``ISO Adds the Missing Ingredient to Make Planets''     


How and over what time scales are planets assembled? Until the recent discovery of extrasolar planets by radial velocity techniques, among the strongest cases for planetary systems outside our own were the dusty `debris' disks detected around young stars such as Vega and beta Pictoris.  Now, a team led by Wing Fai Thi and Ewine van Dishoeck of Leiden University and Geoffrey Blake of Caltech has added a critical ingredient to the planet-making recipe in such disks - substantial amounts of molecular hydrogen - through observations conducted with the European Space Agency Infrared Space Observatory (ISO). This discovery, presented in the January 4th issue of Nature, may well help to resolve a significant problem with Jovian planet formation that had been posed by previous studies.

Molecular hydrogen - H2, two hydrogen atoms joined together by a molecular bond, dominates the primordial matter from which stars and planets are made, only about 1% by mass is present as dust or ice. Though much of the hydrogen is lost as planetary systems are born, the solids coagulate and settle under the influence of gravity, eventually forming into asteroids, comets, and, finally, planets. The dust grains in debris disks are rapidly removed by light from the central star, and so must be continuously regenerated by the collisions of larger bodies.

Small amounts of dust efficiently scatter and reradiate starlight, but H2 is very difficult to measure because of its symmetry and because its principal lines are blocked by the Earth's atmosphere. Previous studies of the gas around young stars have therefore used the trace species carbon monoxide (CO) as a proxy for H2. They have indicated that the gas is lost in just a few million years, and have led to the view that gas-giant planet formation must take place very rapidly, on time scales that are difficult to understand theoretically.

This is where Blake and collaborators enter the picture. Using the Short Wavelength Spectrometer on ISO, they have found large amounts of gas in the debris disks around the stars beta Pictoris, 49 Ceti, and HD135344 by directly observing the lowest transitions of H2. "These mid-infrared wavelength lines from molecular hydrogen are quite weak, which simplifies their interpretation but makes them hard to see. ISO was the first cryogenic telescope equipped with spectrometers that could search for H2, and our data illustrate the powerful capabilities of high resolution infrared spectroscopy from space," says Leiden graduate student Wing Fai Thi, the study's lead author. "We pushed the instrument to its limits."

Why the difference? Before these H2 observations and recent theoretical studies it was generally thought that CO was a robust tracer of the total gas mass. As it turns out, "CO is much easier to remove from disks than was previously assumed. For massive, cold disks the CO can freeze onto grains, while in tenuous disks it is destroyed by ultraviolet light from the young star. Thus, the absence of CO emission may not accurately reflect an absence of gas. Our ISO results provide the first direct studies of the gas around stars several million years in age, and reveal the presence of H2 to much longer time scales than previously believed." according to Ewine van Dishoeck.

While the H2 mass discovered is smaller than estimates for the `minimum mass solar nebula' the total mass required to provide the gas, rock, and ice contained within our Solar System "there is sufficient gas in these disks to significantly alter the dust dynamics" says Geoff Blake. "Our observations mean that dust may be lost even more rapidly, and greatly strengthens the case for the in situ generation of small particles by planetesimal collisions. We have only observed a few sources with ISO, but the fact that the gas is present for much longer periods than previously thought, perhaps as long as 20 million years, means that the theoretical models for Jovian planet formation must be re-examined."

The ability to detect H2 opens a new door to the observational examination of disk dissipation and planetary growth processes around Sun-like stars. Building on the ISO legacy, new instruments aboard the Space Infrared Telescope Facility (SIRTF) and the Stratospheric Observatory for Infrared Astronomy (SOFIA), both scheduled for completion in 2002, and eventually the Next Generation Space Telescope (NGST), to be launched in 2009, will be able to measure the gas and dust dissipation time scales from large samples of circumstellar disks, and thus constrain the time scales for the assembly of planetary systems like our own.

Additional coauthors include G.J. van Zadelhoff of Leiden, A. Sargent of Caltech and V. Mannings of the SIRTF Science Center (SSC), J.M.M. Horn and E.E. Becklin of the University of California at Los Angeles (UCLA), M.E. van den Ancker of the Harvard-Smithsonian Center for Astrophysics, and A. Natta of the Osservatorio Astrofisico di Arcetri.

The SSC is directed by Caltech Professor of Physics Tom Soifer, and has recently selected two Legacy Science teams that will use the SIRTF mid-infrared spectrometer to search for H2 emission from a wide variety of young stars. These teams, led by Profs. N. Evans of the University of Texas and M. Meyer of the University of Arizona, include Caltech Professors of Astronomy Anneila Sargent and Lynne Hillenbrand along with Professors Blake and van Dishoeck. The SOFIA Chief Scientist is Prof. E.E. Becklin of UCLA. For additional details on the various observatories, see

http://isowww.estec.esa.nl     http://sirtf.caltech.edu/

http://sofia.arc.nasa.gov/         http://www.ngst.stsci.edu


For more information about the Nature article, please contact:

Professor Geoffrey A. Blake, Div. Of Geological & Planetary Sciences, Caltech,     (626)-395-6296
gab@gps.caltech.edu

Mr. Wing-Fai Thi, Leiden Observatory,      +31-71-527-5809
thi@strw.leidenuniv.nl

Professor Ewine F. van Dishoeck, Leiden Observatory,      +31-71-527-5814
ewine@strw.leidenuniv.nl


ESA Press Release       (Text only)

Text of the Nature press release and feature.
Download PDF version of the News and Views cover by Dr. J.J. Lissauer, NASA Ames.
Download PDF version of the paper.



Images:

Left column: Scattered light or thermal emission images of the debris disks surrounding beta Pictoris, 49 Ceti, and HD 135344. The relative sizes of the images depict the angular sizes of these disks, located some 63, 200, and 260 light years distant, respectively. The beta Pictoris 1.25 µm scattered light image was acquired with the European Southern Observatory 3.6 m telescope adaptive optics coronograph. The 20 µm thermal emission measurements of 49 Ceti was taken with the MIRLIN mid-infrared camera at the 10 m Keck II telescope, and kindly provided by Prof. David Koerner of the University of Pennsylvania. The 12 µm thermal emission measurement of HD 135344 was acquired with the LWS camera at the Keck I telescope by G. Blake and J. Kessler.

Right column: Infrared Space Observatory Short Wavelength Spectrometer scans of the molecular hydrogen (H2) J = 2 - 0 transition at 28.2 µm from beta Pictoris, 49 Ceti, and HD 135344.



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The ESO 1.25 µm scattered light image of beta Pictoris along with the 17 and 28 µm ISO spectra of molecular hydrogen. From these spectra, a total gas mass of at least 20% that of Jupiter is derived.

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The Nature cover image (ESO 1.25 µm scattered light image with ISO 28 µm H2 spectrum).

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