the suspected planet in this orbit -- far away from a small parent star -- challenges current planet formation theories, according to a press release about the research paper from the Carnegie Institution for Science. The conventional theory is that planets form over tens of millions of years from the slow, persistent accretion of dust, gas and rocks. This process occurs most easily close to the central star, where orbital time scales are short, the release says.
The disk lacks large dust grains in its outer regions, according to a similar press release from the Space Telescope Science Institute at Johns Hopkins University.
“Typically, you need pebbles before you can form a planet,” John Debes, an astronomer at the Space Telescope Science Institute and lead author of the study, is quoted as saying in both releases. “So, if there is a planet there, and there is not even millimeter-sized dust -- roughly the size of a grain of sand -- farther out, the observation is inconsistent with traditional planet formation models.”
According to the STSI release, an alternative planet-formation theory suggests that a portion 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.”
In addition to Debes and Jang-Condell, other co-authors of the paper include: Alycia Weinberger, staff scientist, Department of Terrestrial Magnetism, Carnegie Institution of Washington in Washington, D.C.; Aki Roberge, research astrophysicist, Exoplanets and Stellar Astrophysics Lab, NASA Goddard Space Flight Center in Greenbelt, Md.; and Glenn Schneider, astronomer and project instrument scientist, Steward Observatory, University of Arizona.
Moving from the theoretical
Jang-Condell’s computational astrophysics research focuses on the theoretical study of the origins of planet formation in our solar system and those around other stars. Specifically, her research investigates how planets form as a byproduct of the creation of stars.
While she says most of her research work, to this point, has been theoretical, this is the second paper in which Jang-Condell was able to use her computer models to apply to observations. The research group used images taken by the Hubble Space Telescope and compared them to models of gaps opened by planets to come to its conclusion.
“My part of the project was to run a bunch of models of gaps in disks, and compare them to possible sizes of planets,” she says. “The width of the gap discovered is consistent with what we expect a planet to do.”
TW Hydrae is between 5 million and 10 million years old, which is considered young for a star, Jang-Condell says.
“This star is one of the closest stars -- with a proto-planetary disk -- to us,” says Jang-Condell, a former Carnegie Fellow. “It still has a gaseous disk around it, which means it’s in the stage of giant planet formation. It’s of the right age to be forming planets.”
Jang-Condell says she ran close to 200 models based on known properties of the disk around TW Hydrae, essentially trying out different planets that could create a gap in a star disk similar to that which was revealed by the Hubble images.
“I could create synthetic images of my (gap) models and compare these synthetic images with the Hubble Telescope images,” she explains. “I looked at several different images at seven different wavelengths from the Hubble Telescope.”
In addition to the potential discovery of a planet, Jang-Condell says the modeling research has added significance: TW Hydrae is considered a red dwarf star, or low-mass star (about 60 percent of the sun’s mass). To date, most disks imaged around young stars by researchers have been imaged around stars more massive than TW Hydrae.
“The Hydrae is about the smallest star where this (disk imaging) has been done,” Jang-Condell says.
The research group was cautious in its findings, providing other possible explanations for the gap in the star’s disk.
According to the Carnegie release, Debes says that, even under a disk instability scenario, in which planets can collapse quickly from the disk, it’s not clear whether such a low-mass planet could form.
Jang-Condell proffered another possible scenario.
“Since it’s (gap) very circular, it could just be some kind of warp in the disk, a discontinuity in the shape of the disk at this point,” Jang-Condell says. “Maybe the composition of that disk is changing at that radius.”
But she adds, “We hope it’s a planet.”
To read a copy of the paper, go here.
