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.
Download High Resolution PDF file, 2.2 MB
Download High Resolution Postscript file, 4.4 MB
``ISO Adds the Missing Ingredient to Make Planets''
