Is it a star? Is it a planet? ISO may have discovered the true nature of brown dwarfs
13 November 2001Nature 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 planet?
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 planets.
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.
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:
Osservatorio Astrofisico di Arcetri, Firenze, Italy
Tel: +39 05 52752239
Osservatorio Astrofisico di Arcetri, Firenze, Italy
Tel: +39 05 52752285
Istituto Astrofisica Spaziale, Rome, Italy
Tel: +39 06 49934465
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