Caltech Astronomers Say a Ninth Planet Lurks Beyond Pluto (sciencemag.org)

sciencehabit writes: The solar system may have a new ninth planet. Today, two scientists announced evidence that a body nearly the size of Neptune — but as yet unseen — orbits the sun every 15,000 years. During the solar system's infancy 4.5 billion years ago, they say, the giant planet was knocked out of the planet-forming region near the sun. Slowed down by gas, the planet settled into a distant elliptical orbit, where it still lurks today. Here's a link to the full academic paper published in The Astronomical Journal.

Comments

Re:Well, I guess we'll know in a few thousand year

[xxxJonBoyxxx]

Again, from TFA, we could perform a narrow infrared scan of the possible path until we find it. (We just did one ruling out "Saturn-sized" objects nearby, but this planet is smaller.) The authors expect discovery and confirmation within about 5 years.

Re: Does it count as "evidence"

[Anonymous Coward]

Perturbations of other (dwarf) planets and KBOs is exactly what they are basing this claim off of.

That data strongly supports a massive body well outside Neptune's orbit, but until now no one could say if it was a large planet farther out, or an Earth-sized planet closer in (relatively). Some people even suggested it could be a brown dwarf binary very far out.

These guys have now used the preexisting data to predict that it's a smaller planet "close" in based on computer modeling.

Re:Ninth, mofo.

[Rei]

Assuming it exists, it orbits the sun, it is large enough to be round, and it's big enough it probably has "cleared its neighborhood"

It has as mentioned a semi-major axis of around 700AU. That's 23 times more than Neptune. It has a mass of about 10Me, or 58% of Neptune. Its Margot discriminant would be less than a tenth of Mars' (lowest in the solar system). Plus, it's highly elliptical (e=0.6), meaning it has a far broader neighborhood to clear (something not taken into account in the discriminant).

Hydrostatic equilibrium is part of the definition.

  Planetary scientists wanted a definition based solely around hydrostatic equilibrium; the main group pushing for an orbital dynamics definition was astrophysicists. The original draft was based around hydrostatic equilibrium, so many of them left, content that they'd either get a hydrostatic equilibrium definition or no definition at all.

Some thoughts

[Rei]

An interesting thought. Even at its perihelion (1100 AU), helium won't be getting cold enough to condense out. But hydrogen probably will, condensing to planetwide hydrogen seas. Meaning that - combined with its lower mass - its atmospheric density at perihelion on top of that is probably surprisingly low. However, at aphelion its only about 400AU. That's probably not cold enough to condense hydrogen. So every 15000 years it would go from having hydrogen oceans and low atmospheric pressure to an ice surface under crazy pressures.

What the heck do you call a planet like that?

Such a large planet would certainly have the internal heat for tectonics and volcanism. But I'm still so baffled from trying to picture what such a planet would be like just from that first aspect that I can't even begin to imagine what effect the latter would have on it.

Certainly a lot of energy in play here.

Re:Some thoughts

[Rei]

More thoughts.

1) The atmosphere would be pure - 100% pure helium. The only thing that could contaminate it would be hydrogen, so if it's cold enough for it to be fully condensed out (no hydrogen clouds/rain), then it'll be a monoatomic gas. No clouds.

2) The hydrogen seas would also be pure. There's almost nothing that can float in hydrogen - pretty much just foams gassed with helium, and that doesn't sound likely.

3) Weird nuclear properties: helium is a perfect neutron moderator - it never undergoes neutron capture. It can undergo high energy reactions, but at lower energies, any neutron in helium will become fully thermalized, which - at those temperatures - would make everything interact with it at a very high cross section. Since only helium would be in the atmosphere, that would most likely be 3He. So I would expect 3He depletion.

4) The day length would change when the hydrogen condensed out (like a ballerina pulling her arms in). I'm not sure off the top of my head of the effects of this mass redistribution on any orbital bodies, although I could picture, say, enhanced tidal heating due to the mass redistribution.

5) There's an awful lot of potential non-hydrogen liquids which could exist under the liquid hydrogen (or under the H2/He atmosphere near aphelion) - nitrogen, carbon monoxide, methane and other hydrocarbons, neon, even water. It all depends on the pressure and temperature curves, which one couldn't even begin to speculate on at this point. Most of the latter could potentially form eutectics, but hydrogen is not prone to forming eutectics, so would make its own distinct surface layer.

6) Lava flows of any type (silicate, cryolavas, whatever) would happen underneath the hydrogen ocean. Meaning pillowing. The boiloff of hydrogen could then expose these structures. What do pillow cryolavas formed under hydrogen look like? I haven't the foggiest.

7) Any hot lavas (such as silicates) erupting into liquid hydrogen might have unusual chemistry (metal hydrides and the like? extensive hydrocarbon formation? silanes, stabilized by the low temperatures?). This would then be left exposed on the surface when the hydrogen boils off. That surface could be a really bizarre place.

Any other thoughts?

Re:Ninth, mofo.

[Rei]

The whole "cleared the neighborhood" thing is based on a lie anyway. The vast majority of planets didn't clear their neighborhods. Jupiter**, and to a lesser extent Saturn, did. Mars' lack of influence on its neighborhood can be seen by how low of a percentage of asteroids are in a resonance with it.

Can we stop with the pretending that planets like Mars are responsible for sweeping their orbits clean? No models support this.

It's funny, but you see almost the exact same reason given by everyone interviewed who voted for the IAU definition - always a variant of "I don't want my daughter to have to memorize the names of 50 planets". As if that's even remotely any sort of scientific argument, as if we should say there's only 8 rivers in the world or 8 bones in the human body and all others are "dwarf rivers" and "dwarf bones" that aren't really rivers and bones, in order to make it easier for schoolkids.

They had their preconceived concept - they wanted a low, memorizeable number of planets - and tried to create a definition to fit it. And failed miserably at it. Now we've got a definition where a "Dwarf X" is not an X, despite the fact that in astronomy (and almost everywhere else) "Dwarf X" always denotes a type of X - dwarf stars, dwarf galaxies, etc. We've got a definition based on poorly defined concepts like "neighborhood". We've got a definition that arbitrarily excludes exoplanets from being planets, which is a terminology disaster. We have a definition that runs contrary to what people associate with the word "planet" - they expect "big round object floating through space around a star" - if it's pulled itself into a sphere, they think "planet", if it's lumpy then they think "not a planet".

We had a perfectly good dividing line: hydrostatic equilibrium. It's not just what the public expects the word to mean. Collapse into hydrostatic equilibrium produces altered minerals, releases of energy, fluids, and all sorts of things - they're the place you'd go to study planetary evolution, search for life, etc. Bodies that have not collapsed into hydrostatic equilibrium are where you'd go to study primordial materials, the origins of the solar system, etc. They're fundamentally different bodies.

And for that matter, what sort of nonsensical grouping is it that says that Mercury is more like Jupiter than it is Ceres? Want to pinch off some bodies from the list of planets? Go all the way. We have the inner planets, we have the gas giants, we have the ice giants.... IMHO I really like Stern's multi-classification approach. You have an adjective which describes the size and whether it's in hydrostatic equlibrium - say, superdwarf, dwarf, giant, supergiant, etc; you have a compositional term, such as terrestrial, gas/hydrogen, ice, etc - and you have an orbital term, such as "planet" (body that orbits around a star), "moon" (body that orbits around a planet"), and so forth. When describing a body, you can use as many or as few of the components as you need to.

(** Hell, if I really wanted to nitpick, I could point out that the definition requires planets orbit the sun. Jupiter orbits the Sun-Jupiter barycentre, which is not inside the sun. You can say "close enough", but where do you draw the cutoff line?)

Re:Ninth, mofo.

[pugugly]

Yeah that was the first thing that went through my mind - Under current planetary evolution theories, despite it's size it has almost certainly not cleared it's neighborhood.

Which is of course the *exact* problem many people had with the definition, and I really hope this turns out to be real and they have to deal with a 'Dwarf Planet' ten times Earth Mass.