Neither Star nor Planet They are often eclipsed by more attractive topics, like black holes or exoplanets. Even the name itself is less than sensational: brown dwarfs. But Viki Joergens and her colleagues from the Max Planck Institute for Astronomy in Heidelberg have gained fascinating insights in this research field. PHYSICS & ASTRONOMY_Brown Dwarfs TEXT THOMAS BÜHRKE he Milky Way probably has it. As Japanese theoreticians Chu-shiro Stars are formed when individual re- roughly as many brown dwarfs Hayashi and Takenori Nakano discov- gions in the interior of a large cloud of as it has planets. Astronomers ered back in 1963, this process requires gas and dust contract under the effects don’t know the precise num- a star to have at least 7 to 8 percent of of gravity. The core of such a cloud ro- ber, as these celestial bodies the solar mass, corresponding to 75 tates and forms a disk. The material in T are small and very dim, and thus diffi- times the mass of the planet Jupiter. the cloud center becomes more and cult to observe. They have, however, re- But bodies that are just below this more compressed until hydrogen fusion ally shaken our tried and tested defini- threshold should initially still be suffi- begins. The young star is then stable. tions of the terms “star” and “planet” ciently hot to fuse heavy hydrogen Dust particles collide with each oth- of which we have grown so fond. Spots (deuterium) to helium-3 and thus gen- er in the disk that still surrounds them, form on their surface like the spots on erate energy. However, the raw materi- clump together and finally grow into the Sun, and clouds form as if they were al deuterium is present only in small asteroids and planets. Earth and the gas planets. “This is one of the most impor- quantities, so that the fire is extin- planet Jupiter were also formed in this tant characteristics of our research field: guished after just a few million years. way. And the terms “star” and “planet” there are countless overlaps with the From then on, the celestial body slow- are defined in accordance with this sce- properties of both planets and stars,” ly cools down. “Eventually, all brown nario: a star is a stable ball of gas that explains Viki Joergens, who has been dwarfs are approximately the size of Ju- generates energy in its interior through investigating these celestial bodies for piter,” says Joergens. nuclear fusion; planets aren’t able to do more than ten years now. this, are smaller, and orbit their sun. Back in 1962, Shiv Kumar, who was THE HYDROGEN FUSION STARTS But which formation path do brown then working as a post-doc at the God- DEEP IN THE INTERIOR dwarfs choose? dard Space Flight Center of the US “Since young stars are initially still space agency NASA, set the ball rolling. In order for this deuterium burning to surrounded by a disk of dust, the ob- He asked himself how small a star can start, the body must have at least 13 vious thing was to look at whether actually be, and what properties bod- times the mass of Jupiter. This repre- brown dwarfs can also be surrounded ies that are just below this threshold sents the lower mass threshold of by a disk,” explains Viki Joergens. The possess. The fundamental characteris- brown dwarfs according to the current Heidelberg-based astronomers used tic of stars consists in the multi-step definition of the International Astro- Herschel, the space telescope of the Eu- fusion of hydrogen to helium in their nomical Union. These objects thus ropean Space Agency (ESA), for this central region. form the link between planets and project. Equipped with a 3.5-meter di- This process releases energy that, in stars in a range of around 13 to 75 Ju- ameter main mirror, it was the largest the form of heat, creates an outward piter masses. telescope ever launched into space. It pressure against the gravity acting in- Astronomers spent more than 30 was in operation from June 2009 for ward. If these two pressures balance, years looking in vain for these failed stars nearly four years, at which point the the star is stable. Our Sun has been in until they finally found the first repre- helium for cooling the instruments was this phase for around 4.5 billion years. sentative of this class of objects in 1995. exhausted. To reach this state in the first place, Approximately 2,000 brown dwarfs Herschel observed exclusively in the the celestial body must have a certain have since been identified – some with region of mid- to far infrared at wave- minimum mass; otherwise, the pressure surprising properties. “One of the most lengths ranging from 70 to 500 mi- and temperature aren’t sufficient to ig- topical questions concerns their birth,” crometers. “This is where the thermal nite the hydrogen fusion and maintain says Joergens. radiation of cool dust, among other View of a brown dwarf: These linking elements between stars and planets have been keeping astronomers busy for over half a century. Graphic: ESO/Ian Crossfield 3 | 14 MaxPlanckResearch 47 above: Presentation in the team: Viki Joergens things, can be observed, as we expect it planets such as Jupiter to form in the (standing) and her colleagues Ian Crossfield, to be emitted by the disks of brown disks of brown dwarfs, but smaller Niall Deacon and Esther Buenzli (from left). dwarfs,” says Joergens. rocky planets may very well do so. But below: The cosmos in art: The object PSO J318.5-22 (top) has around seven times the mass PACS, one of the three instruments to date none have been discovered. of Jupiter and travels through space alone, aboard Herschel, was built under the di- Surprisingly, brown dwarfs follow a meaning without a parent sun. The picture of rection of the Max Planck Institute for law that was discovered for young stars: OTS44 (bottom) illustrates that this object, just Extraterrestrial Physics with crucial the disks of dust always have around 1 two million years young, formed in a similar contributions from the astronomers in percent of the stellar mass. Viki Joer- way to stars, namely from a disk of gas and dust. Even today, considerable quantities of matter Heidelberg. In return, they received gens’ observations show that this ap- still fall onto OTS44. guaranteed observation time with plies down to a central mass of only 12 PACS. Working with colleagues from Jupiter masses. Thus, in this respect, the University of Texas, they com- brown dwarfs don’t differ from their menced a program to search for dust big brothers. They are apparently also disks around brown dwarfs – with formed in the same way and not like great success. planets. If this relation also existed for “For 36 of 47 carefully selected our Sun, it is possible to conclude from brown dwarfs, we found infrared emis- this that only around 10 percent of the sions that originate from such disks,” dust was converted into planets. says Viki Joergens. And the initiator of Since brown dwarfs appear to over- the project, Max Planck Director Thom- lap seamlessly with the range of the as Henning, adds: “We therefore suc- planets at the lower threshold of around ceeded in carrying out the first survey 13 Jupiter masses, the Max Planck as- for such disks in the infrared region, tronomers from Heidelberg searched and in narrowing down their masses.” through the Herschel observations to “We find that disks around brown see whether any of the representatives dwarfs have masses ranging from just with the lowest mass had a disk – and below one Earth mass up to one Jupiter found a celestial body at a distance of mass,” says Joergens; Jupiter, on the 530 light-years with the designation other hand, has 300 times more mass OTS44. The intensity of its far-infrared than Earth. However, not all the disk radiation had to originate from a disk material is used to form planets. It of at least 10 Earth masses. In addition, therefore isn’t possible for large gas the researchers discovered that the ob- Graphics: MPI for Astronomy/V. Ch. Quetz (top) – A. M. (bottom); photo: Thomas Hartmann 48 MaxPlanckResearch 3 | 14 PHYSICS & ASTRONOMY_Brown Dwarfs ject, only two million years young, was The important mass determination of of known age in the Chameleon con- still picking up matter from the disk, as a brown dwarf is a particularly tricky stellation in the southern sky. With young stars do. task and can’t be managed with the PSO J318.5-22, on the other hand, the Interestingly, OTS44 has only around old, familiar rules of stellar physics. astronomers were able to measure the 12 Jupiter masses and is thus still in the Whereas there is a clear relationship motion in space. In the process, they classical mass range of the planets. How- between the measurable luminosity ascertained that it used to belong to a ever, the object doesn’t orbit a star, but and the mass of a star during the phase group of young stars that formed moves freely through space – a key piece of stable burning, this isn’t the case for around 12 to 21 million years ago. But of information for theories of star for- brown dwarfs: They form as hot balls the researchers can’t always hope for mation. “OTS44 can be designated ei- of gas and cool down over time. Their such fortunate circumstances, and this ther as a very low-mass brown dwarf or light becomes weaker and weaker as makes the interpretation of observa- as a free-floating planet.