Blurring the lines between stars and planets: Lonely planets offer clues to star formation

Things used to be so simple. On the one hand, there were stars: gigantic incandescent nuclear furnaces, emitting substantial amounts of light. On the other hand, there were planets, with much lower masses than stars, reflecting their host stars' light. Stars formed from the collapse of gigantic clouds of gas; planets formed in disks of gas and dust around their nascent host stars. In between there was the somewhat more ambiguous class of brown dwarfs: an intermediaries between planet and star, more massive than a planet, but with insufficient mass for nuclear fusion to ignite in the object's core, turning it into a star. Now, two new discoveries have blurred the border between these kinds of object even further, as they show that free-floating objects with planet-like masses very likely form like stars.

Using the Pan-STARRS 1 (PS1) telescope on Hawai'i, an international team of astronomers led by Michael Liu from the University of Hawaii has discovered an exotic young object with a mass six times that of the gas giant Jupiter, which is floating in space on its own -- no host star in sight. The object, dubbed PSO J318.5-22, is located just 80 light-years away from Earth, in the constellation Capricornus. Its properties are similar to those of giant gas planets found orbiting around young stars. At an estimated age of 12 million years, it is an adolescent in terms of planetary or stellar ages.

Between 1995 and now, astronomers have found nearly a thousand extrasolar planets -- but mostly by indirect methods, detecting a wobbling or dimming of the host stars induced by the planet. Only a handful of planets have been directly imaged, all of which are around young stars (less than 200 million years old). PSO J318.5-22 is very similar in mass, color, and energy output to those directly imaged planets. Niall Deacon (Max Planck Institute for Astronomy), who is a co-author of the study, explains why this is a stroke of luck for astronomers: "Planets found by direct imaging are incredibly hard to study, since they are right next to their much brighter host stars. PSO J318.5-22 is not orbiting a star so it will be much easier for us to study. It is going to provide a wonderful view into the inner workings of gas-giant planets like Jupiter in an early phase of their evolution."

With just six times the mass of Jupiter, PSO J318.5-22 is one of the lowest-mass free-floating objects known, perhaps the very lowest. Ordinary planets are formed in a disk of gas and dust surrounding the embryo of the star they will eventually orbit. But how do solitary objects like this come into existence? Can objects this low-mass -- free-floating planets and brown dwarfs in general -- form in the same way as ordinary stars? An in-depth study of a different object, published at the same time by another group of astronomers which is led by MPIA's Viki Joergens, provides valuable evidence that they do.

Joergens and her team studied an object called OTS44, which is only about 2 million years old -- in terms of stellar or planetary time-scales a newborn baby. The object has an estimated 12 Jupiter masses (marginally more massive than PSO J318.5-22). It is also floating through space without a close companion -- but in a particularly interesting part of space: OTS44 is a member of the Chamaeleon star forming region in the Southern constellation Chamaeleon, at a little over 500 light-years from Earth, where numerous new stars are born from the collapse of dense clouds of gas and dust.

Just like a young star, OTS44 is surrounded by a disk of gas and dust. And, as Joergens and colleagues were able to show, the birth of this object isn't quite over yet: the astronomers split the light of OTS44 into its component colors using the SINFONI spectrograph at ESO's Very Large Telescope in Chile, discovering features that indicate OTS44 is still drawing in material from its disk at a substantial rate. Joergens explains: "Our observations show that, even now, there is still gas falling onto OTS44, increasing its mass."

Furthermore, combining data from numerous telescopes, including the Herschel Space Observatory, and carefully constructing a model of the free-floating planet, Joergens and her colleagues were able to show that the disk surrounding OTS44, traces of which had been found earlier, is quite substantial, with at least 30 times the mass of Earth. Both the substantial disk and the infalling material (accretion) are telltale signs of the standard mode of star formation -- an indication that there is no fundamental difference between the formation of a low-mass objects such as OTS44 and an ordinary star. OTS44 probably has the lowest mass of all objects where disk and infalling material have been detected.

Joergens continues: "If PSO J318.5-22 is an adolescent object, OTS44 is a newborn baby -- and we can see that it has been born just like an ordinary star. For the researchers working on star formation, knowing that the same processes are at work all the way down to planetary-mass objects is key information."

Such objects do not fall clearly into any of the existing categories. Solitary planets or extremely low-mass brown dwarfs -- if you want to play it safe, you can talk, more generally, about free-floating planetary-mass objects. Hubert Klahr (MPIA), an expert in simulations of star and planet formation, who was not involved in the research, comments: "This is another indication that our traditional categories of planets and stars, which are based on mass values, tell us nothing about the inner structure or the formation history of these objects."