Astronomers have known for many years that stars form when the cores of
giant clouds of cold molecular material fragment and collapse. However,
the details of the star formation process are poorly understood. In
particular, the smallest fragments that can collapse to form very low
mass stars or sub-stellar objects have not yet been identified.
Dr. Philip Lucas (University of Hertfordshire) and Dr. Patrick Roche
(Oxford University) controversially announced last year that they had
directly observed 13 faint points of light in Orion (a giant stellar
nursery where thousands of stars are being born) which appeared to have
masses closer to those of the giant planets — a few times more massive
than Jupiter in our Solar System — than the stars.
On Tuesday 3 April, the same pair will announce at the UK National
Astronomy Meeting in Cambridge that they have confirmed their discovery
of ‘free floating planets’ in the Orion Nebula. Their measurements of
the spectrum of the infrared light from 20 objects in the Nebula show
the characteristic signature of water vapour. This confirms that these
objects are indeed young, low-mass bodies, and that the faintest of
them are of planetary mass. These planetary mass objects can only be
seen because they are very young and still warm after the process of
formation.
Dr Lucas commented, “It’s exciting to find these planet-sized objects
floating around in space, unlike planets such as our Earth which orbit
a star. Our new results provide the first steps in the exploration of
their physical properties.”
“The identification and study of these objects is extremely interesting
in itself,” added Dr. Roche, “but it can also aid our understanding of
the star formation process, which is one of the major mysteries in
astronomy.”
This proof is sure to add fuel to the controversy of how such objects
were born. Are they actually planets, thrown out of their solar systems
and now floating in space, or have they been formed directly from a gas
cloud in space, much like a normal star is?
There is also controversy over how to classify these “inbetween” objects.
Some astronomers say that as these may have been formed like a star,
they should not be called planets. The authors suggest that a new term —
planetars — may be a good compromise.
FINDING THE EVIDENCE
Lucas and Roche were following up their discovery of these objects last
year, when they measured faint points of light in the Orion Nebula — a
vast cloud of gas and dust that can be seen with the naked eye as the
middle ‘star’ in the sword of the constellation of Orion. Their
observations were made in infrared light, using the United Kingdom
Infrared Telescope at Mauna Kea Observatory, Hawaii.
This sparked a wide controversy among scientists, with some arguing that
the result was a mistake because the planets might be normal stars far
behind Orion and just happened to look like very young planets.
Now Lucas and Roche have used the same telescope to analyse the light
from these giant worlds at different frequencies and shown that they
must be inside the Orion Nebula. By studying the spectrum of the light
from the planets, they were able to measure the temperature of the
objects, and then, with the use of theoretical models, derive their
masses.
The spectroscopic analysis shows strong absorption features due to water
vapour in the atmospheres of these objects, which shows that they are not
hot enough to be distant stars.
The results have been strengthened with the help of theoretical work
on the atmospheres by France Allard and Isabelle Baraffe of the Ecole
Normale Superieure at Lyon in France and by Peter Hauschildt at the
University of Georgia in the United States. Lucas, Roche, Allard and
Hauschildt analysed 20 of the brown dwarfs and planet candidates in the
Orion Nebula to confirm the result.
The so-called ‘planets’ float in space by themselves, not orbiting any
star. They are thought to be between 5 and 13 times as massive as the
planet Jupiter, so they are rather large by the standards of our Solar
System and are most unlikely to support life.
However, these worlds are not massive enough to shine by nuclear fusion,
the process that powers the Sun and the stars. Nor are they massive
enough for even the meagre nuclear reactions that occur in ‘brown dwarfs’,
objects with masses between those of planets and stars.
The scientists admit that there is a slight chance that they are
misinterpreting the data. If all the theoretical calculations for these
poorly understood objects are wrong, or if they are far older than the
stars and brown dwarfs around them, there is an outside chance that they
are actually 20 or 30 times the mass of Jupiter — too big to be called
planets. However, all the evidence found so far points to them being
free-floating planets, and other astronomers in Japan and Spain are
beginning to find signs of more and more planets in other nebulae like Orion.
CONTACT:
Dr Phil Lucas
Dept of Physical Sciences
University of Hertfordshire
College Lane
HATFIELD
AL10 9AB
Phone: +44 (0)1707-286070
Fax: +44 (0)1707-286142
E-mail: pwl@star.herts.ac.uk
Dr Pat Roche
Dept of Astrophysics
Nuclear and Astrophysics Lab.
Keble Road
OXFORD
OX1 3RH
Phone: +44 (0)1865-273338
Fax: +44 (0)1865-273418
E-mail: pfr@astro.ox.ac.uk
IMAGES ARE AVAILABLE ON THE WEB AT
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