A stellar association is a loose grouping of stars of similar spectral type and age that share a common motion. About 90 percent of all stars are thought to originate as members of associations. The TW Hydrae association (TWA) is a case in point: The group is made up of about thirty young stars, each thought to be roughly ten million years old. This is the youngest grouping of stars in the neighborhood of the Sun. You may recall 2M1207, which has turned up in these pages before, a brown dwarf member of the TWA that has a companion of planetary mass. Now we learn of another exotic find, a young, bright free-floating planet-like object.
Jonathan Gagné (Carnegie Institution for Science) used the FIRE spectrograph on Carnegie’s Baade 6.5-m telescope in Chile to measure the line-of-sight velocity of the object, known as 2MASS J1119-1137. This along with the sky motion of the object allowed researchers to make the definitive call that 2MASS J1119-1137 is indeed a member of the TW Hydrae association. The identification allowed the team to peg the object’s age at no more than 10 million years.
Between four and eight times Jupiter’s mass, 2MASS J1119-1137 is somewhere between large planet and small brown dwarf. Carnegie’s Jacqueline Faherty notes the challenge of distinguishing such free-floating objects in nearby space from more distant counterparts:
“Much more commonly, distant old and red stars residing in the far corners of our galaxy can display the same characteristics as nearby planet-like objects,” says Faherty. “When the light from the distant stars passes through the large expanses of dust in our galaxy on its way to our telescopes, the light gets reddened so these stars can pose as potentially exciting nearby young planet-like objects in our data, when they actually are not that at all.”
Lead author Kendra Kellogg (Western University, Ontario) says that the object’s strong infrared signature was an early clue to its youth. Follow-up work using the Gemini South instrument in Chile allowed the team to confirm the object’s proximity to the solar neighborhood. It is thought to be about 95 light years from the Earth. The linkage to the TW Hydrae association nailed down the object’s age, exposing its position as the nearest isolated member of the TWA.
Video: An animated view of the TW Hydrae association with the young object depicted within it. Credit: David Rodriguez, using visualization software Uniview by SCISS and the American Museum of Natural History’s Digital Universe data.
Finding free-floating objects like these can be a significant help to our studies of exoplanets, as the paper explains:
Young brown dwarfs, especially at the latest spectral types, have masses and atmospheres similar to those of directly imaged gas giant exoplanets. Isolated young brown dwarfs offer a way to study cool, low-pressure atmospheres of exoplanets without the inherent difficulties of isolating the planet flux from that of a brighter host star. Most of the known isolated planetary-mass brown dwarfs have been found through their unusually red optical and near-infrared colors, often in the regions of young stellar associations… Over the past few years, targeted searches have also encompassed the position-velocity phase spaces of nearby young stellar associations… These have helped recognize or discover the lowest-mass isolated brown dwarfs in the solar neighborhood…
No bright star overwhelming the light of 2MASS J1119-1137. No wonder such objects are useful.
The paper is Kellogg et al., “The Nearest Isolated Member of the TW Hydrae Association is a Giant Planet Analog,” accepted for publication at the Astrophysical Journal Letters (abstract). A Carnegie news release is also available.
So would this object have formed on its own, or would it have been ejected from its orbit around a star? Is there a minimal size at which such objects can coalesce?
My non-rocket scientist understanding is that such objects form on their own. That is, they form in the same way as the stars in the same association but without the accretion of enough mass to form a larger object/system.
Figuratively, they form from a wisp of a tendril of interstellar gas, collapsing in on itself but without the mass to form even a full-sized brown dwarf.
See, for example the “sub-brown dwarf” discussion in these entries:
https://en.wikipedia.org/wiki/Brown_dwarf#Low-mass_brown_dwarfs_vs._high-mass_planets
https://en.wikipedia.org/wiki/Sub-brown_dwarf
Makes me wonder how many of these things are out and about in interstellar space generally — given that they’re formed by the same processes that form stars throughout the cosmos.
When we talk about traveling one day at relativistic speed, there’s usually mention of the likely deleterious effects of impacting a grain of interstellar dust at that speed.
I’m thinking that instead hitting something with four to eight Jupiter masses “might leave a mark.”
Hitting one would be astronomically unlikely, no matter how many there are. Space is incredibly empty.
The lower mass limit for an object to form by the star formation process is held to be about one Jupiter mass. Below that, it has to form as a planet.
There was a paper a few years ago that found that 14% of the objects formed in a star-forming cloud appeared to be planetary mass objects.
These small massed objects could also be ‘seeds’ for the formation of brown dwarfs and other stars in dense gas and dust clouds.
George: there’s thought to be far fewer “sub brown dwarfs” as there are “proper stars”.
There could well be a substantial population of smaller planetary mass objects that were formed in stellar systems and then ejected. Current models suggest it is commonplace for planets to be ejected early in the history of a stellar system.