Nearby brown dwarf turns out to be a free-floating planetary-like object
A new study has found that one of the nearest brown dwarfs to our Solar System, designated SIMP J013656.5+093347 (SIMP0136 for short), might actually be a planetary-mass object. A team of astronomers, led by Jonathan Gagne of the Carnegie Institution for Science in Washington, has presented evidence suggesting the object’s planetary nature. The findings were recently published in The Astrophysical Journal Letters.
Located some 20 light-years from our homeworld, SIMP0136 was located in 2006 by a near-infrared proper motion survey known as SIMP (Sondage Infrarouge de Mouvement Propre). It was initially classified as a bright T-type brown dwarf star. Three years later, it was discovered that SIMP0136 exhibits photometric variability, which is interpreted as a signature of weather patterns coming in and out of view over the object’s rotational period.
Now, Gagne and his team – comprising scientists from the Institute for Research on Exoplanets (iREx) at Universite de Montreal, the American Museum of Natural History, and the University of California, San Diego – revealed new insights into the nature of this variable object.
Researchers employed a new Bayesian analysis tool known as the Bayesian Analysis for Nearby Young AssociatioNs (BANYAN); they also conducted radial velocity measurements of SIMP0136 with the Near InfraRed Spectrometer (NIRSPEC) installed on the Keck II Telescope at the Keck Observatory on Maunakea, Hawaii. The study allowed the team to determine that SIMP0136 is a planetary-like member of a 200-million-year-old group of stars called Carina-Near.
“Our tool flagged SIMP0136 as a potential member of Carina-Near, and we realized that its distance was already measured in the literature and its radial velocity was also measured. We were thus able to put all these measurements together and get the velocity of SIMP0136 in 3-D, which confirmed that our tool had correctly identified that the velocity of SIMP0136 is consistent with the Carina-Near moving group,” Gagne told Astrowatch.net.
Groups of similarly-aged stars moving together through space like Carina-Near are considered by astronomers as prime regions to search for free-floating planetary-like objects. The key to properly classifying SIMP0136 was to estimate its mass. However, in order to do so, it was necessary to calculate its age. One of the methods to precisely determine the ages of brown dwarfs is to identify those that are members of young stellar associations.
“Because we now think SIMP0136 is a member of Carina-Near, we also have a good estimate for its age, which we can use as the final needed piece to estimate its mass when we compare all its properties with physical models of brown dwarfs,” Gagne said.
The researchers estimated that, at the age of about 200 million years, SIMP0136 has a model-dependent mass of approximately 12.7 Jupiter masses. Therefore, the object is right at the boundary that separates brown dwarf-like properties – primarily the short-lived burning of deuterium (a “heavy” form of hydrogen) in the object’s core – from planet-like properties.
However, Gagne noted the estimation of mass is dependent on our current understanding of the physics that govern the core and the atmosphere of brown dwarfs, and how they evolve as the brown dwarf ages. It is hoped that when brown dwarf models become more sophisticated, the mass estimate of SIMP0136 will change.
Etienne Artigau (University of Montreal, Canada), the co-author of the study, said the object’s low-mass and young age imply that, unlike more massive brown dwarfs, it should not have burned most of its initial deuterium.
“There are lots of known brown dwarfs that have temperatures close to that of SIMP0136 and that could be used for comparison. Confirming the presence of deuterium on SIMP0136 and its absence in field brown dwarfs would provide an excellent test for evolutionary models for these objects and bulletproof confirmation of its low mass. There is a similar test that exists with lithium for low mass stars and young brown dwarfs,” Artigau told Astrowatch.net.
Although SIMP0136’s other fundamental properties, like its radius (1.22 Jupiter radii), rotational period (2.4 hours), and effective temperature (1,098 K) are known, there is still much to uncover when it comes to its composition.
Researchers believe future observations might allow them to precisely measure the chemical composition of the stellar members of Carina-Near and, therefore, give them more information about the composition of SIMP0136 itself. They also hope to get a much better understanding of the object’s atmosphere.
“We already know a few things about the composition of SIMP0136’s atmosphere from separating its light in all its colors, but that doesn’t tell us the whole picture. For instance, it does not tell us about its composition deep below the atmosphere,” Gagne said.
More detailed information about SIMP0136 could be delivered by the James Webb Space Telescope (JWST). This space-based observatory, which is being developed jointly by NASA, ESA, and the Canadian Space Agency, is scheduled to be launched in October 2018. One of the main goals of the JWST is to study exoplanets and their atmospheres.
“Actually, a few months ago we had a meeting in Montreal to organize our team’s ‘guaranteed time’ on JWST. The 450 hours come for the Canadian contribution to the project. Four hours were allocated to observe SIMP0136,” Artigau said.
Artigau said the object will be monitored over a little more than a rotational period with a spectrograph that covers a large wavelength domain in the near-infrared. Although this mode is mostly used for exoplanet transit spectroscopy, it should also be perfect to get very high accuracy monitoring of SIMP0136.
“This will allow us to get a much better understanding of the clouds patterns that lead to its variability. We should get this data set sometime between July and September 2019,” Artigau said.
Tomasz Nowakowski is the owner of Astro Watch, one of the premier astronomy and science-related blogs on the internet. Nowakowski reached out to SpaceFlight Insider in an effort to have the two space-related websites collaborate. Nowakowski's generous offer was gratefully received with the two organizations now working to better relay important developments as they pertain to space exploration.