The exoplanet with a million year orbit
Human lives are normally measured in decades and years. The passage of the seasons provides a nice unit to mark this sort of timescale. Since homo sapiens arose several hundred thousand years ago, Earth has orbited the Sun several hundred thousand times, yielding several hundred thousand winters, springs, summers and autumns.
87 light-years away, however, lies a exoplanet which in that timespan hasn’t even completed a single orbit.
Brown dwarf or exoplanet?
2MASS J2126–8140, at first glance, isn’t a strange object. It is a giant planet roughly 13 times the mass of Jupiter, massive enough that it was originally unclear whether it was an exoplanet or a brown dwarf. It orbits a red dwarf, TYC 9486–927–1, that is much fainter and less massive than the Sun. In the sky, the star is several magnitudes too dim to see by the naked eye — not too peculiar.
What is peculiar about 2MASS J2126–8140 is its orbit. It circles TYC 9486–927–1 at a distance 6,900 times the distance Earth orbits the Sun. Kepler’s third law shows that longer semi-major axes imply longer orbital periods. This is slightly exacerbated by the fact that the red dwarf is somewhat light, as stars go, less than half the mass of the Sun.
2MASS J2126–8140 was first discovered by Cruz et al. (2009), as part pf a search to identify low-mass brown dwarfs. The authors used data from the Two Micron All-Sky Survey (2MASS), which targeted objects that emit primarily in the infrared. Brown dwarfs emit primarily infrared light because they’re rather cool, usually only a couple thousand Kelvin.
Specific spectral criteria were used to initially filter out candidates in the 2MASS data, excising certain regions of infrared light. More specific criteria, including elemental abundances, both confirmed certain candidates and also yielded clues to their ages. For instance, the abundance of lithium in a brown dwarf is a function of its age. The astronomers were able to confirm that these objects also have low surface gravities, another key property of this type of objects.
New data and a new planetary system
That was in 2009. Seven years later, another group (Deacon et al. (2016)) made some key observations. Both of the objects in the system were known to exist, and their basic properties had been studied. However, they were thought to be separate, and unrelated. Deacon et al. looked at the 2MASS data again, as well as that from other surveys, such as WISE. However, they added their own observations, using the Phoenix spectrograph at the Gemini Observatory in Chile.
Using this data, the team was able to measure the objects’ radial velocity, the speed with which they are moving away from an observer on Earth. Radial velocity can be used to determine whether or not objects are associated with one another; if they have similar radial velocities and are near each other, they’re likely traveling together through space. This was used by Schweizer et al. (1987) to determine that the core and ring of Hoag’s Object are related, as I talked about in a recent blog post.
Both the red dwarf and the second body are moving away from us at roughly 8 to 11 km/s; the chance of a coincidental alignment like this was less than 5%. Age estimates also indicated that they are both about the same age, and presumably formed together. The data was conclusive: The two objects must be related. This opened some possibilities. Could 2MASS J2126–8140 be an exoplanet?
Deacon et al. fit some evolutionary models to the data. The temperature was likely between 1700 and 1900 K, and the age no more than 50 million years. Various models yielded different mass ranges, anywhere from 11.6 to 15 times the mass of Jupiter. This put the object firmly between high-mass exoplanets and low-mass brown dwarfs. The boundary between the two types of objects is quite fuzzy, and these results couldn’t put 2MASS J2126–8140 firmly in either category. However, comparisons with other exoplanetary systems supported the exoplanet hypothesis. In particular, the system resembles Beta Pictoris and Beta Pictoris b.
The future
If 2MASS J2126–8140 is indeed an exoplanet, it would have the largest semi-major axis — and orbital period — of any exoplanet known. GU Piscium b, with a semi-major axis of 2,000 AU and an orbital period of 163,000 years, would be relegated to second — by a lot. We might never be able to definitively call 2MASS J2126–8140 an exoplanet or a brown dwarf. As is often the case in astronomy, we need new data. For now, though, I’m content to call it the planet with a million year orbit.
