Big gas planet or “failed” star? This question is difficult to answer for some massive celestial bodies. Because gas giants and brown dwarfs are very similar in many ways – and both can orbit stars as lighter partners. But now astronomers could have found a distinguishing feature: The orbit of brown dwarfs is on average significantly more eccentric than that of gas giants, as observation data show. This in turn indicates that these two types of celestial bodies are created in different ways.
According to the current hypothesis, planets can arise in two ways: either they grow gradually in a large disk of dust and gas orbiting a young star. This accretion created the Earth and most of the other planets in our solar system. Theoretically, very massive gas planets could also be created in a similar way to stars, in that locally unstable parts of the primary cloud collapse and condense into a lump of matter. This planet formation due to gravitational instability only works in relatively massive, cool protoplanetary disks. So far, what gas giants have actually arisen through this process has been disputed.
Gas planet or brown dwarf?
“One of the overriding motivations for observing exoplanets is therefore to determine the dominant pathways through which gas giants and brown dwarfs emerge and evolve,” said Brendan Bowler of the University of Texas at Austin and his colleagues. This applies in particular to celestial bodies that orbit their star at a very great distance of more than a hundred astronomical units. Because with them it is often not clear how they were formed, nor whether they are very large gas planets or brown dwarfs – “failed” stars, the mass of which was not sufficient to ignite the nuclear fusion inside. Common assumption is that brown dwarfs with more than 15 masses of Jupiter are significantly heavier than gas giants. If they are companions of normal stars, they would also have to arise like double stars: not through the local collapse in the protoplanetary disk of the star, but in a separate gas cloud.
But how can this be distinguished if you don’t “catch” the celestial bodies when they are formed? Bowler and his team have now found a possible solution for this. “One way to find out is to examine the dynamics of such systems – by looking at their orbits,” Bowler explains. Because if a celestial body was created in the accretion disk around a star, it would have to orbit it in a rather circular orbit. However, if the object comes from its own small gas cloud, much more eccentric orbits are possible. “Even if such celestial bodies are millions of years old, the memory of their path of origin should still be preserved in their eccentricity,” explains co-author Eric Nielsen from Stanford University.
Clear differences
To test this assumption, astronomers have now examined the orbital parameters of 27 gas giants and brown dwarfs far from their star. Because these celestial bodies sometimes take centuries to orbit, they used an astrophysical model to reconstruct the entire orbit and its shape from a few position data. “Instead of waiting for decades or centuries, we can compensate for the short observation intervals,” explains Nielsen. The evaluations showed that there is actually a difference between gas giants and brown dwarfs. “The objects we see as planets actually tend to have more circular orbits,” Bowler reports. The brown dwarfs, which weigh more than 15 Jupiter masses, on the other hand predominantly move on eccentric orbits – similar to the lighter partners in normal binary star systems.
According to astronomers, this confirms that gas giants are likely to form like planets even at a great distance from their star: in the protoplanetary disk of their star. Systems consisting of a star and a brown dwarf orbiting it, on the other hand, form like double stars through the collapse of two separate gas clouds. Bowler and his team now want to add further examples to their previously small sample of only 27 such objects. To do this, they search among other things in the data from the Gaia space observatory for systems with planets or brown dwarfs that orbit a central star at a great distance.
Source: Brendan Bowler (The University of Texas, Austin) et al., Astronomical Journal, doi: 10.3847 / 1538-3881 / ab5b11