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THE STEPPENWOLF: A PROPOSAL FOR A HABITABLE PLANET …

The Astrophysical Journal Letters, 735:L27 (4pp), 2011 July 10 2011. The American Astronomical Society. All rights reserved. Printed in the steppenwolf : A PROPOSAL FOR A HABITABLE PLANET IN INTERSTELLAR SPACED. S. Abbot1and E. R. Switzer2,31 Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USAR eceived 2011 February 5; accepted 2011 May 18; published 2011 June 16 ABSTRACTR ogue planets have been ejected from their planetary system.

the steppenwolf: a proposal for a habitable planet in interstellar space D. S. Abbot 1 and E. R. Switzer 2 , 3 1 Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA; abbot@uchicago.edu

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Transcription of THE STEPPENWOLF: A PROPOSAL FOR A HABITABLE PLANET …

1 The Astrophysical Journal Letters, 735:L27 (4pp), 2011 July 10 2011. The American Astronomical Society. All rights reserved. Printed in the steppenwolf : A PROPOSAL FOR A HABITABLE PLANET IN INTERSTELLAR SPACED. S. Abbot1and E. R. Switzer2,31 Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USAR eceived 2011 February 5; accepted 2011 May 18; published 2011 June 16 ABSTRACTR ogue planets have been ejected from their planetary system.

2 We investigate the possibility that a rogue planetcould maintain a liquid ocean under layers of thermally insulating water ice and frozen gas as a result of geothermalheat flux. We find that a rogue PLANET of Earth-like composition and age could maintain a subglacial liquid ocean ifit were times more massive than Earth, corresponding to 8 km of ice. Suppression of the melting point bycontaminants, a layer of frozen gas, or a larger complement of water could significantly reduce the planetary massthat is required to maintain a liquid ocean. Such a PLANET could be detected from reflected solar radiation, and itsthermal emission could be characterized in the far-IR if it were to pass withinO(1000) AU of words:astrobiology conduction convection planetary systems planets and satellites: surfaces1.

3 INTRODUCTIONAs a planetary system forms, some planets or planetesimals,referred to as rogue planets, can enter hyperbolic orbits andbe ejected from the system as a result of gravitational inter-actions with gas giant planets (Lissauer1987). Furthermore,interaction with passing stars can eject planets from maturesystems (Laughlin & Adams2000). The ability of a rogueplanet to support life is of interest as a sort of pathologicalexample of planetary habitability, because such a PLANET couldpotentially represent a viable option for interstellar panspermia(Durand-Manterola2010), and because such a PLANET could bethe closest source of extrasolar life for exploration by humanityin the distant future.

4 Since some sort of starting point is re-quired to discuss the issue, a PLANET is often defined as habitableif it can sustain liquid water at its surface (Kasting et ).Stevenson (1999) argued that if a rogue PLANET had an extremelyhigh-pressure hydrogen atmosphere, pressure broadening of far-infrared absorption by molecular hydrogen could support liquidwater on the PLANET s surface as a result of the geothermal heatflux alone, making the PLANET potentially HABITABLE . Debes &Sigurdsson (2007) showed that terrestrial planets can be ejectedwith moons and that the resulting tidal dissipation could increasethe geothermal heat flux by up to two orders of magnitude forO(108yr).

5 Subglacial liquid water oceans sustained by internal heat fluxon icy bodies represent an alternative type of habitat. It is wellknown that subglacial oceans are possible on moons aroundgiant planets and on trans-Neptunian objects in the solar system(Hussmann et ), as well as water-rich exoplanets indistant orbits (Ehrenreich et ; ). A possibleterrestrial analog is Lake Vostok, a 125 m deep lake whichis sustained by geothermal heat flux under 4 km of ice onAntarctica (Kapitsa et ). Laughlin & Adams (2000)have even argued that a terrestrial rogue PLANET , not attachedto any star and receiving negligible energy at its surface, couldsustain a subglacial liquid ocean if it had a thick enough icelayer.

6 We wish to consider this point in more depth, includingissues such as the potential for solid-state convection of ice,the potential effect of a thermally insulating frozen gas layerfrom outgassing of the mantle on an Earth-like rogue PLANET , theeffects of melting point suppression due to contaminants, andobservational prospects. By Earth-like, we mean specificallywithin an order of magnitude in mass and water complement,similar in composition of radionuclides in the mantle, and ofsimilar age. A subglacial ocean on a rogue PLANET is interestingbecause it could serve as a habitat for life which could, forexample, survive by exploiting chemical energy of rock thatis continually exposed by an active mantle.

7 We will refer toa rogue PLANET harboring a subglacial ocean as a Steppenwolfplanet, since any life in this strange habitat would exist like alone wolf wandering the galactic can imagine that the ice layer on top of an ocean on aSteppenwolf PLANET will grow until either it reaches a steady statewith the ice bottom at the melting point, or all available waterfreezes. Geothermal heat from the interior of the Steppenwolfplanet will be carried through the ice layer by conduction, andpotentially by convection in the lower, warmer, and less viscousportion of the ice layer. Since convection transports heat muchmore efficiently than conduction, the steady-state ice thicknesswill be much larger if convection occurs, making it harder tomaintain a subglacial we will calculate steady-state ice thicknesses when thereis conduction only and when there is convection in the lowerportion of the ice, and make the conservative assumption that thethicker solution is valid.

8 We must acknowledge, however, thatit is very difficult to establish definitively whether convectionwould occur, and the resulting ice thickness if it were to occur,without detailed knowledge of conditions in and microscalecomposition of the ice (Barr & Showman2009). More generally,we will make many simplifications, including considering thequestion within the framework of a one-dimensional (vertical)model, since our primary objective is to establish whether or nota steppenwolf PLANET is GEOPHYSICAL CONSIDERATIONSF irst we calculate the conductive steady-state thickness,Hcond. Above 10 K, the temperature dependence of the thermalconductivity of water ice is well approximated byk(T)=AT 1,whereTis the temperature in Kelvin andA=651 W m 1(Petrenko & Whitworth2002).

9 Dimensional analysis shows thatthermal steady state is reached in 106years, much shorter thanthe timescale of decay of the geothermal heat flux. Geothermal1 The Astrophysical Journal Letters, 735:L27 (4pp), 2011 July 10 Abbot & Switzerheat flux through the shell will be constant at steady state, sinceno heat is produced within the ice, as would occur by tidalheating of a frozen moon. Since the steppenwolf planets weconsider would be much larger and drier than the icy moons onwhich subglacial oceans are typically studied, we can assumethat the ice thickness is much less than the planetary radius,yielding an exponential temperature profile through the ice andsteady-state thickness, so thatHcond=AFlog(THT0),(1)whereTHis the temperature at the ice water interface (themelting temperature)

10 ,T0is the temperature at the top of theice, andFis the geothermal heat of radioactive elements in Earth s interior and primor-dial heat remaining from Earth s formation lead to an averagegeothermal heat flux emanating from Earth s surface ofF = W m 2(Pollack et ). This heat flux decays withtime such that Earth s geothermal heat flux may have beenroughly twice its present value 3 Gyr ago (Turcotte1980). In or-der to consider steppenwolf planets of different sizes, we use theradius mass scalingR Mvfor super-Earths withv= (Valencia et ). Heuristically, this yields a geothermalheat flux that scales as (M/M )1 2v, or roughly as the squareroot of the pressure at the bottom of the ice layer is 9 MPa for eachkilometer of ice, scaling with mass as (M/M )1 2v.


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