The Planet That Should Not Exist

[|Glitch|]

While studying a seemingly inconsequential young star and its large planetary debris field, around 300 light years from Earth, they made an interesting observation. They discovered an exoplanet that is 654 ± 3 AU from its star. As a comparison, Pluto, at its furthest point in its orbit (semi-major axis) is 49.31 AU away from the sun, which makes this planet more than 13 times further from its star than Pluto is from our sun.

The observations were made using the new Magellan Adaptive Optics in conjunction with the Clio2 infrared camera system at the Las Campanas Observatory in Chile. It was further confirmed, using data from the Hubble Advanced Camera for Surveys Astrometry over a period of 8.3 years, that both the star and planet are moving together in space.

HD 106906
Constellation...: Crux
Spectral Type...: F5V C
Distance....: 300 ± 18 light years (92 ± 6 parsecs)
Apparent Magnitude...: 7.8
Effective Temperature: 6,516° ± 165° K
Mass....: 1.5 ± 0.1 M☉
Diameter....: 1.6 ± 0.2 M☉
Current Age...: 13 million years
Life Expectancy...: 3.6 billion years

HD 106906 b
Discovery Date.: December 4, 2013 (published)
Mass...: 11 ± 2 Jupiter masses
Radius...: Unknown
Density...: Unknown
Semi-Major Axis: 654 ± 3 AU
Eccentricity...: Unknown

As previously mentioned, there is a large planetary debris field orbiting this star. The inner edge of this debris field is 17.5 ± 2.5 AU from the star, and the outer edge is ~120 AU from the star.

If this planet had a perfectly circular orbit (an eccentricity of zero), it would take 13,651 years traveling at a constant speed of 1.42 km/s to complete one orbit. However, some astronomers are speculating that the eccentricity may be greater than 0.65, so that it would get close to the outer edge of the debris field.

In order for the planet to get as close as ~120 AU to the outer edge of the debris field it would need an eccentricity of 0.80. Furthermore, a planet with 11 Jupiter masses with such an eccentric orbit would severely disturb the debris field with every orbit. With an eccentric orbit of 0.80 it would take 13,608 years to complete one orbit, however, the planet's orbital velocity would vary from as little as 0.476 km/s at the semi-major axis to as much as 4.26 km/s at its closest approach.

As the planet makes its closest approach, the outer edge of the debris field will be gravitationally attracted to the planet. While most of the disturbed debris field will remain in orbit around the star, there should still be a detectable amount of debris around the planet itself. Therefore, it should be relatively easy to determine if the planet has a highly eccentric orbit or not. The more eccentric the orbit, the more the debris field should be visibly distorted.

The following is a simulation I created using the above data, giving the planet an eccentricity of 0.80. The simulation runs for 156,870 years (11+ orbits). The debris field is composed of 100,000 objects. I used different colors to represent different layers of the debris field, with red being the furthest from the star at 120 AU, and pink being the closest to the star at 17.5 AU.

https://www.youtube.com/watch?v=5ESLDN3GPqk
HD 106906 b Simulated Orbit

If the planet does not have a highly eccentric orbit (> 0.65), then the debris field will not be distorted. In either case, how this planet could have formed completely defies explanation.

Deuterium burning begins when there is ~14 Jupiter masses, which is what constitutes a brown dwarf. Could this be a failed brown dwarf, or just a loose collection of debris with 11 Jupiter masses?

Could this exoplanet have formed separately from its star in the solar nebula, and yet still be captured into an orbit around the star?

Or could it be just a loose knot of debris from the debris field that has not yet formed into a planet? After all, the solar system is still very young at 13 million years.

I just love it when the universe gives us such seemingly impossible situations and defies us to explain how such things are possible. It helps keep us humble by reminding us of how little we know.


Sources:
Astronomers discover planet that shouldn't be there --- Astronomy.com
HD 106906 b: A Planetary-Mass Companion Outside a Massive Debris Disk --- arXiv:1312.1265 (PDF)
http://exoplanet.eu/catalog/hd_106906_b/ --- The Extrasolar Planet Encyclopaedia

 

[Simon Bridge]

...how this planet could have formed completely defies explanation.

That's a strog statement - what are you suggesting?

You managed several possible explanations - it it doesn;t completely defy explanation does it?
I'd add the possibility that a large lump of matter from the debris field or near to it got knocked out by a collision or many. That would allow "normal" planetary formation.

For discovered exoplanets, high-eccentricity orbits appear to be the norm (iirc) - it could be that we happen to notice the exoplanets that are highly eccentric because of how we detect planets of course but this has been changing the way we think about the formation of planetary systems.

I just love it when the universe gives us such seemingly impossible situations and defies us to explain how such things are possible. It helps keep us humble by reminding us of how little we know.

... quite: although in this case, is there really enough information to be able to tell if there is anything outside understood physics happening here?

 

[|Glitch|]

That's a strog statement - what are you suggesting?

I am not the one suggesting anything. Vanessa Bailey is the one who said "This system is especially fascinating because no model of either planet or star formation fully explains what we see. This extreme mass ratio is not predicted from binary star formation theories — just like planet formation theory predicts that we cannot form planets so far from the host star."

You managed several possible explanations - it it doesn;t completely defy explanation does it?

Actually, all I managed was pure speculation. There is no reasonable explanation, based upon our current understanding of solar system formation, as to how a planet could form 650+ AUs from its star.

I'd add the possibility that a large lump of matter from the debris field or near to it got knocked out by a collision or many. That would allow "normal" planetary formation.

For discovered exoplanets, high-eccentricity orbits appear to be the norm (iirc) - it could be that we happen to notice the exoplanets that are highly eccentric because of how we detect planets of course but this has been changing the way we think about the formation of planetary systems.

... quite: although in this case, is there really enough information to be able to tell if there is anything outside understood physics happening here?

As I posted above, if the planet's orbit is highly eccentric (> 0.65) then it should be easy enough to determine if that is the case by looking at the debris field. If an 11 Jupiter mass object were to pass close to the outer edge of the debris field, then there should be a detectable distortion of the debris field. Furthermore, the orbiting planet should have a detectable amount of debris in its orbit. If the planet's orbit is not highly eccentric, then the debris field will not be distorted, and there will not be a significant amount of debris around the planet.

 

[nikkkom]

Actually, all I managed was pure speculation. There is no reasonable explanation, based upon our current understanding of solar system formation, as to how a planet could form 650+ AUs from its star.

It could have migrated there...

 

[Simon Bridge]

There is no reasonable explanation, ...

... for a narrow value of "reasonable".
I agree about the speculation ... there are ways within known physics to get a planet there, we just don't know which, if any, of these is correct for that planet. This is due to a lack of data. Meanwhile theories will proliferate.

It's a big Universe.

 

[|Glitch|]

... for a narrow value of "reasonable".
I agree about the speculation ... there are ways within known physics to get a planet there, we just don't know which, if any, of these is correct for that planet. This is due to a lack of data. Meanwhile theories will proliferate.

It's a big Universe.

If it is even a planet. It could be just a loose collection of debris with ~11 Jupiter masses. Perhaps from the surrounding massive debris field, or perhaps captured from outside the solar system. Again, since there is a lack of data I am merely speculating.

In order to keep our current understanding of solar system formation unaltered, the planet (or whatever it is) would almost have to be captured. If it was not captured, then we need to revisit our understanding of solar system formation.

 

[Phoenix59]

I agree that 650 AU is a rather extreme distance for a planet to have formed. Is there a strong reason to believe that this is not a binary system, perhaps with the companion being a brown dwarf?

 

[mfb]

The measured motion has an uncertainty of .6 mas/year or 900m/s with an observation arc of just two years. Re-measuring the position in a few years will give a good idea about the eccentricity.

Edit: Oh wait, the observations are not ordered by time. They have 9 years of observations. Okay, bad prospects, and the thing is too dim for Gaia.

 

[Damo ET]

Please forgive my ignorance, but a planet of "11 Jupiter masses" is not going to be a "loose collection of debris", it will be a fully formed planet whether gas or rocky. As for forming in a highly eccentric orbit, there is no reason to think it would have. The logical solution would be a 3 body problem where it was ejected after forming inside the debris field of that star.

Damo

 

[|Glitch|]

Please forgive my ignorance, but a planet of "11 Jupiter masses" is not going to be a "loose collection of debris", it will be a fully formed planet whether gas or rocky. As for forming in a highly eccentric orbit, there is no reason to think it would have. The logical solution would be a 3 body problem where it was ejected after forming inside the debris field of that star.

Damo

Normally I would agree. However, taking into consideration that the age of the solar system is only ~13 million years old, and the extremely large debris field (more than 100 AUs across), we could be seeing a section of that debris field that broke off into a highly eccentric orbit. It may be a planet, or it may eventually form into a planet, but it also may have not done so yet.

"This system is especially fascinating because no model of either planet or star formation fully explains what we see. This extreme mass ratio is not predicted from binary star formation theories — just like planet formation theory predicts that we cannot form planets so far from the host star." --- Vanessa Bailey​