An object earlier described as a free-floating, big Jupiter analog is actually two objects, each with the mass of several Jupiters. This binary system has the lowest mass we’ve ever discovered.

Key takeaways

  • A previously identified single Jupiter-like object is actually two objects, each with several Jupiter masses.
  • These two objects form the lowest-mass binary system ever discovered.
  • Brown dwarfs cool over time, making it hard to determine their age and mass from observations alone.
  • By studying brown dwarfs in moving groups, scientists can estimate their age based on nearby stars.
  • The binary system’s components have very low masses, making them significant for studying star and planet formation.

Tracking Down Ages

Brown dwarfs are stars with masses below ~75-80 Jupiter masses, unable to fuse hydrogen. Observing these objects presents us with a unique chance to learn about star development and atmosphere models; but, in order to properly appreciate these observations, we must first calculate the dwarfs’ masses and ages.

However, this is quite tough. Brown dwarfs cool constantly as they age, resulting in observational degeneracy: dwarfs of various masses and ages might have the same brightness, making it impossible to deduce their physical attributes from observations.

We can fix this dilemma by taking independent measurements of the dwarfs’ masses. One strategy is to look for brown dwarfs that belong to neighboring star groupings known as “moving groups”. Because the stars in the association are around the same age, the age of a brown dwarf may be approximated using the ages of other stars in the group that are simpler to distinguish.

An Unusual Binary

A team of scientists led by William Best (Institute for Astronomy, University of Hawaii) investigated such an object: the highly red, low-gravity L7 dwarf 2MASS J11193254-1137466, which might be a member of the TW Hydrae Association. However, using the sophisticated adaptive optics on the Keck II telescope in Hawaii, the team determined that this Jupiter-like object was hiding something: it was really two objects of equal flux around each other.

To understand more about this peculiar binary, Best and coworkers used measured features such as sky location, proper motion, and radial velocity to determine the possibility that 2MASS J11193254-1137466AB is a member of the TW Hydrae Association of stars. They discovered an 80% likelihood that it belonged to this group.

The authors estimated the binary’s separation to be roughly 3.9 AU based on its distance to the group, which is around 160 light-years away. The supposed membership in the TW Hydrae Association also reveals binary’s age, which is around 10 million years. This enabled Best and colleagues to estimate the masses and effective temperatures of the components using luminosities and evolutionary models.

Keck images of 2MASS J11193254–1137466 reveal that this object is actually a binary system. A similar image of another dwarf, WISEA J1147-2040, is shown at bottom left for contrast: this one does not show signs of being a binary at this resolution. [Best et al. 2017]

Planetary-Mass Objects

The scientists discovered that each component has a tiny ~3.7 Jupiter masses, putting them in the ambiguous category between planets and stars. While the International Astronomical Union considers objects with masses less than the minimum required to fuse deuterium (about 13 Jupiter masses) to be planets, other classifications differ based on composition, temperature, and origin. According to the authors, the binary consists of two objects with planetary masses.

The positions of 2MASS J11193254–1137466A and B on a color-magnitude diagram for ultracool dwarfs. The binary components lie among the faintest and reddest planetary-mass L dwarfs. [Best et al. 2017]
Regardless of the criterion, 2MASS J11193254-1137466AB is the lowest-mass binary identified to date. The individual masses of the components also make them among the lowest-mass free-floating brown dwarfs discovered. As a result, this system will serve as an important benchmark for future testing of evolutionary and atmospheric models for low-mass stars.

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