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June 01, 2013
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When can a moon harbor life? Scientists get down to nitty-gritty
Jan. 10, 2013
Courtesy of Leibniz-Institut für Astrophysik
Potsdam, University of Oxford
and World
Science staff
With the notion of life on moons
of distant solar systems drawing growing interest from astronomers, some are starting to research exactly what might make such a
moon habitable. Artist’s conception of
Earth-like moons orbiting a giant gaseous planet. (Credit: R.
Heller, AIP)
The answer turns out to be more complex than you might expect.
Just as with planets, moons have their own “habitable zones,” areas in which they can reside without losing the chance to have liquid water or life, said Rory Barnes of the University of Washington and the NASA Astrobiology Institute.
But for moons the habitable zone is “a little different” and in some ways more restricted,
added Barnes, co-author of a new study on the subject. For instance, a moon’s habitable zone depends a good deal on the distance to its host planet, a factor unlikely to greatly affect the planet’s own livability.
Astronomers are intrigued by the idea of life on moons orbiting planets outside our solar system, called “exomoons.” Several factors make the possibility enticing. One is that a single planet can host several moons, any of which with luck might be habitable—even if the planet itself is not, which is considered a common scenario considering big planets are usually made of gas.
“Jupiter has several large, water-rich moons. Imagine dragging that
system into the comfortably warm region where the Earth is. If such a
planet had Earth-size moons, we'd see... worlds with rivers, lakes and all sorts of
habitats—a surprising scenario that might just be common,” said
astronomer Chris Lintott of Oxford University. He is a collaborator
in a project in which citizen volunteers can help find planets
using a website, Planethunters.org.
There are about 20 known planets in planetary “habitable zones,”
and of these worlds might have moons, Lintott added. Just this week, Planet Hunters
volunteers added one to that list, a confirmed planet dubbed PH2 b,
he said. The finding has been submitted to The Astrophysical
Journal and posted online.
A problem with finding life on moons is that due to their relatively small size,
they’re hard to detect. And they generally can’t retain an atmosphere if they’re much smaller than Earth, said Barnes and his co-author, René Heller of Germany’s Leibniz Institute for Astrophysics Potsdam. Our solar system’s largest moon, Jupiter’s Ganymede, weighs a puny one-fortieth of Earth, raising the question of whether big enough moons even exist—but nothing rules out the possibility, Heller and Barnes said.
The fine precision of NASA’s Kepler space telescope now makes the detection of a Mars-to-Earth sized moon possible, indeed imminent, the two researchers added. Their analysis on exomoon habitablity appears in the January issue of the journal
Astrobiology and is also online here.
The climate on exomoons will likely differ from those on the planets they are orbiting, or “exoplanets,”
they wrote. For one thing, moons are typically tidally locked to their host planet. This means that, as with our moon,
the same side of the moon always faces the planet.
Moons would also have two sources of light: the star and the planet. But the planet would sometimes block out their “sun.” This could create drastic and climate-altering eclipses, especially considering that the blocking object might be an immense planet. “An observer standing on the surface of such an exomoon would experience day and night in a totally different way than we do on Earth,” explained Heller. “Stellar eclipses could lead to sudden total darkness at noon.”
Another disruptive influence for a moon is tidal heating, Heller and Barnes said. Our moon creates relatively mild tidal effects on Earth, but if there were a much heavier and closer body in our sky, that could be a different story: the tides might be strong enough to literally bend the ground beneath our feet, creating friction that heats our whole world. Moons that orbit their host planet too closely will undergo strong tidal heating that would boil away surface water and render them uninhabitable, Barnes and Heller explained.
That’s why a moon can’t be more than so close to its host planet without becoming unliveable, they added. That minimum distance separating the two bodies, they have dubbed the “habitable edge”—determined by a complicated formula that depends on factors including the planet’s mass, the moon’s reflectivity and the circularity of its orbit. This concept will allow future astronomers to evaluate the habitability of
distant moons, the researchers propose.
In addition to these conditions, moons face another influence on habitability that also affects their host planets: the distance to the host star of both bodies, and the properties of that star. These factors bear on whether the moon-planet system has warmth suitable to support
liquid water.
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With the notion of life on far-off moons drawing increasing interest from astronomers, some are starting to research exactly what might make such a world habitable. The answer turns out to be more complex than you might expect. Just as with planets, moons have their own “habitable zones,” areas in which they can reside without losing the chance to have liquid water or life, said Rory Barnes of the University of Washington and the NASA Astrobiology Institute, co-author of a new study on the subject. But for moons the habitable zone is “a little different” and in some ways more restricted, he added. For instance, a moon’s habitable zone depends a good deal on the distance to its host planet, a factor unlikely to greatly affect the planet’s own livability. Astronomers are intrigued by the idea of life on moons orbiting planets outside our solar system, called “exomoons.” Several factors make the possibility enticing. One is that a single planet can host several moons, any of which with luck might be habitable—even if the planet itself is not, which is considered a common scenario considering big planets are usually made of gas. But a problem is that exomoons, due to their relatively small size, are hard to detect. And they generally can’t retain an atmosphere if they’re much smaller than Earth, said Barnes and his co-author, René Heller of Germany’s Leibniz Institute for Astrophysics Potsdam. Our solar system’s largest moon, Jupiter’s Ganymede, weighs a puny one-fortieth of Earth, raising the question of whether big enough moons even exist—but nothing rules out the possibility, Heller and Barnes said. The fine precision of NASA’s Kepler space telescope now makes the detection of a Mars-to-Earth sized moon possible, indeed imminent, the two researchers added. Their analysis on exomoon habitablity appears in the January issue of the journal Astrobiology and is also published online here. The climate on exomoons will likely differ from those on the planets they are orbiting, or “exoplanets,” moons are typically tidally locked to their host planet, they wrote. This means that, as with our moon, one side of the moon permanently faces the planet. Moons would also have two sources of light—that from the star and the planet they orbit. But the planet would sometimes block out their “sun.” This could create drastic and climate-altering eclipses, especially considering that the blocking object might be an immense planet. “An observer standing on the surface of such an exomoon would experience day and night in a totally different way than we do on Earth,” explained Heller. “Stellar eclipses could lead to sudden total darkness at noon.” Another disruptive influence for a moon is tidal heating, Heller and Barnes said. Our moon creates relatively mild tidal effects on Earth, but if there were a much heavier and closer body in our sky, that could be a different story: the tides might be strong enough to literally bend the ground beneath our feet, creating friction that heats our whole world. Moons that orbit their host planet too closely will undergo strong tidal heating that would boil away surface water and render them uninhabitable, Barnes and Heller explained. That’s why a moon can’t be more than so close to its host planet without becoming unliveable, they added. That minimum distance separating the two bodies, they have dubbed the “habitable edge”—determined by a complicated formula that depends on factors including the planet’s mass, the moon’s reflectivity and the circularity of its orbit. This concept will allow future astronomers to evaluate the habitability of extrasolar moons, the researchers propose. In addition to these conditions, moons face another influence on habitability that also affects their host planets: the distance to the host star of both bodies, and the properties of that star. These factors bear on whether the moon-planet system has warmth suitable to support liqud water. About 850 planets outside the solar system are known. Most are believed to be sterile gas giants, similar to Jupiter. Only a handful are thought to have a solid surface and orbit their host stars in the habitable zone.