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Spanish and German astronomers are to report today in "Science", on their discovery of isolated giant planets undergoing formation

Birth of lonely giant planets observed

Oct. 5, 2000

Researchers from the Instituto de Astrofisica de Canarias (IAC), the California Institute of Technology and the Max Planck Institut für Astronomie, co-ordinated by Professor Rafael Rebolo (IAC/CSIC), have discovered in the Orion region three giant planets and another fifteen bodies, whose planet status could be confirmed once analyses are completed. The planets detected are reported to have masses between 5 and 15 times the mass of Jupiter, the largest planet in the Solar System. The results to be published by the specialised journal Science, include unprecedented images and spectra of bodies whose planetary masses are not associated to a given star. The superjupiters examined roam freely in Orion’s Sigma cluster, a very active star formation region located at approximately 1000 light years from Earth. The age of these extraordinarily young planets is expected to be less than five million years.



Images of these solitary planets have been obtained, in the visible range, with the 2.5-m Isaac Newton Telescope, at the IAC’s Spanish Observatorio del Roque de Los Muchachos (La Palma), and in the infra-red, with the 3.5-m telescope at Calar Alto Observatory (Almeria, Spain). The combination of these data has allowed identification of a large concentration of very dim, exceedingly red objects, in a small region surrounding Orion’s stellar system known as Sigma. These features are characteristic of giant planets currently undergoing a formation process. Subsequently, the spectra obtained with the world’s largest telescope - the 10-m Keck telescope on Mauna Kea Observatory (Hawaii)– confirmed these findings.

Although the existence of Jupiter-like bodies orbiting stars has been known since 1995, images of these giants have not been obtained to date, essentially because they are as much as one thousand million times fainter than the stars they are orbiting. The contraction process affecting these newly detected planets is in full swing – which means that their size is diminishing due to gravity – and they irradiate about ten thousand times more energy than is to be expected once they reach the size of Jupiter, i.e. when they become more stable.

To capitalise fully on this circumstance, researches began exploring in 1998, surveying the Orion region –renowned for hosting huge numbers of young stars– in the search for giant planets. The results to be released today by Science show, for the very first time, images and spectra of bodies showing planetary masses which oddly enough are not linked to any of the surrounding stars.

These so-called superjupiters float freely within a star cluster, but at distances sufficiently large to allow them to avoid the gravitational attraction of other stars. Of the eighteen candidates detected so far, three have been scrutinised using spectroscopic techniques and have been confirmed as gaseous objects with surface temperatures in the range 1,500 degrees Celsius, as expected for planets slightly less massive than Jupiter undergoing very early evolutionary phases.

In the words of Prof Rafael Rebolo, "this discovery is a challenge for current theories. In fact, a definitive explanation is still lacking. These bodies appear to be far too numerous and young to have formed in protoplanetary disks and later ejected as a result of the collisions between stars present in the disks. A more plausible hypothesis is that they emerged directly from the fragmentation and collapse of clouds of dust, a process that may well occur in a few million years time". However, the fragmentation scenario poses difficulties from the theoretical point of view when attempting to explain the formation of bodies with masses so close to Jupiter’s, and hence a definitive explanation for their existence is still pending.

The objects detected in Orion will cool down progressively – according to Víctor Sánchez Béjar, a PhD student and team member at the IAC – and in a few hundred million years will reach surface temperatures in the range 0 to 100 degrees centigrade. They will never develop rocky regions and temperatures will continue to drop until they fall in the range of Jupiter’s".

It is still premature to affirm how many of these giant planets may be present in the Galaxy. However, if the statistics inferred for Orion were representative of the entire Milky Way, hundreds of millions of isolated superjupiters would be found populating interstellar space. According to the researchers involved in the study, there are indications that they could be as numerous as solar-type stars. In the Sun’s neighbourhood (i.e. in a radius of 20 light years) there could be 30 or 40 such objects. Their discovery is clearly a challenge for current technologies.


  • María Rosa Zapatero Osorio (IAC- California Institute of Technology)
  • Víctor Sánchez Béjar (IAC)
  • Eduardo Martín (California Institute of Technology- Institute of Astronomy, University of Hawaii)
  • Rafael Rebolo (IAC-Spanish Council of Scientific Research)
  • David Barrado y Navascués (Max Planck Institut für Astronomie - Universidad Autónoma de Madrid)
  • Coryn Bailer-Jones (Max Planck Institut für Astronomie)
  • Reinhard Mundt (Max Planck Institut für Astronomie)

More information:

Physics of Low Mass Stars, the Search and Characterization of Brown Dwarfs and Extrasolar Planets

Complet list of press releases | More press releases of year 2000

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