Nature enjoys teasing us. Stars are stars and planets are planets, you
may think. In reality it is not as clear-cut as that with the discovery
of more and more objects that are neither star nor planet. An Italian
team, using observations by ESA’s Infrared Space Observatory, ISO, has
obtained the first detailed evidence that these ambiguous star-planet
‘missing links’ form in the same manner as stars, tipping the balance
in favour of a stellar origin.

For decades astronomers have suspected the existence of a ‘missing link’
between stars and planets, objects more massive than giant planets like
Jupiter, but not massive enough to be stars. In 1995 they found the
first one. It was called a ‘brown dwarf’ and dubbed a ‘failed star’.
In terms of mass it was closer to a star than a planet. It seemed clear
that brown dwarfs were simply objects that began to form in the same
way as stars but simply couldn’t make it in the end.

But then something unexpected happened: more and more of these objects
were found, and some of them had so little mass that they could
actually be considered to be giant planets instead of failed stars.
Astronomers are still debating how to classify these very low mass
objects. For some they are ‘free-floating’ planets — because they
are floating free in space instead of orbiting a star — while others
don’t like this term because they argue these objects don’t form like
planets. Or, do they?

The truth is, no one can say yet. Could it be that the most massive
of these ‘missing links’ form like stars, and the less massive like
planets? Is there a clear-cut division in the mass scale which leads
on the one hand to the formation of a star, and on the other to a

Based on observations made with ISO Antonella Natta and Leonardo Testi
from the Osservatorio Astrofisico di Arcetri in Firenze, Italy, found
that objects down to at least 4 per cent of the mass of the Sun, or 40
Jupiter masses, form in the same way as stars. This is about half-way
along the star-planet mass scale.

The difference between stars and planets

Stars form in large clouds of gas: the gas contracts due to the inward
force of gravity, and a gaseous rotating sphere forms. The more the
sphere grows — by attracting more gas from the cloud — the stronger
the inward pull of gravity and the more ‘squeezed’ the gas becomes at
the centre of the sphere. The intense compression triggers nuclear
reactions, which release the energy we see as the star’s light.

So stars need to have a certain amount of mass, otherwise gravity won’t
compress the gas sufficiently to trigger nuclear reactions and hence to
produce light. That is the ‘tragedy’ of brown dwarfs: they have less
than 8 per cent of the mass of the Sun, or 80 times the mass of the
planet Jupiter, which is the minimum mass for a star to shine. From
that perspective, brown dwarfs are ‘failed stars’.

Planets, on the other hand, also form by accreting material, but they
take it, not from the cloud directly, but from a disc that surrounds
newborn stars at the stage when they are still sucking-in gas. Planets
are therefore expected to be found orbiting a central star, although
it is also possible that they are ejected from the system due to
gravitational interactions with other bodies. So in principle there’s
nothing against brown dwarfs actually being giant ejected planets,
rather than ‘failed stars’.

The answer lies in the disc

Astronomers already have some clues on how to solve this mystery. They
can focus their research on the disc surrounding newborn stars. Because
planets form ‘within’ a disc, they shouldn’t have one of their own. So
if young brown dwarfs do have circumstellar discs, it is strong evidence
in favour of the idea that they form like stars, and therefore are more
closely related to stars than to planets.

That is what Natta and Testi have tested. They chose three brown
dwarfs — originally detected with ISO by Paolo Persi (Istituto
Astrofisica Spaziale, Rome) — showing the strongest evidence for the
presence of a disc, and compared their signal with that of true young
stars which are known to have a disc, to check their similarity. Their
results showed that the signal coming from the three brown dwarfs is
identical to that of the young stars. Natta and Testi also showed that
the brown dwarfs have discs, and therefore formed as stars, and not as

Later, Fernando Comeron (European Southern Observatory, Garching,
Germany) and his collaborators found the masses of these objects to be
between 4-9 per cent of the mass of the Sun.

“Our result provides the first indication that objects with only 4 per
cent of the mass of the Sun form like stars. The study of discs around
very low mass objects is of crucial importance to understanding their
formation,” observes Natta. “If discs are present, and if their
properties are identical to those of discs around young stars, we can
conclude that both kinds of object form in a similar way. In this
respect, ISO provides a unique possibility to constrain the disc
properties of brown dwarfs and, in turn, their mechanism of origin,”
Testi adds.

It is not possible yet to extrapolate these results to the lower mass
‘free-floating’ planets — the less massive brown dwarfs. But Natta
and Testi prove that future ESA missions such as ESA’s Herschel, to be
launched in 2007, and NGST, that will follow in 2009, will be able to
observe discs around objects down to at least 5 Jupiter masses.

Comments by other experts

Comeron, a brown dwarf expert, comments on the result: “Indications
that young brown dwarfs are like stars in having large amounts of dust
in their surroundings had been obtained from the ground over ten years
ago. But with ISO, and in particular with its camera ISOCAM, we could
see regions more and more distant from the central brown dwarf, which
allowed us to analyse the shape of the surrounding dust and find out
whether it formed an extended disc.”

In his view, “What we know now suggests that the most massive brown
dwarfs form essentially like stars, by contraction of gas and dust
clouds under their own gravity. However, the discovery of lighter and
lighter brown dwarfs not orbiting a star, which came somewhat as a
surprise, opened the door to alternative formation mechanisms, such as
the ejection of planets in the very early stages of planetary systems.
In the short term, before future space missions arrive, we might find
out if this really happens once we have identified enough extremely
low mass objects and traced their trajectories back through space.”

35 new young brown dwarfs discovered by ISO

Due to their faintness, brown dwarfs are very difficult to study, and
since the detection of the first one in 1995 no more than several
hundred have been found. Of these, an especially interesting subgroup
is that made up of the ‘newborn’ brown dwarfs. In old brown dwarfs the
circumstellar disc may have dissipated already, but not in the young
brown dwarfs. And since the discs are a sign of a stellar origin,
young brown dwarfs are crucial to studying how brown dwarfs form. “In
this respect ISO has been a unique tool in the search for young brown
dwarfs,” says Paolo Persi.

The three brown dwarfs whose discs have been confirmed by Natta and
Testi are just a few million years old. They are in a well known
active stellar nursery called Chamaeleon I, a star forming region
located in the Southern hemisphere only 520 light-years away. In this
region, Persi says, ISO has detected 34 new young brown dwarfs, whose
nature has been confirmed by follow-up observations. Searches
conducted with ISO in other nearby regions of star formation, such as
the rho Ophiuchi cloud and Serpens, have also revealed the presence
of many young brown dwarf candidates. (See also the ISO Photo Release:
A game reserve for brown-dwarf hunters — ISO finds 30 ‘failed stars’
in nearby stellar nursery)

About ISO — the Infrared Space Observatory

The European Space Agency’s infrared space telescope, ISO, operated
from November 1995 till May 1998. As an unprecedented observatory for
infrared astronomy ISO made nearly 30,000 scientific observations.

Additional information

This ESA news note is based on the following papers:

* Exploring brown dwarf disks by A. Natta and L.Testi, published in
Astronomy and Astrophysics 376, L22-L25 (2001).

* ISOCAM observations of the Chamaleon I dark cloud by P.Persi et al.,
published in Astronomy and Astrophysics, 357, 219-224 (2000).

* Probing the brown dwarf population of the Chamaeleon I star forming
region by F.Comeron et al., published in Astronomy and Astrophysics,
359, 269-288 (2000).

For further information please contact:

Antonella Natta

Osservatorio Astrofisico di Arcetri, Firenze, Italy

Tel: +39 05 52752239


Leonardo Testi

Osservatorio Astrofisico di Arcetri, Firenze, Italy

Tel: +39 05 52752285


Paolo Persi

Istituto Astrofisica Spaziale, Rome, Italy

Tel: +39 06 49934465


Fernando Comeron

European Southern Observatory, Garching, Germany

Tel: +49 89 320 06 531


Leo Metcalfe, ISO project scientist

European Space Agency, Spain

Tel: +34 91 8131372


ESA Science Communication Service

Tel: +31 71 565 3223


* More about ISO

* A game reserve for brown-dwarf hunters — ISO finds 30 ‘failed stars’ in
nearby stellar nursery


[Image 1:]
Is it a star? Is it a planet? Objects with less than 80 Jupiter masses
(80 MJ) never become true stars, they are ‘sub-stellar’ objects: brown
dwarfs and the so-called ‘free-floating’ bodies of planetary mass. The
sizes and masses of sub-stellar objects are not directly related. Giant
planets such as Jupiter may be much less massive than brown dwarfs, but
are about the same diameter.

Young substellar objects, like ‘Teide 1’ and the objects in Sigma Orionis,
are much hotter than old ones. They are still forming. Once their
formation process is over they will probably be the same size as Jupiter.
There are not yet any direct measurements of the size of these objects,
so astronomers use models to derive them. [In this diagram MS is the mass
of the Sun, MJ is the mass of Jupiter]

The data used to prepare this diagram was provided by Rafael Rebolo,
Instituto de Astrofísica de Canarias (IAC), Spain. Rebolo’s group
discovered Teide 1, the first brown dwarf confirmed by spectroscopic
measurements, in 1995.

[Image 2:]
ISO also located brown dwarfs in the region of rho Ophiuchi.