Finding an Interstellar Wanderer

by Paul Gilster on May 17, 2011

Imagine a planet far more massive than Jupiter and spinning faster than Jupiter’s 10 hour rotation. Throw in a large nearby moon and the associated auroral effects that would occur as the moon moved through fields of plasma trapped in the planet’s magnetic field. The scenario isn’t all that different from what we see happening between Jupiter and Io. But here’s the kicker: Put planet and moon far away from any star, a rogue planet scenario of the kind recently discussed by Dorian Abbot and Eric Switzer, who called such rogue planets ‘Steppenwolfs.’ I jumped on that idea in a Centauri Dreams post last February because interstellar planets have always fascinated me.

Abbot and Switzer were interested in whether a rogue planet could support life, finding in their paper that a planet just 3.5 times as massive as the Earth, and with the same basic composition and age, could sustain a liquid ocean under layers of insulating water ice and frozen atmosphere. But our rogue gas giant offers something the smaller world cannot. The auroral effects created by planet and moon could be detected out to a distance of 185 light years, provided the planet were big enough and we had the appropriate equipment, which in this case means the Square Kilometer Array radio telescope. So says Heikki Vanhamaki of the Finnish Meteorological Institute in Helsinki.

Image: Aurora on Jupiter, as detected by the Hubble Space Telescope. Can we use the emissions of such events to detect a planet wandering between the stars? Credit: NASA and the Hubble Heritage Team (STScI/AURA) Acknowledgment: NASA/ESA, John Clarke (University of Michigan).

You may recall that we also looked recently at Jonathan Nichols’ work at the University of Leicester on how an exoplanet aurora could be used as a detection tool. Vanhamaki’s calculations show that such an aurora could be produced by a rogue planet in two different ways. Movement through interstellar plasma could generate an aurora, but the scenario above, in which a moon moves through magnetically trapped plasma, produces a signature 100 times stronger. The scientist calls detection of a rogue Jupiter extremely unlikely — “perhaps nearly impossible” — in the forseeable future, but he adds that there may be 2800 interstellar planets within 185 light years, the distance within which he calculates a Square Kilometer Array might be effective.

One or more of them will have to be large enough, at least eight Jupiter masses, and orbited by a moon inducing auroral activity, for any of this to work. It’s the slimmest of chances, but earlier studies of known ‘hot Jupiters’ have predicted that some of them should produce cyclotron radiation of between 1014 and 1016 W, orders of magnitude higher than the values Vanhamaki discusses for typical interstellar planets, although these emissions have not yet been detected. Nonetheless, exoplanet aurorae seem like a promising area of research, and the suspicion that biological processes could go to work on a rogue planet makes the hunt more enticing. Says Vanhamaki:

“It has been speculated that Earth-like rogue planets could have very thick atmosphere that keeps them relatively warm, or moons of giant rogue planets could experience tidal heating and have oceans beneath their icy surface.”

It’s a scenario that once again turns our notion of the habitable zone on its head. We’ve gone from describing a habitable zone as a region around a star where liquid water could be maintained at the surface to seeing the possibilities of life under frozen oceans, a zone that could extend all the way to the Kuiper Belt. A rogue planet would be the ultimate habitable zone extension. If it has no reflected starlight by which to spot it, will the auroral method come into its own with a breakthrough detection?

Recall, too, that Christopher McKay (NASA Ames) has looked at a variety of scenarios involving Titan-like worlds, placing them around several red dwarf stars to see where they could host oceans of liquid methane. One idea his team looked at was a rogue Titan in interstellar space. The conclusion: A Titan with 20 times the geothermal heat of Earth could keep its current surface temperature in the absence of any star. Alternatively, an atmosphere 20 times thicker than Titan’s could retain enough heat to make a surface ocean viable. These are tall orders, but a larger Titan with a thicker atmosphere might sustain a liquid methane sea.

The Vanhamaki paper is “Emission of cyclotron radiation by interstellar planets,” Planetary and Space Science, published online 17 April 2011 (abstract). Christopher McKay’s paper is “Titan under a red dwarf star and as a rogue planet: requirements for liquid methane,” Planetary and Space Science, published online 2 April 2011 (abstract). Astrobiology Magazine covers the story here.

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Tulse May 17, 2011 at 9:59

Do models of solar system formation offer any predictions of how common interstellar planets would be? And is it assumed that all interstellar planets would have been ejected from a forming solar system, or are there mechanisms for sub-stellar-mass gas clouds to form “planets” independently?

djlactin May 17, 2011 at 13:41
Paul Gilster May 17, 2011 at 14:00

Interesting, djlactin! A sub-brown dwarf or a rogue planet. Had never heard of this until you sent the link. Thanks!

James M Essig May 17, 2011 at 15:14

Hi Folks;

The discovery of rogue planets would be awesome indeed. We would need to send probes at first and then follow-up with manned missions. A rogue planet within 4 light years of Earth would be an awesome first step.

Sending submarines perhaps nuclear powered to ply the depths of any aquatic environments on such planets would be an absolutely whimsical blast in my opinion.

Any ETI on such planets might have eyes that operate in the infrared range typical of temperate black body spectrums here on Earth.

I hear that NASA has three finalist projects for another round in an ongoing competition to propose robotic missions. One of these projects would involve our first extraterrestrial ocean exploration. The ocean would be the vast liquid hydrocarbon oceans on Titan.

All of Paul’s threads on planetary geology and hydrology as of late have got my thinking that I should enroll in some planetary science courses. I took Atmospheric Physics I and II as an undergrad at George Mason University here in Fairfax Virginia about 1 1/2 decades ago. My wanderlust for exo-planets and the exotic lifeforms they may harbor has been restoked by all the the latest news comming out of NASA regarding planned robotic missions within our solar system. My hope and faith is that sometime in the lifetimes of many of us who comment here at TZ-CD, plans will start for real missions targeted at on site exploration of other solar systems.

I can imagine the cover of National Geographic being adorned with beautiful photographic images of any lifeforms on Titan, Europa, and other moons in our solar system within the next 30 years. If such lifeforms are present here in our solar system, they are likely ubiquitous throughout our universe.

FrankH May 17, 2011 at 15:15

Some problems with this or any theory that puts forth a habitable moon around a gas giant. There seems to be a maximum size for an icy moon and at least one paper proposed a maximum size that is less than Mars. Mars is not habitable now and would lose most of what little atmosphere it has if it warmed up to Earth temperatures.
Also, icy moons are just that – ices – which implies a low density, volatile compounds and low surface gravities. If you bring them up to 273K they will lose whatever atmosphere they have as well as some mass. So you’ll end up with a much smaller, airless rocky moon.

The only way these habitable moons would work is if the gas giant captures a rocky, Earth sized planet (or a super Earth) on it’s migration in towards it’s sun (or expulsion out of its planetary system).

Kenneth Harmon May 17, 2011 at 19:52

Question for All,

Since it now seems possible for individual Rogue Planets, and Rogue Stars to exist is there any evidence indicating that entire Rogue Star Systems may exist and be wandering between Galaxies? Rather then a Single Star or perhaps a single planet ejected into inter-Galactic Space is there any evidence that there could be entire Star Systems such as something like the Alpha Centauri Triple Star System floating between Galaxies or is this impossible given what we current know about Star System formation and dynamics?

Colin Weaver May 18, 2011 at 2:34
Ashley May 18, 2011 at 4:17

@ Kenneth Harmon

I don’t think I have ever read anything about rogue star systems or rogue planetary systems, but that’s an interesting question. Considering the mechanisms we think to be responsible for the existence of rogue stars/planets, it seems to me very unlikely that any rogue system could exist.

However, there is this paper, quite interesting, discussing the possibility of a rogue planet with a moon, both ejected from their parent system: http://iopscience.iop.org/1538-4357/668/2/L167/

For sure, that seems an interesting and exciting possibility to explore.

Tobias Holbrook May 18, 2011 at 6:23

Current models for the ejection of stars from the galaxy involve a three body interaction, in which one of the bodies (the lighter one) is ejected. However, I don’t know whether those “bodies” could be fully fledged gravitationally bound systems, though – say, a sextuple star system encountering a binary neutron star system.

I take it rogue Jovians *can* support such plasma loops? Perhaps a body half the size of Mars could gain a thin atmosphere of Oxygen as a result, as it’s irradiated surface ice breaks down and the hydrogen escapes? It would be interesting to see if there are any worlds that are biocompatible for Terran life as a result…

I also note that this [plasma rings around rogue Jovians and Brown Dwarfs] was an important background element in Permanence…

Simfish InquilineKea May 18, 2011 at 22:56

Does anyone know if any work has been done on 3D modelling of the atmospheres of rogue planets? (perhaps a result that could eventually be empirically verifiable?) This is something I’d like to do for future research (although I’ll first start off with doing 3D models of the Earth’s atmosphere around various main sequence stars)

Bassey E. May 19, 2011 at 8:22

Just to dip a finger into the impact of this story on the general public, the NYT published an article based on this today. (Disclaimer: I work at NYT) The most recommended comments: http://community.nytimes.com/comments/www.nytimes.com/2011/05/19/science/space/19planets.html?sort=recommended

The blog post above has more delicious beef than the story, for obvious reasons, but I do enjoy the illustration they dug up: http://www.nytimes.com/imagepages/2011/06/10/science/10planets1.html

Bounty May 19, 2011 at 11:43

If we are talking about Mars sized moons around rogue Jupiters, what is the proposed mechanism for the atmosphere to vanish? It shouldn’t be be solar wind or comets if it’s rogue? There would be interstellar wind, but I don’t know how well a rogue Jupiter would deflect it. Also, I wonder now if that would cause “auroral effects.”

Rob Henry May 20, 2011 at 19:25

I note that the above talk, in response to Kenneth Harmon’s question, of whether stars ejected from galaxies can keep their planets seems to be strangely avoiding a question that, to my mind seems central. While it may be possible to model a galaxy that is so very tightly gravitationally bound that stars must be ejected by such extreme encounters as to strip them of all their planets, it is not possible to produce such a model where two equally mass galaxies merge. In such mergers, before the first stars are ejected, the average binding energy can be only slightly more than the average kinetic energy of each star, opening a window for ejection of many star systems by a series of gentle slingshots.

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