Planet 9 takes shape

April 7, 2016, University of Bern
Simulated structure of planet candidate 9. Credit: Esther Linder, Christoph Mordasini, Universität Bern

Astrophysicists at the University of Bern have modelled the evolution of the putative planet in the outer solar system. They estimate that the object has a present-day radius equal to 3.7 Earth radii and a temperature of minus 226 degrees Celsius.

How big and how bright is Planet 9 if it really exists? What is its temperature and which telescope could find it? These were the questions that Christoph Mordasini, professor at the University of Bern, and his PhD student Esther Linder wanted to answer when they heard about the possible additional planet in the solar system suggested by Konstantin Batygin and Mike Brown of the California Institute of Technology in Pasadena.

The Swiss scientists are experts in modelling the evolution of . They usually study the formation of young exoplanets in disks around other stars light years away and the possible direct imaging of these objects with future instruments such as the James Webb Space Telescope. Therefore, Esther Linder says: "For me candidate Planet 9 is a close object, although it is about 700 times further away as the distance between the Earth and the Sun." The astrophysicists assume that Planet 9 is a smaller version of Uranus and Neptune – a small ice giant with an envelope of hydrogen and helium. With their planet evolution model they calculated how parameters like the planetary radius or the brightness evolved over time since the has formed 4,6 billion of years ago. The study was financed by the research project of the Swiss National Science Foundation PlanetsInTime and the National Center for Competence in Research (NCCR) PlanetS.

Heated from the inside

In their paper accepted by the journal Astronomy & Astrophysics, the scientists conclude that a planet with the projected mass equal to 10 Earth masses has a present-day radius of 3.7 Earth radii. Its temperature is minus 226 degrees Celsius or 47 Kelvin. "This means that the planet's emission is dominated by the cooling of its core, otherwise the temperature would only be 10 Kelvin," explains Esther Linder: "Its intrinsic power is about 1000 times bigger than its absorbed power." Therefore, the reflected sunlight contributes only a minor part to the total radiation that could be detected. This also means that the planet is much brighter in the infrared than in the visual. "With our study candidate Planet 9 is now more than a simple point mass, it takes shape having physical properties," says Christoph Mordasini.

The researchers also checked if their results explain why planet 9 hasn't been detected by telescopes so far. They calculated the brightness of smaller and bigger planets on various orbits. They conclude that the sky surveys performed in the past had only a small chance to detect an object with a mass of 20 Earth masses or less, especially if it is near the farthest point of its orbit around the Sun. But NASA's Wide-field Infrared Survey Explorer may have spotted a planet with a mass equal to 50 Earth masses or more. "This puts an interesting upper mass limit for the planet," Esther Linder explains. According to the scientists, future telescopes like the Large Synoptic Survey Telescope under construction near Cerro Tololo in Chile or dedicated surveys should be able to find or rule out candidate Planet 9. "That is an exciting perspective," says Christoph Mordasini.

Explore further: Four new giant planets detected around giant stars

More information: E. F. Linder et al. Evolution and magnitudes of candidate planet nine, Astronomy & Astrophysics (2016). DOI: 10.1051/0004-6361/201628350

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18 comments

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mitcheroo
4 / 5 (7) Apr 07, 2016
What if Planet IX is not one large world, but a smaller world with a large moon (or two, or more)?
Nanook
3.3 / 5 (8) Apr 07, 2016
The reason planet IX has to be a large planet (approximately nine earth masses) has to do with it's ability to shepherd the orbits of the other dwarf planet Kuiper belt objects. A smaller world would not have sufficient gravitational pull to have this effect upon them.
billpress11
3.2 / 5 (5) Apr 07, 2016
Wouldn't radioactive decay also contribute to higher surface temperature?

Excerpt for article:
"Its temperature is minus 226 degrees Celsius or 47 Kelvin. "This means that the planet's emission is dominated by the cooling of its core, otherwise the temperature would only be 10 Kelvin," explains Esther Linder:

Read more at: http://phys.org/n...html#jCp
JongDan
5 / 5 (3) Apr 07, 2016
What if Planet IX is not one large world, but a smaller world with a large moon (or two, or more)?

Even if it is a binary world, at 5 earth masses and such low temperature there shouldn't be a big difference in structure, at 5 earth masses. Having more than two objects of similar mass, however, is a well known chaotic problem in classical mechanics (there are approximate solutions that are long term stable if they group in pairs neatly, as you can see in many multiple stars, or if two of the objects are much heavier than the rest), so sooner or later, they'd either collide or eject each other until just two are left.
tblakely1357
5 / 5 (4) Apr 07, 2016
It doesn't sound like they've considered that planet IX might be a large rocky planet formed in the inner solar system and then ejected by the chaotic orbits of the gas giants early in the solar system's life.
Mark Thomas
2.1 / 5 (7) Apr 07, 2016
A good scientist must reserve judgement until all the evidence is in, but that doesn't mean one can't have a gut feeling. My gut feeling is Mike Brown's planet 9 between 5-15 Earth masses and between 200 AU and 1,200 AU does not exist. I hope Mike Brown gets plenty of Subaru Telescope time and proves me wrong, but something tells me he won't. Maybe Mike's next book should be, "How I Killed Pluto, Second Biggest Blunder of my Career." :-)
wduckss
1 / 5 (5) Apr 08, 2016
Science today.
There is nothing on the horizon and is completely questionable whether there will be, but already everything we know about it as it is in orbit around the Earth.
Imaginary = scientific realities.
Da Schneib
5 / 5 (6) Apr 08, 2016
Amusingly, I can actually pretty much understand the names of the layers even though they're not in my language:

Atmosphäre is pretty obvious,
Gasschicht is the layer of liquid, slushy, and solid volatiles (methane, ammonia, CO₂, etc., but no water, see below),
Eisschicht is the layer of water ice,
Silikatmantel is the silicate mantle, and
Eisenkern is the ice core.

It's pretty interesting that models of planetary formation are this advanced. And the fact that its intrinsic radiation (that is how bright it is due to its own heat) is overwhelmingly greater than any sunlight it might reflect answers the obvious question why we never detected it before: infrared astronomy isn't all that old, and there haven't been a lot of infrared sky surveys before because we simply didn't have the instrumentation.
antialias_physorg
5 / 5 (10) Apr 08, 2016
Amusingly, I can actually pretty much understand the names of the layers even though they're not in my language:

You got it almost right, except:

Gasschicht is the layer of liquid, slushy

"Gasschicht" means gaseous layer

Eisenkern is the ice core.

"Eisen" is iron (i.e. iron core)
FredJose
1.4 / 5 (9) Apr 08, 2016
They usually study the formation of young exoplanets in disks around other stars light years away

I wonder how they "study" these things since they have NEVER been observed, EVER.
Furthermore, if they use simulation, how does the simulation get over the fact that in the vacuum of space it's near impossible, bordering on the sheer miraculous, to have any dust or rocks accumulate into a bigger and bigger conglomerate that eventually end up as a planet.
The idea that rocks can bump into each other and stick together is simple story telling, ESPECIALLY in the vacuum of space. Such accumulation of material has NEVER been observed. EVER.
Mark Thomas
3 / 5 (6) Apr 08, 2016
nkalanaga, I agree with everything you wrote, except ironically, I actually do want to be proven wrong and hope Planet 9 exists. It would be great for humanity to have an easy to articulate reason to send a probe that far.

What do you think is more likely? 1. Planet 9 exists and we have been unable to detect or precover Planet 9 from existing telescope data because it has the right location, size and temperature to avoid detection by everything done to date in all wavelengths, or 2. We have not fully considered other reasons for the KBO effects Mike discovered including all the close passes by other stars and rogue planets over the past 4.5 billion years. Check out Scholz's Star, for example, there must have been a lot more. Could a rogue Jupiter-sized planet passing by 2 billion years ago have caused this?

Mark Thomas
2.6 / 5 (5) Apr 08, 2016
@Phys1; How is that bad?

The rationale for demoting Pluto is that it is only one of many KBOs, yet a decade after demotion it is still the largest known body in the solar system beyond Neptune. Except for Eris, most of the KBOs discovered are much smaller. Furthermore, this argument effectively precludes a planet of any size in the Kuiper Belt because the neighborhood is not cleared and thus defines a planet by location, which makes no sense.
Eikka
5 / 5 (4) Apr 08, 2016
The idea that rocks can bump into each other and stick together is simple story telling, ESPECIALLY in the vacuum of space. Such accumulation of material has NEVER been observed. EVER.


Gravity has been proven to exist, not only between planetary bodies, but scientists back in the 1800's actually measured the mutual force of two lead cannonballs in a torsion pendulum. The force between small objects is incredibly tiny, but not neglible.

Which means "such accumulation of material" HAS been observed and proven. Had those two cannonballs been in free space, they would have bumped together. Of course they would have bounced apart instantly, but since the bodies are not perfectly rigid, each bounce loses a portion of the gravitational potential energy into heat and radiation, and the next bounce will be smaller - until the objects eventually stick together.

Eikka
5 / 5 (5) Apr 08, 2016
https://en.wikipe...periment

Actually, it was done in late 1700's

The apparatus constructed by Cavendish was a torsion balance made of a six-foot (1.8 m) wooden rod suspended from a wire, with a 2-inch (51 mm) diameter 1.61-pound (0.73 kg) lead sphere attached to each end. Two 12-inch (300 mm) 348-pound (158 kg) lead balls were located near the smaller balls, about 9 inches (230 mm) away, and held in place with a separate suspension system.[8] The experiment measured the faint gravitational attraction between the small balls and the larger ones.


A cloud of gas and dust eventually has no option but to fall together due to its own gravity. The conservation of momentum prevents it from doing so instantly, but every collision also loses momentum, which means the density of the cloud will keep increasing until it becomes a more or less solid clump of stuff.
Mark Thomas
3.3 / 5 (7) Apr 09, 2016
nkalanaga, fair enough, you feel it more likely that Planet 9 exists and I feel it more likely it does not. We both acknowledge we need more info, but the passage of time favors my viewpoint. If no planet 9 is found in a couple years, I will declare victory and go home. :-)


torbjorn_b_g_larsson
4.4 / 5 (7) Apr 09, 2016
I do hope the 2100 K ice is normal ice at the top, i.e. there could be a habitable water layer beneath the H/He atmosphere.

As for PN existence, it is now a given as far as shepherding constraints goes. They just found another shepherded KBO, making the likelihood against something like 0.001, see Mike Brown's twitter feed. (Also, a paper on Cassini observations of Saturn orbit improves predictions significantly if PN exists.)

What if Planet IX is not one large world, but a smaller world with a large moon (or two, or more)?

might be a large rocky planet formed in the inner solar system


The reason is the same, it likely was an early ejected planet since it was confined to a relatively close orbit to Sun, i.e. a lone body due to gravitational effects and a mini Neptune due to early ejection before the open cluster Sun was born in scattered much. (Gas giants forms fastest.)

Mark Thomas
2.6 / 5 (5) Apr 10, 2016
"As for PN existence, it is now a given as far as shepherding constraints goes."

Statistically-speaking, something caused this effect on the KBOs, but it has not been proven the only possible explanation is a 5-15 Earth mass planet in a 200-1,200 AU orbit as Mike Brown predicted.

"Also, a paper on Cassini observations of Saturn orbit improves predictions significantly if PN exists."

Unfortunately, the reported variances for Cassini just disappeared.
"Saturn spacecraft not affected by hypothetical Planet 9"
http://phys.org/n...net.html

That ol' gut feeling that Mike Brown's Planet 9 does not exist is getting stronger, but as nkalanaga wisely pointed out, we are likely to learn something here, regardless.
Mark Thomas
1 / 5 (3) Apr 10, 2016
nkalanaga, good article. You might enjoy: Star Trek: Enterprise, Season 1, Episode 18, "Rogue Planet."

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