One trillion kilometres apart—a lonely planet and its distant star

January 26, 2016
n artist's impression of 2MASS J2126. Credit: University of Hertfordshire / Neil Cook

A team of astronomers in the UK, USA and Australia have found a lonely planet, until now thought to be a free floating or lonely planet, in a huge orbit around its star. Incredibly the object, designated as 2MASS J2126, is about 1 trillion (1 million million) kilometres from the star, or about 7000 times the distance from the Earth to the sun. The researchers report the discovery in a paper in Monthly Notices of the Royal Astronomical Society.

In the last five years a number of free floating planets have been found. These are gas giant worlds like Jupiter that lack the mass for the nuclear reactions that make stars shine, so cool and fade over time. Measuring the temperatures of these objects is relatively straightforward, but it depends on both mass and age. This means astronomers need to find out how old they are, before they can find out if they are lightweight enough to be planets or if they are heavier 'failed stars' known as .

US-based researchers found 2MASS J2126 in an infrared sky survey, flagging it as a possible young and hence low mass object. In 2014 Canadian researchers identified 2MASS J2126 as a possible member of a 45 million year old group of stars and brown dwarfs known as the Tucana Horologium Association. This made it young and low enough in mass to be classified as a free-floating planet.

In the same region of the sky, TYC 9486-927-1 is a star that had been identified as being young, but not as a member of any known group of . Until now no one had suggested that TYC 9486-927-1 and 2MASS J2126 were in some way linked.

False colour infrared image of TYC 9486-927-1 and 2MASS J2126. The arrows show the projected movement of the star and planet on the sky over 1000 years. The scale indicates a distance of 4000 Astronomical Units (AU), where 1 AU is the average distance between the Earth and the sun. Credit: 2MASS/S. Murphy/ANU

Lead author Dr Niall Deacon of the University of Hertfordshire has spent the last few years searching for young stars with companions in wide orbits. As part of the work, his team looked through lists of known young stars, brown dwarfs and free-floating planets to see if any of them could be related. They found that TYC 9486-927-1 and 2MASS J2126 are moving through space together and are both about 104 light years from the sun, implying that they are associated.

"This is the widest planet system found so far and both the members of it have been known for eight years," said Dr Deacon, "but nobody had made the link between the objects before. The planet is not quite as lonely as we first thought, but it's certainly in a very long distance relationship."

When they looked in more detail, the team were not able to confirm that TYC 9486-927-1 and 2MASS J2126 are members of any known group of young stars.

"Membership in a group of young stars is great for establishing an age," said study co-author Josh Schlieder of the NASA Ames Research Center, "but when we can't use that we need to resort to other methods."

The team then looked at the spectrum – the dispersed light – of the star to measure the strength of a feature caused by the element lithium. This is destroyed early on in a star's life so the more lithium it has, the younger it is. TYC 9486-927-1 has stronger signatures of lithium than a group of 45 million year old stars (the Tucana Horologium Association) but weaker signatures than a group of 10 million year old stars, implying an age between the two.

Based on this age the team was able to estimate the mass of 2MASS J2126, finding it to be between 11.6 to 15 times the mass of Jupiter. This placed it on the boundary between planets and brown dwarfs. It means that 2MASS J2126 has a similar , age and temperature to one of the first planets directly imaged around another star, beta Pictoris b.

"Compared to beta Pictoris b, 2MASS J2126 is more than 700 times further away from its host star," Dr Simon Murphy of the Australian National University, also a study co-author, "but how such a wide planetary system forms and survives remains an open question."

2MASS J2126 is around 7000 Earth-sun distances or 1 trillion kilometres away from its parent star, giving it the widest orbit of any planet found around another star. At such an enormous distance it takes roughly 900,000 years to complete one orbit, meaning it has completed less than fifty orbits over its lifetime. There is little prospect of any life on an exotic world like this, but any inhabitants would see their 'sun' as no more than a bright star, and might not even imagine they were connected to it at all.

Explore further: Newly discovered star offers opportunity to explore origins of first stars sprung to life in early universe

More information: A nearby young M dwarf with a wide, possibly planetary-mass companion. Monthly Notices of the Royal Astronomical Society, Oxford University Press, in press. www.ras.org.uk/images/stories/press/Exoplanets/young_planet_paper.v2.pdf

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19 comments

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Phys1
3.8 / 5 (8) Jan 26, 2016
To call this a planet is maybe not justified. The question is whether this object was formed in a protoplanetary disk around the star, or, alternatively, was formed independently by contraction from the same nebula as the star. This could be a double star where one partner was too light.
TheGhostofOtto1923
4 / 5 (8) Jan 26, 2016
to call this a planet
Science has already determined how to classify a planet
http://missionsci...net.html

-and apparently they don't make the distinction as to whether it was captured or whether it formed along with the star.

The astronomers cited in the article would be very familiar with how to classify such objects, and they have called it a planet.

Correct?
Phys1
3.4 / 5 (5) Jan 27, 2016
This is the first object of its kind, because it is gravitationally bound to a star.
Models of planet formation do not apply to this case.
It is better to extend the definition of star to this category because they are likely to have formed in the same way. The debate is ongoing as you can read on the best website of the world.
"some researchers call them free-floating planets whereas others call them planetary-mass brown dwarfs.
https://en.wikipe...wn_dwarf
TechnoCreed
4.4 / 5 (7) Jan 27, 2016
This is the first object of its kind, because it is gravitationally bound to a star.
Models of planet formation do not apply to this case.

There are two processes to the formation of planets:
1- Core accretion, off course; this is the one everybody knows about.
2- Gravitational instabilities; this one happens during the molecular cloud collapse and it is the one you have got to learn about.
It is better to extend the definition of star to this category because they are likely to have formed in the same way. The debate is ongoing as you can read on the best website of the world.

A celestial body can only be called a star when it is massive enough to maintain the fusion process. A brown dwarf is not a star.
TechnoCreed
4.5 / 5 (8) Jan 27, 2016
Edit
A celestial body can only be called a star when it is massive enough to maintain the fusion process.
A celestial body can only be called a star when it is massive enough to sustain hydrogen-1 fusion.
Phys1
3.4 / 5 (5) Jan 27, 2016
Edit
A celestial body can only be called a star when it is massive enough to maintain the fusion process.
A celestial body can only be called a star when it is massive enough to sustain hydrogen-1 fusion.

That is a definition. Definitions can change, see Pluto. It is very impractical to call this object a planet for then there are two radically different kinds of planets, one formed from stellar accretion and the other by gravitational collapse of a molecular cloud. Also it likely fails the third requirement for being a planet: "has cleared its neighbouring region of planetesimals".
I prefer sub brown dwarf
https://en.wikipe...wn_dwarf
Phys1
3 / 5 (4) Jan 27, 2016
Dwarf is short for "dwarf star". The fusion criterion is arbitrary, especially if restricted to 1H fusion. This restriction is ad hoc, to maintain definitions. I am all in favour of common language but science must be remain flexible when new phenomena are found.
TechnoCreed
4.4 / 5 (7) Jan 27, 2016
Dwarf is short for "dwarf star". The fusion criterion is arbitrary, especially if restricted to 1H fusion. This restriction is ad hoc, to maintain definitions. I am all in favour of common language but science must be remain flexible when new phenomena are found.

Au contraire, the fact that brown dwarfs cannot burn H1 gives them their distinctiveness and clearly sets them apart.

Coming back to 2MASS J2126, it is not clear whether it is a brown dwarf or not; it sits right at the hypothetical limit (13 Jupiter mass) my guess is that JWST will be able to give an answer to that; meanwhile, it is a planet. It is gravitationally bound to TYC 9486-927-1, so its origin is most probably the result of a fragmentation of the same molecular cloud.
Phys1
3 / 5 (4) Jan 27, 2016
Why do you attach absolute value to 13 Jupiter masses? It is a rule of thumb. It is not a planet as in a stellar system. It has a different origin. This is a summary of the pov of astronomers:
"A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a mass below the limiting mass for thermonuclear fusion of deuterium (about 13 MJ).[1] Some researchers call them free-floating planets whereas others call them planetary-mass brown dwarfs."
Phys1
3 / 5 (4) Jan 27, 2016

Au contraire, the fact that brown dwarfs cannot burn H1 gives them their distinctiveness and clearly sets them apart.

It sets them apart from sub brown dwarfs that can not fuse 1H but can fuse 2H. The 1H fusers you call stars and the 2H fusers planets. Why ? Fusion is fusion.
Suppose two stars collide. Two bodies emerge one star and the other smaller the 13 Jupiter masses. Are going to call that a planet too?
These are rules of thumb, not laws. If you stretch them, they lose their utility.
Phys1
3 / 5 (4) Jan 27, 2016
Here is a 2014 peer reviewed paper on an object of 3-10 Jupiter masses and T=250K. The author refers to it as a brown dwarf.
http://arxiv.org/abs/1404.6501
This will converge to either free-floating planet or (sub / planetary mass) brown dwarf.
My bet is on an extension of the notion of brown dwarf.
The HR diagram will be extended to include them since they are obviously a stage of stellar evolution.
TechnoCreed
3.4 / 5 (5) Jan 28, 2016
It is a rule of thumb… "…Some researchers call them free-floating planetary-mass brown dwarfs."
The general rule (not of thumb), when paraphrasing or quoting a source, is to give the source, otherwise it is called plagiarizing and it makes you look really bad. I have nothing against Wikipedia except that they are not the governing body of the astronomy. It is not you nor me and not even K. L. Luhman (the author of the arxiv paper you have linked ) that will give the designation of a celestial body; It is the IAU and I can assure you they are not playing rock, paper, scissors.
Phys1
2.3 / 5 (3) Jan 28, 2016
@TC
Are you accusing me of plagiarism? That is silly.
I already gave the source of that, the wiki page on sub brown dwarfs.
Also I referred to the paper reporting the discovery.
If you don't have an opinion of your own by all means follow the "governing body". You'll come around when the astro government does.
TechnoCreed
3.7 / 5 (3) Jan 28, 2016
Here is a 2014 peer reviewed paper on an object of 3-10 Jupiter masses and T=250K. The author refers to it as a brown dwarf.
http://arxiv.org/abs/1404.6501
As I am sure that you have nothing against Wikipedia, I will invite you to read their article about WISE J085510.83-071442.5 https://en.wikipe...8%920714

TechnoCreed
5 / 5 (2) Jan 28, 2016
Are you accusing me of plagiarism? That is silly.
I already gave the source of that, the wiki page on sub brown dwarfs.
Alright you did; sorry about that. You should refer back to it whenever it is appropriate. It would avoid this kind of misunderstanding.
Phys1
2.3 / 5 (3) Jan 28, 2016
@TC
Wikipedia is great, its founders should get a Nobel prize.
Thanks for the link. I read
"A lighter object and one orbiting another object is considered a planet. So far this WISE object is alone, though it could be a rogue planet, something first identified in 2004 in the case of Cha 110913-773444."
When the classification was decided these objects were still very rare or unknown. I am 100% confident that they are just an extension to the HR diagram, which planets are not.
But feel free to differ.
TechnoCreed
5 / 5 (2) Jan 28, 2016
If you don't have an opinion of your own by all means follow the "governing body".
Individual opinions does not count. I know that some astronomers would like to debate about that, but the IAU stands firm and it represents the majority. http://cs.astrono...arf.aspx
TechnoCreed
5 / 5 (2) Jan 28, 2016
I am 100% confident that they are just an extension to the HR diagram, which planets are not.
But feel free to differ.
On this I would strongly recommand that you listen to the following lecture. It is given by Dr. Gibor Basri; he is a prominent person on the subject. https://www.youtu...Dsb4n4KU
Phys1
3.7 / 5 (3) Jan 28, 2016
@TC
Interesting indeed.

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