Young Stars Poised for Production of Rocky Planets
VLT Interferometer Studies the Inner Region of Circumstellar Discs [1]
One of the currently hottest astrophysical topics – the hunt for
Earth-like planets around other stars – has just received an important
impetus from new spectral observations with the MIDI instrument at the
ESO VLT Interferometer (VLTI).
An international team of astronomers [2] has obtained unique infrared
spectra of the dust in the innermost regions of the proto-planetary
discs around three young stars – now in a state possibly very similar
to that of our solar system in the making, some 4,500 million years
ago.
Reporting in this week’s issue of the science journal Nature, and
thanks to the unequalled, sharp and penetrating view of
interferometry, they show that in all three, the right ingredients are
present in the right place to start formation of rocky planets at
these stars.
The full text of this press release, with three photos and all
weblinks, is available at:
http://www.eso.org/outreach/press-rel/pr-2004/pr-27-04.html
“Sand” in the inner regions of stellar discs
The Sun was born about 4,500 million years ago from a cold and massive
cloud of interstellar gas and dust that collapsed under its own
gravitational pull. A dusty disc was present around the young star, in
which the Earth and other planets, as well as comets and asteroids
were later formed.
This epoch is long gone, but we may still witness that same process by
observing the infrared emission from very young stars and the dusty
protoplanetary discs around them. So far, however, the available
instrumentation did not allow a study of the distribution of the
different components of the dust in such discs; even the closest known
are too far away for the best single telescopes to resolve them. But
now, as Francesco Paresce, Project Scientist for the VLT
Interferometer and a member of the team from ESO explains, “With the
VLTI we can combine the light from two well-separated large telescopes
to obtain unprecedented angular resolution. This has allowed us, for
the first time, to peer directly into the innermost region of the
discs around some nearby young stars, right in the place where we
expect planets like our Earth are forming or will soon form”.
Specifically, new interferometric observations of three young stars by
an international team [2], using the combined power of two 8.2-m VLT
telescopes a hundred metres apart, has achieved sufficient image
sharpness (about 0.02 arcsec) to measure the infrared emission from
the inner region of the discs around three stars (corresponding
approximately to the size of the Earth’s orbit around the Sun) and the
emission from the outer part of those discs. The corresponding
infrared spectra have provided crucial information about the chemical
composition of the dust in the discs and also about the average grain
size.
These trailblazing observations show that the inner part of the discs
is very rich in crystalline silicate grains (“sand”) with an average
diameter of about 0.001 mm. They are formed by coagulation of much
smaller, amorphous dust grains that were omnipresent in the
interstellar cloud that gave birth to the stars and their discs.
Model calculations show that crystalline grains should be abundantly
present in the inner part of the disc at the time of formation of the
Earth. In fact, the meteorites in our own solar system are mainly
composed of this kind of silicate.
Dutch astronomer Rens Waters, a member of the team from the
Astronomical Institute of University of Amsterdam, is enthusiastic:
“With all the ingredients in place and the formation of larger grains
from dust already started, the formation of bigger and bigger chunks
of stone and, finally, Earth-like planets from these discs is almost
unavoidable!”
Transforming the grains
It has been known for some time that most of the dust in discs around
newborn stars is made up of silicates. In the natal cloud this dust is
amorphous, i.e. the atoms and molecules that make up a dust grain are
put together in a chaotic way, and the grains are fluffy and very
small, typically about 0.0001 mm in size. However, near the young star
where the temperature and density are highest, the dust particles in
the circumstellar disc tend to stick together so that the grains
become larger. Moreover, the dust is heated by stellar radiation and
this causes the molecules in the grains to re-arrange themselves in
geometric (crystalline) patterns.
Accordingly, the dust in the disc regions that are closest to the star
is soon transformed from “pristine” (small and amorphous) to
“processed” (larger and crystalline) grains.
VLTI observations
Spectral observations of silicate grains in the mid-infrared
wavelength region (around 10 micron) will tell whether they are
“pristine” or “processed”. Earlier observations of discs around young
stars have shown a mixture of pristine and processed material to be
present, but it was so far impossible to tell where the different
grains resided in the disc.
Thanks to a hundred-fold increase in angular resolution with the VLTI
and the highly sensitive MIDI instrument, detailed infrared spectra of
the various regions of the protoplanetary discs around three newborn
stars, only a few million years old, now show that the dust close to
the star is much more processed than the dust in the outer disc
regions. In two stars (HD 144432 and HD 163296) the dust in the inner
disc is fairly processed whereas the dust in the outer disc is nearly
pristine. In the third star (HD 142527) the dust is processed in the
entire disc. In the central region of this disc, it is extremely
processed, consistent with completely crystalline dust.
An important conclusion from the VLTI observations is therefore that
the building blocks for Earth-like planets are present in
circumstellar discs from the very start. This is of great importance
as it indicates that planets of the terrestrial (rocky) type like the
Earth are most probably quite common in planetary systems, also
outside the solar system.
The pristine comets
The present observations also have implications for the study of
comets. Some – perhaps all – comets in the solar system do contain
both pristine (amorphous) and processed (crystalline) dust. Comets
were definitely formed at large distances from the Sun, in the outer
regions of the solar system where it has always been very cold. It is
therefore not clear how processed dust grains may end up in comets.
In one theory, processed dust is transported outwards from the young
Sun by turbulence in the rather dense circumsolar disc. Other theories
claim that the processed dust in comets was produced locally in the
cold regions over a much longer time, perhaps by shock waves or
lightning bolts in the disc, or by frequent collisions between bigger
fragments.
The present team of astronomers now conclude that the first theory is
the most likely explanation for the presence of processed dust in
comets. This also implies that the long-period comets that sometimes
visit us from the outer reaches of our solar system are truly pristine
bodies, dating back to an era when the Earth and the other planets had
not yet been formed.
Studies of such comets, especially when performed in-situ, will
therefore provide direct access to the original material from which
the solar system was formed.
More information
The results reported in this ESO PR are presented in more detail in a
research paper “The building blocks of planets within the
“terrestrial” region of protoplanetary disks”, by Roy van Boekel and
co-authors (Nature, November 25, 2004). The observations were made in
the course of ESO’s early science demonstration programme.
Notes
[1]: This ESO press release is issued in collaboration with the
Astronomical Institute of the University of Amsterdam, The Netherlands
(NOVA PR
Max-Planck-Institut fuer Astronomie (Heidelberg, Germany (MPG PR
http://www.mpia.de/Public/Aktuelles/PR/PR_de.html).
[2]: The team consists of Roy van Boekel, Michiel Min, Rens Waters,
Carsten Dominik and Alex de Koter (Astronomical Institute, University
of Amsterdam, The Netherlands), Christoph Leinert, Olivier Chesneau,
Uwe Graser, Thomas Henning, Rainer Koehler and Frank Przygodda
(Max-Planck-Institut fuer Astronomie, Heidelberg, Germany), Andrea
Richichi, Sebastien Morel, Francesco Paresce, Markus Schöller and
Markus Wittkowski (ESO), Walter Jaffe and Jeroen de Jong (Leiden
Observatory, The Netherlands), Anne Dutrey and Fabien Malbet
(Observatoire de Bordeaux, France), Bruno Lopez (Observatoire de la
Cote d’Azur, Nice, France), Guy Perrin (LESIA, Observatoire de Paris,
France) and Thomas Preibisch (Max-Planck-Institut fuer
Radioastronomie, Bonn, Germany).
[3]: The MIDI instrument is the result of a collaboration between
German, Dutch and French institutes. See ESO PR 17/03 and ESO PR 25/02
for more information.
Contacts
Michiel Min
Astronomical Institute
University of Amsterdam
The Netherlands
Phone : +31-20-525-7476
Email : mmin@science.uva.nl
Francesco Paresce
European Southern Observatory
Garching, Germany
Phone : +49-89-3200-6297
Email : fparesce@eso.org
Christoph Leinert
Max-Planck-Institut fuer Astronomie
Heidelberg, Germany
Phone : +49-6221-528264
Email : leinert@mpia.de
Richard West
ESO Education and Public Relations Dept.
Karl-Schwarzschild-Strasse 2
D-85748 Garching (Germany)
e-mail: rwest@eso.org
Tel +49-89-32006276
Fax +49-89-3202362
