A new study published in the Astrophysical Journal (arXiv.org version) provides evidence that an extrasolar planet may be forming quite far from a small red dwarf star known as TW Hydrae.
TW Hydrae protoplanetary disk (NASA / ESA / J. Debes, STScI / H. Jang-Condell, University of Wyoming / A. Weinberger, Carnegie Institution of Washington / A. Roberge, Goddard Space Flight Center / G. Schneider, University of Arizona, Steward Observatory / A. Feild, STScI, AURA)
TW Hydrae is located some 176 light-years away in the constellation Hydra. It is only about 55% of the mass of the Sun.
The study authors led by Dr Glenn Schneider from the University of Arizona used NASA’s Hubble Space Telescope to detect a deficit of material in the TW Hydrae’s protoplanetary disk at about 80 AUs (astronomical units), or 7.5 billion miles. If the planet orbited in our Solar System, it would be roughly twice Pluto’s distance from the Sun.
Astronomers’ models indicate that the depression is about 20 AUs wide, just slightly wider than necessary for a planet-opening gap and consistent with a planet of between 6 and 28 Earth masses. The feature is seen at all wavelengths indicating it is structural and not a local compositional difference. The astronomers believe the evidence is strong for planet formation causing the gap.
“TW Hydrae is between 5 and 10 million years old, and should be in the final throes of planet formation before its disk dissipates,” said study co-author Dr Alycia Weinberger of the Carnegie Institution.
“It is surprising to find a planet only 5 to 10% of Jupiter’s mass forming so far out since planets should form faster closer in. In all planet formation scenarios, it’s difficult to make a low-mass planet far away from a low mass star.”
The goal of the study was to understand not only whether planets have formed, but also what conditions can result in planet formation and what chemical constituents are available for new planets. Models showed that the disk was brighter than expected, which indicates that very small dust grains are being lifted high above the midplane. This is surprising because observations with radio telescopes have previously shown that the disk contains dust that has conglomerated into pebbles.
The scientists designed the observations to be able to detect large water ice grains in the surface layer of the disk. These grains weren’t seen, which probably means that they have grown and sunk to the midplane of the disk where they can aggregate into water-rich planets.
Planet formation far away from a small parent star is at odds with the conventional planet-making dogma. Under the most accepted scenario, planets form over tens of millions of years from the slow accretion of dust, rocks, and gas. That happens most easily close to the central star, where orbital timescales are short. Even under a disk instability scenario, in which planets can collapse quickly from the disk, it’s not clear such a low mass planet could form.
“Typically, you need pebbles before you can form a planet. So, if there is a planet in the gap and there is no dust larger than a grain of sand farther out, we have provided a challenge for traditional planet formation models,” concluded study lead author Dr John Debes from the Space Science Telescope Institute.
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Bibliographic information: John H. Debes et al. 2013. The 0.5-2.22 μm Scattered Light Spectrum of the Disk around TW Hya: Detection of a Partially Filled Disk Gap at 80 AU. ApJ 771, 45; doi: 10.1088/0004-637X/771/1/45
