June 16th, 2013 by James Ayre
An exoplanets in the process of formation? NASA’s Hubble Space Tlescope may have recently spotted just such a thing — an unexplained gap in an enormous protoplanetary disk of gas and dust rotating around the red dwarf star TW Hydrae.
TW Hydrae is located about 176 light-years away in the constellation Hydra — the Sea Serpent. The only plausible explanation — yet put forward — for the gap in the protoplanetary disc that surrounds the star is that it is the result of a growing planet that is gravitationally gathering up much of the material that would otherwise be there. Of course such a planet has yet to be observed, and there are other possibilities…
Something to note, the distance between the gap and the parent star is rather large — 7.5 billion miles. If the proposed planet was within our solar system that would put it at about two times the distance from the Sun that Pluto is — that’s quite a distance.
The location of the proposed planet seems strange though, to those in field — challenging most popular theories on how planets form. The most popular theory claims that over a period of tens of millions of years that the material of a protoplanetary disc slowly aggregates — concentrating the dust, rocks, and gas, of the disc into a proto planet. TW Hydrae and its newly proposed planet don’t match up with this theory though — the star system 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.”
There are other planet-formation theories though, one of which suggests that planets may form rapidly. That one theorizes that a portion of the disc may rapidly become gravitationally unstable and collapses in on itself. If this theory is true a planet could form as quickly as a couple of thousand years.
“If we can actually confirm that there’s a planet there, we can connect its characteristics to measurements of the gap properties,” states John Debes, of the Space Telescope Science Institute in Baltimore. “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.”
Further complicating this already rather strange and unexpected finding is the reality that the red dwarf star is only about 55% the mass of our Sun. “It’s so intriguing to see a system like this,” Debes notes. “This is the lowest-mass star for which we’ve observed a gap so far out.”
The disc also — somewhat strangely — seems to lack any large dust grains in the outer reaches. “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 states.
The Space Telescope Science Institute has more:
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 explains. “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.
The new research was published June 14th in The Astrophysical Journal.
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