Herschel's new view on giant planet formation



by Staff Writers



Paris (ESA) Mar 15, 2012



Astronomers have used ESA's Herschel Space Observatory to observe 2M1207, a peculiar brown dwarf with its own circumstellar disc and a planetary companion five times more massive than Jupiter. These new data provide the first image of this system taken at sub-millimetre wavelengths and show that the disc's mass amounts to a few times the mass of Jupiter. The presence of such a massive disc around this ten-million-year old brown dwarf suggests that its planetary companion formed directly from the disc's fragmentation. This reopens the debate on how giant planets form around stellar and sub-stellar objects.

Brown dwarfs are a puzzling class of objects suspended between stellar and planetary status. Not massive enough to ignite hydrogen fusion in their cores, like stars do, they are more massive than planets and thus able - unlike planets - to burn deuterium for a brief period in their interiors.

Since the first brown dwarf was discovered, in 1995, astronomers have detected hundreds of them in our Galaxy, the Milky Way, and have speculated about the origin of these curious celestial bodies.

Like stars, a number of brown dwarfs are surrounded by discs of gas and dust, very similar to circumstellar discs seen around young stars. A handful of them are also known to possess planetary companions with the properties of gas giants and masses similar to or larger than Jupiter's.

Since the separation between these planetary-mass objects and their parent bodies is larger than the typical values observed for most gas giant exoplanets, these brown dwarf planetary companions are valuable when investigating how giant planets form at rather large distances from their parent bodies.

Two main physical mechanisms are known to result in the formation of giant gaseous planets around stars: the 'standard' scenario of core accretion and the 'alternative' one invoking disc fragmentation.

In the core accretion mechanism, a rocky core forms first from the coagulation of dust grains in the disc, and only later does this solid core start accreting gas from its surroundings, eventually growing into a giant gaseous planet.

In the disc fragmentation model, the circumstellar disc collapses due to its own weight into smaller fragments, whose minimum mass depends on the disc's total mass, thus forming proto-planets that later evolve into proper planets. Both mechanisms could well be at play around brown dwarfs as well.

Theory suggests that disc fragmentation is more efficient at forming giant planets at large radial distances from the parent body: brown dwarfs with planetary companions are thus a crucial testbed to probe the validity of this alternative model for giant planet formation.

For this purpose, a team of astronomers led by Basmah Riaz from the University of Hertfordshire, UK, have exploited ESA's Herschel Space Observatory to study a peculiar brown dwarf which is known to possess both a disc and a planetary companion.

Named 2MASSW J1207334-393254, or 2M1207 for short, this brown dwarf is quite evolved, with an age of about 10 million years, and has a mass that amounts to 25 times that of Jupiter. It is located at a distance of about 170 light years from us and belongs to the low-density group of stars known as the TW Hydrae association.

The brown dwarf's planetary companion, a gaseous giant five times more massive than Jupiter, was the first extrasolar planet to have been directly imaged and lies at a very large distance from 2M1207 - the projected distance between the two bodies measuring 55 astronomical units (AU).

"We have been looking forward to using SPIRE, the long-wavelength instrument on Herschel, to obtain the first view of this brown dwarf in the sub-millimetre portion of the spectrum," explains Riaz. By tracing emission from cold dust, sub-millimetre observations provide crucial information about the disc around 2M1207.

Although only a trace component in terms of the disc's mass, which mainly consists of gas, cold dust extends out to the far reaches of the disc and thus allows astronomers to probe its entire extent.

"After modelling the system with the new data, we find that the disc's total mass amounts to about 3 to 5 times the mass of Jupiter and that its radius ranges between 50 and 100 AU," she adds.

Since 2M1207 is the oldest known brown dwarf possessing a disc, the astronomers inquired into a possible correlation between disc masses and the age of their parent brown dwarfs.

"Interestingly, the mass of 2M1207's disc is similar to those of other discs observed around much younger brown dwarfs, with ages of only 1 million years," notes Riaz.

This suggests that dissipation processes of discs around brown dwarfs may take place on much slower time scales as opposed to discs around fully-fledged stars, where a trend of decreasing disc masses for increasingly older objects is recorded.

By knowing the mass and size of the disc around this brown dwarf, the astronomers have also gained crucial insight into the origin of its planetary companion.

"We knew that the planetary companion to 2M1207 did not form through core accretion, because such a process would have taken much longer than the system's age," explains Riaz. For the first time, the new Herschel data allowed the alternative formation scenario to be tested quantitatively.

"The mass of the disc indicates that this planetary-mass body has likely arisen directly from fragmentation of the disc, which may have been more massive in the brown dwarf's early days," she adds.

The system comprising 2M1207 and its companion represents the first evidence showing that planetary-mass bodies can indeed originate from disc fragmentation, thus challenging the currently leading scenario for the formation of giant planets.

However, there are also other possible scenarios that could explain the formation of this system - for example, both the brown dwarf and its planetary-mass companion might have formed at the same time as a binary system - and the issue remains open.

"This is the first detection of the brown dwarf 2M1207 in the sub-millimetre range", comments Goran Pilbratt, ESA's Herschel Project Scientist. "The present study showcases Herschel's capability to investigate the mechanisms that lead to the formation of planetary-mass companions to stellar and sub-stellar objects."

The results presented here are based on observations of the brown dwarf 2MASSW J1207334-393254, or 2M1207 for short, that have been performed with the SPIRE instrument on Herschel at wavelengths of 250, 350 and 500 micron.

The observations have been performed as part of the Herschel Open Time Programme "Disk masses for ~10 Myr old brown dwarf disks" of which B. Riaz is Principal Investigator. The other astronomers involved in this study are G. Lodato (Dipartimento di Fisica, Universita degli Studi di Milano, Italy), D. Stamatellos (School of Physics and Astronomy, Cardiff University, UK) and J.E. Gizis (Department of Physics and Astronomy, University of Delaware, USA).

The system consists of 2M1207A, a brown dwarf with a mass of 25 Jupiter masses, and 2M1207B, a planetary companion with a mass of 5 Jupiter masses. The two bodies have a projected separation of 55 astronomical units (AU). To date, 2M1207A is the oldest brown dwarf known to possess a circumstellar disc.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

The SPIRE instrument contains an imaging photometer (camera) and an imaging spectrometer. The camera operates in three wavelength bands centred on 250, 350 and 500 m, and so can make images of the sky simultaneously in three sub-millimetre colours. SPIRE has been developed by a consortium of institutes led by Cardiff Univ. (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC (UK); and NASA (USA).

B. Riaz, et al., "First sub-millimeter detection of the TWA brown dwarf disc 2MASSW J1207334-393254", 2012, Monthly Notices of the Royal Astronomical Society: Letters, doi:10.1111/j.1745-3933.2012.01225.x, in press