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An international team of astronomers [2] is announcing today the
discovery of dusty disks surrounding numerous very faint objects that
are believed to be recently formed Brown Dwarfs in the Orion Nebula [3].

This finding is based on detailed observations with SOFI, a specialised
infrared-sensitive instrument at the ESO 3.5-m New Technology Telescope
at the La Silla Observatory. It is of special interest because it sheds
light on the origin and nature of substellar objects, known as “Brown

In particular, these results suggest that Brown Dwarfs share a common
origin with stars and that Brown Dwarfs are more similar in nature to
stars than to planets and, like stars, have the potential to form with
accompanying systems of planets.

Moreover, the presence of dusty protoplanetary disks around the faintest
objects in the Orion Nebula cluster confirms both the membership
of these faint stars in the cluster and their nature as bona-fide
substellar objects, making this the largest population of Brown Dwarf
objects yet known.

These important results are being reported today to the American
Astronomical Society Meeting in Pasadena (California, USA).

PR Photo 22a/01: Infrared picture of the Orion Nebula (NTT + SOFI).

PR Photo 22b/01: “Finding Chart” for Very Young Brown Dwarfs in the
Orion Nebula.

PR Photo 22c/01: Animated GIF presentation of PR Photos 22a+b/01.

Faint substellar objects in the Milky Way

Over the past 5 years, several groups of astronomers have identified a type
of very faint, substellar objects within our Milky Way galaxy. These gaseous
objects have very low masses and will never shine like normal stars because
they cannot achieve central temperatures high enough for sustained thermal
nuclear reactions to occur in their cores.

Such objects weigh less than about 7% of our Sun and have been variously
called “Brown Dwarfs”, “Failed Stars” or “Super Planets”. Indeed, since they
have no sustained energy generation by thermal nuclear reactions, many of
their properties are more similar to those of giant gas planets in our own
solar system such as Jupiter, than to stars like the Sun.

For example, even though their masses range between 10-70 times that of
Jupiter (the largest and most massive planet in our solar system), the sizes
of Brown Dwarfs are still comparable to that of Jupiter, approximately
140,000 km, or roughly 10 times smaller than the Sun.

Are Brown Dwarfs giant planets or failed stars?

Among the most fundamental issues raised by the existence of Brown Dwarfs is
the question of their origin and genetic relationship to planets and stars.
Are Brown Dwarfs giant planets or small, failed stars, or perhaps something
completely different?

The critical test needed to resolve this very basic question is to learn
whether Brown Dwarfs form by a process similar to what produces stars or
rather to one which produces planets.

Stars are thought to form when gravity causes a cold, dusty and rarefied
cloud of gas to contract. Such clouds are inevitably rotating so the gas
naturally collapses into a rotating disk before it falls onto the forming
star. These disks are called circumstellar or protoplanetary disks. They
have been found around virtually all young stars and are considered to be
sites of planet formation. Gravity helps planets form too, but this occurs
by condensation and agglomeration of material contained in the circumstellar
disk around a young star.

Thus, stars form with a disk around them while planets form within disks
around young stars. The planets in our own solar system were formed in such
a circumstellar disk around the young Sun about 4.6 billion years ago.

To date, the most important observations bearing on the question of Brown
Dwarf origin have been:

* the observed lack of Brown Dwarf companions to normal stars (something
astronomers have called the “Brown Dwarf desert”), and

* the existence of free-floating Brown Dwarfs in the Milky Way galaxy.

Both facts would appear to imply a stellar, rather than a planet-like origin
for these objects. However, one might also explain these observations if
most Brown Dwarfs initially formed as companions to stars (within
circumstellar disks), but were later ejected from the systems, e.g., because
of gravitational effects during encounters with other stars. So the issue of
Brown Dwarf origin is still unsettled.

NTT observations of substellar objects in the Orion Nebula

ESO PR Photo 22a/01

Caption: PR Photo 22a/01 shows a colour composite of near-infrared images
of the central regions of the Orion Nebula, obtained on March 14, 2000,
with the SOFI instrument at the ESO 3.5-m New Technology Telescope (NTT)
at La Silla. Three exposures were made through J- (wavelength 1.25 _m
here colour-coded as “blue”), H- (1.65 _m; “green”) and Ks-filters (2.16
_m; “red”), respectively. The central group of bright stars is the famous
“Trapezium”. The total effective exposure time was 86.4 seconds per band.
The sky field measures about 4.9 x 4.9 arcmin2 (1024 x 1024 pix2). North
is up and East is left.

ESO PR Photo 22b/01

Caption: PR Photo 22b/01 contains the corresponding “finding chart” with
the positions of the very young Brown Dwarfs in the Orion Nebula that were
studied during the present investigation. The starlike symbols represent
the brightest stars in PR Photo 22a/01 and are plotted for reference. In
this chart, very young Brown Dwarfs are represented by a double open
circle (if a dusty disk was detected) or with a single open circle (if no
dusty disk was detected). The scale is exactly as in PR Photo 22a/01.

ESO PR Photo 22c/01
[Animated GIF: 482 x 465 pix – 248k]

Caption: PR Photo 22c/01 is an animated GIF-composite of PR Photo 22a/01
and PR Photo 22b/01 for easy comparison.

To resolve this mystery, an international team of astronomers [2] has
obtained sensitive near-infrared observations of young Brown Dwarf
candidates in the Trapezium cluster, at the centre of the Orion Nebula. For
this, they used the state-of-the-art near-infrared SOFI instrument on the
ESO 3.5-m New Technology Telescope (NTT) at the La Silla Observatory

The Trapezium Cluster is a group of young stars that appears to the unaided
eye as a faint central ‘star’ in the Orion Nebula. This cluster is located
at a distance of about 1200 light-years and contains nearly 1000 stars, most
of which are younger than 1 million years. The stars in this cluster are in
their infancy when compared to our middle-aged Sun that is about 4.6 billion
years old (reduced to a human timescale, they would be just 3 days old,
compared to the Sun’s 40 years). Among the hundreds of normal stars in the
Trapezium Cluster, astronomers have previously identified a population of
objects so faint that they have been considered as prime candidates for very
young Brown Dwarfs.

The observations obtained with the NTT benefitted from superb atmospheric
conditions (e.g., a seeing of 0.5 arcsec) and allowed the astronomers to
examine the near-infrared light of more than 100 of the Brown Dwarf
candidates in the cluster. More than half of them were found to have excess
near-infrared light, compared to that a normal young Brown Dwarf should
emit. The only plausible explanation is that this extra light originates
from glowing, hot dust within protoplanetary disks surrounding these
objects. It was the same method, albeit at longer infrared wavelengths, that
first led to the discovery of dust disks around several normal stars, some
of which have later been studied in much detail, e.g., that at the southern
star Beta Pictoris.

In fact, and strongly supporting this explanation, twenty-one of the Brown
Dwarf candidates detected via the NTT observations are also identified with
optical “proplyds”, the famous dusty disks first imaged in 1994 by the
Hubble Space Telescope (HST) at optical wavelengths, cf. the corresponding
HST Press Release and images [4].

Dusty disks

The presence of such hot and dusty disks around these objects is a clear
sign of their extreme youth — this in turn confirms both their membership
in the young cluster and their nature as bona-fide substellar objects.
Thus, the Trapezium Cluster contains the largest population (approximately
100) of Brown Dwarfs yet known.

Indeed, only about 80 freely floating Brown Dwarfs have so far been
positively identified outside this cluster. “Brown Dwarfs are considerably
easier to detect and study when they are young, because they are ten times
larger and thousands of times brighter during their early youth than during
their middle age” says Elizabeth Lada from the University of Florida and a
member of the team.

Her colleague August Muench explains that “even at their brightest, however,
most Brown Dwarfs are still 100 or more times intrinsically fainter than
our Sun, explaining why astronomers have great difficulties in detecting
such objects”.

A common origin of normal stars and Brown Dwarfs

“The high incidence of disks around both young stars and Brown Dwarfs in
this cluster strongly suggests that both stars and Brown Dwarfs trace their
origin to a common physical process and that Brown Dwarfs are more similar
in nature to stars than to planets” says Charles Lada from the Smithsonian
Astrophysical Observatory.

Moreover, as is the case for stars, the disks that surround Brown Dwarfs may
be capable of forming systems of planets. According to Joao Alves from ESO,
“it is entirely possible that the Milky Way Galaxy contains numerous
planetary systems that orbit cold and dark, failed stars. Whether these
disks can indeed form planetary systems, however, still remains to be

Even if Brown Dwarfs do have planetary systems, their planets would not have
a stable climate and thus would be inhospitable to life as we know it. This
is because Brown Dwarfs do not generate their own energy for any substantial
period of time but instead fade rapidly as they age.

The next steps

For the moment being, the detection of disks around the Brown Dwarf
candidates in the Trapezium Cluster rests entirely on the measurements of
the near-infrared colours of these objects. Additional confirmation of the
presence of such dust disks can be obtained with sensitive infrared
observations made at longer wavelengths. Such observations are possible
with the largest ground-based telescopes like the VLT [5] or with the
upcoming NASA infrared satellite mission (SIRTF).


[1]: This ESO Press Release is issued in parallel with a Press Release on
the same subject by the American Astronomical Society (AAS). The indicated
embargo corresponds to the time of release at the AAS meting in Pasadena.

[2]: The team consists of Joao F. Alves (ESO, Garching, Germany), Charles J.
Lada (Smithsonian Astrophysical Observatory, Cambridge MA, USA), Elizabeth
A. Lada and August A. Muench (both Department of Astronomy, University of
Florida, Gainesville FL, USA). The research reported here was supported in
part by the US National Science Foundation.

[3]: Other ESO Press Communications about Brown Dwarfs include PR 07/97, PR
14/99 and PR 16/00. Discoveries of exoplanets and other small objects, some
of which have masses near the borderline between Brown Dwarfs and planets,
are reported in PR 18/98, PR 13/00 and PR 07/01. A spectacular infrared
image of the Orion Nebula with the VLT and the ISAAC instrument was
published earlier this year (PR Photo 03a/01) with a discussion about small
objects within this nebula.

[4]: More information about “proplyds” (PROto-PLanetarY DiskS) is available
in ESO PR 06/97 that discusses the discovery of the first such object
outside the Orion nebula.

[5]: The VLT is already equipped with one instrument suited for such
measurements, the Infrared Spectrometer And Array Camera (ISAAC) –
examples of mid-infrared observations of the giant planet Jupiter have
just been published as ESO PR Photos 21a-f/01. The NAOS-CONICA adaptive
optics multi-mode instrument will enter into operation later in 2001, to
be followed by the VLT Mid Infrared Spectrometer/Imager (VISIR). Another
powerful mid-infrared facility at ESO is the Thermal Infrared Multimode
Instrument (TIMMI2), now in operation at the ESO 3.6-m telescope on La Silla
and with which observations of the central part of the Orion Nebula were
recently made, cf. PR Photos 12a-e/01.


Joao F. Alves

European Southern Observatory

Garching, Germany

Tel.: +49-89-3200-6503


Charles J. Lada

Smithsonian Astrophysical Observatory

Cambridge, MA, USA

Tel.: +1-617-495-7017


Elizabeth A. Lada

Department of Astronomy

University of Florida

Gainesville, FL, USA

Tel.: +1-352-372-0361


August A. Muench

Department of Astronomy

University of Florida

Gainesville, FL, USA

Tel.: +1-352-392-2052 ext 268