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Earthlike Planets: The Visibility of Youth

Directly imaging a terrestrial planet is going to be a tough challenge. Suppose you were thirty light years from the Sun, looking back at our star in the hope of seeing the Earth. You would face the problem that the Earth and its star show an angular separation of 100 milliarcseconds, a spacing so tiny that the far brighter Sun would render its third planet (and all the others) invisible. Indeed, in optical wavelengths the Earth is ten billion times less bright than the Sun. How to go about seeing it?

Observing at other wavelengths offers some help. The Sun is only a million times brighter than the Earth in the mid-infrared, which is why our first glimpse of planets like ours will probably be in this range.

And it may be that our first catch is not a mature, established planet potentially offering a habitat to living organisms. Instead, it may be a clump of molten rock still glowing brightly from the heat of formation. Even after surface magma solidifies — and new work suggests this could take five million years, rather than the hundreds of thousands previously thought — the planet might stay hot enough to be an unusually bright target in the infrared for tens of millions more.

This is the conclusion of Linda Elkins-Tanton (MIT), whose work implies that a glowing, molten planetary surface may be the most feasible find for early terrestrial planet hunter missions. As to the processes producing all that magma, they’re initially the result of radioactivity in the planet’s interior and the heat of planetary formation created by millions of rocky collisions in the early system. But a second process, causing iron-rich materials to sink toward the core, may force hotter materials from within back up to the surface, keeping the landscape molten much longer.

So we may have a ‘magma ocean’ that lasts at least a few million years longer than had previously been thought. It’s an interesting model, and one that clearly has implications for detectability since it lengthens the window for observation. Surprisingly, the theory may gain support when the MESSENGER mission settles in around Mercury. Earth’s crust is too dynamic for material from such early epochs to survive, but Mercury’s surface may offer up minerals that Elkins-Tanton’s model says should be there. Even better, of course, would be the direct detection of a molten, young Earth analog, but for now Mercury may have to do.

Comments on this entry are closed.

  • andy October 17, 2008, 13:13

    Makes sense – after all the candidates for imaged extrasolar giant planets are also around young stars. I wonder how many terrestrial-type planets we will find at several hundreds of AU from their parent stars (perhaps ejected from orbit by interactions among the growing planets).

  • James M. Essig October 18, 2008, 20:30

    Hi Folks;

    As I was reading this article, the mental image of the planet with perpetual molten lava rivers in the last movie of the Star Wars series where the character of Darth Vader is developed suddenly jumped into my mind.

    I can imagine the variety of beautiful worlds whose surfaces are simply superheated lava with a whole variety of shades from deep red through brilliant white depending on the chemical composition of the lava and its temperatures.

    A study of how radioactivity heats planets and maintains molten cores has implications for planetary habitats for human and ETI civilizations long after the stars burn out.

    Some meta stable radioactive isotopes have half lives of at least 10 EXP 20 years. These isotopes could in theory keep the enteriors of planets warm for trillions if not quadrillions of years provided that they were included within or instilled within the cores of such planets in high enough concentrations.

    The idea here is to enable sub-surface planetary habitats to exist for periods long after the stellar period of our universe has ended and where adequate energy and warmth can be provided for sub-surface dailly living and infrastrusture.

    If our universe is going to have a heat death wherein the mattergy density of our universe becomes ever more reduced with time, a study of how radioactive heat warms the planetary cores and lower portions of solid crust may be very important considerations for comfortable living at these distant future eras. Thus, the production and/or collection of interstellar very long half lived radionuclides and methods to instill them within terrestrial-like planetary bodies might not only be necessary for super long lived human and ETI civilizations, but may provide a comfortable and palitable way to provide habitats for humans and ETIs long after the last stars have burned out.



  • ljk October 27, 2008, 0:00

    Direct Imaging and Spectroscopy of a Planetary Mass Candidate Companion to a Young Solar Analog

    Authors: David Lafrenière, Ray Jayawardhana, Marten H. van Kerkwijk (University of Toronto)

    (Submitted on 8 Sep 2008 (v1), last revised 24 Oct 2008 (this version, v2))

    Abstract: We present Gemini near-infrared adaptive optics imaging and spectroscopy of a planetary mass candidate companion to 1RXS J160929.1-210524, a roughly solar-mass member of the 5 Myr-old Upper Scorpius association. The object, separated by 2.22″ or 330 AU at ~150 pc, has infrared colors and spectra suggesting a temperature of 1800(-100/+200) K, and spectral type of L4(-2/+1).

    The H- and K-band spectra provide clear evidence of low surface gravity, and thus youth. Based on the widely used DUSTY models, we infer a mass of 8(-2/+4)Mjupiter. If gravitationally bound, this would be the lowest mass companion imaged around a normal star thus far, and its existence at such a large separation would pose a serious challenge to theories of star and planet formation.

    Comments: Revised accepted version, ApJL, in press

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0809.1424v2 [astro-ph]

    Submission history

    From: David Lafreni\`ere [view email]

    [v1] Mon, 8 Sep 2008 20:00:05 GMT (218kb)

    [v2] Fri, 24 Oct 2008 19:31:52 GMT (294kb)


  • ljk October 28, 2008, 23:12

    The mass-radius relationship from solar-type stars to terrestrial planets: a review

    Authors: G. Chabrier, I. Baraffe, J. Leconte (ENS-Lyon), J. Gallardo (Universidad de Chile) T. barman (Lowell Obs.)

    (Submitted on 28 Oct 2008)

    Abstract: In this review, we summarize our present knowledge of the behaviour of the mass-radius relationship from solar-type stars down to terrestrial planets, across the regime of substellar objects, brown dwarfs and giant planets.

    Particular attention is paid to the identification of the main physical properties or mechanisms responsible for this behaviour. Indeed, understanding the mechanical structure of an object provides valuable information about its internal structure, composition and heat content as well as its formation history.

    Although the general description of these properties is reasonably well mastered, disagreement between theory and observation in certain cases points to some missing physics in our present modelling of at least some of these objects.

    The mass-radius relationship in the overlaping domain between giant planets and low-mass brown dwarfs is shown to represent a powerful diagnostic to distinguish between these two different populations and shows once again that the present IAU distinction between these two populations at a given mass has no valid foundation.

    Comments: Cool Stars, Stellar Systems and the Sun 15, invited review

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0810.5085v1 [astro-ph]

    Submission history

    From: Gilles Chabrier [view email]

    [v1] Tue, 28 Oct 2008 17:01:40 GMT (139kb)


  • ljk January 19, 2009, 12:51

    Smallest known exoplanet may actually be Earth-mass

    00:00 19 January 2009 by Stephen Battersby

    The smallest planet around a normal star other than the Sun may be even smaller than first thought. A new analysis suggests the rocky body weighs just 1.4 Earths – less than half the original estimate.

    Observations over the next few months should test the prediction.

    Full article here: