Steppenwolf: Rogue Planet Teeming With Alien Life And Guess Where Astronomers Say It's Headed

Dorian S. Abbot  (Department of the Geophysical Sciences) and E. R. Switzer (Kavli Institute for Cosmological Physics and Department of Astronomy and Astrophysics) say recent developments in exoplanet characterization have raised the possibility that in the relatively near future, we may be able to search for biosignatures in the atmospheres of terrestrial-mass planets

Abbot, assistant professor of geophysical sciences, published a paper (pdf) in Astrophysical Journal Letters in July 2011 with then postdoctoral researcher Eric Switzer, AB’03, about the possibility of “Steppenwolf planets”—planets hurtling through interstellar space “like a lone wolf wandering the galactic steppe,” as Abbot and Switzer put it—that might support life under an insulating ocean of ice.

Credit: NASA

Another way a planet can go rogue is by interaction with passing stars whichy can eject planets from mature systems.  The ability of a rogue planet to support life is of interest as a sort of pathological example of planetary habitability, because such a planet could potentially represent a viable option for interstellar panspermia and because such a planet could be the closest source of extrasolar life for exploration by humanity in the distant future.

A Steppenwolf planet with mean ocean depth greater than the steady-state ice thickness, accounting for the ice-water density difference, will have an ocean under its ice layer.

Writing in the prestigious Astro  journal Icarus if a planet with equilibrium temperature lower than that of Earth has multiple bars of hydrogen in its atmosphere early in its evolution (either due to direct capture from the nebula or outgassing after accretion) but subsequently loses it due to escape, it will almost certainly pass through a transient state where liquid water can form on its surface. It has been shown that the duration of habitable conditions can range from thousands to hundreds of millions of years, although planets left with exactly the right amount of hydrogen for permanent habitable conditions only occur in rare cases.

Credit: NASA

Assistant professor of geophysical sciences Fred Ciesla is building computer models of protoplanetary disks, the loose agglomeration of material that surrounds a star in its earliest days, to determine what physical properties cause planetary systems to form.

A Steppenwolf planet’s lifetime will be limited by the decay of the geothermal heat flux, which is determined by the halflife of its stock of radioisotopes (40K, 238U, 232Th) and by the decay of its heat of formation. As these decay times are ∼1–5 billion years, its lifetime is comparable to planets in the traditional habitable zone of main-sequence stars Earth

If a Steppenwolf planet harbors life, it could have originated in a more benign era before ejection from the host star. Alternatively, after ejection, life could originate around hydrothermal vents, which are a proposed location for the origin of life on Earth.  If life can originate and survive on a Steppenwolf planet, it must be truly ubiquitous in the universe.

Finally if you guessed it’s headed our way,  you aren’t right but you aren’t wrong.  It is an unknown thing.  NASA has found there are billions of rogue planets roaming the Milky Way, but so far none have been detected in our neighborhood. 

So astronomers say at this point it is still a theoretical world, but such a planet could be detected from reflected solar radiation, and its thermal emission could be characterized in the far-IR if it were to pass within 1000 astronomical units (AU) or 93 billion miles of Earth.

The University of Chicago’s new focus on exoplanets stems in part from its commitment to “big glass,” says astronomy and astrophysics chair Rocky Kolb, referring to the University’s commitment to the 6.5-meter Magellan Telescopes in Chile and their successor, the 24.5-meter Giant Magellan Telescope. Once complete in 2019, the telescope should be powerful enough to detect Earth-sized planets in the habitable zones around sun-like stars. Future telescopes and satellite missions should be able to discern not just where the Earth-like planets are but also the kind of climate they have, the composition of their atmospheres, and perhaps even the gaseous signatures of life. Detecting life on other worlds would be a major advance for biochemistry and evolutionary biology, among other fields, and, as Fabrycky says, “that ‘gee-whiz’ factor can’t be underestimated.”