IN A distant planetary system, a dozen giant moons buzz around a planet five times the diameter of Jupiter. Several of the moons are monsters as large as Earth, their surfaces continually rocked by violent earthquakes and volcanic eruptions that would put Krakatoa to shame. Perhaps they seem a vision of hell. Yet a growing number of researchers believe such moons could be cosmic oases. 鈥淭hey might be the most likely places to find life in the galaxy,鈥� says Caleb Scharf, an astrobiologist at Columbia University in New York.
鈥淓xomoons might be the most likely places to find life in the galaxy鈥�
Although no one has ever seen a moon orbiting an extrasolar planet, observing the satellites around Jupiter and Saturn has led some astronomers to think that 鈥渆xomoons鈥� capable of supporting life are almost certainly out there. What鈥檚 more, Scharf believes that life could thrive much farther away from stars than planetary scientists ever thought.
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This is great news for astrobiologists hoping to find hospitable worlds outside the solar system. Life is abundant on Earth because our planet lies at just the right distance from the sun for liquid water. This so-called 鈥渃lassical habitable鈥� zone is disappointingly narrow. The heat from the sun would boil any water on the surface of a planet orbiting slightly closer than Earth, while any water on the surface of Mars, which lies farther away from the sun, would freeze (see Diagram).
Now the idea that life will only be found in the habitable zone is looking a little simplistic. Many planetary scientists who have been studying Jupiter and its four largest moons Ganymede, Callisto, Io and Europa think that despite Europa鈥檚 frozen surface, this moon could be one of the most promising locations for life in our solar system.
That is a dramatic consequence of the four large moons orbiting very close to Jupiter and each other. Gravitational interactions between the moons are strong and this sets up orbital 鈥渞esonances鈥�. For instance, for every time Ganymede circles Jupiter, Europa completes two orbits and Io four. This means the moons regularly end up in the same arrangement relative to one another and repeatedly yank each other, causing their orbits to be elliptical. In such orbits, they swing in close to the huge mass of Jupiter, which puts them through enormous squeezing and stretching, warming their interiors through a process called tidal heating.
鈥淐onsequently, we see Io with many active volcanoes,鈥� says Scharf, 鈥渨hile Europa very probably has a giant ocean beneath its global ice sheet.鈥� That has big implications for the hunt for extraterrestrial life, not only on Europa but also in other solar systems.
Beyond the zone
鈥淪ince planetary scientists recognised the possibility of life on Europa, they have realised that satellites of giant planets could be habitable,鈥� says Richard Greenberg of the University of Arizona鈥檚 Lunar and Planetary Laboratory in Tucson. 鈥淪uch bodies, which lie far outside the habitable zone, may be even more common platforms for life than planets in the habitable zones.鈥�
Tidal heating makes Jupiter鈥檚 moons far warmer than you would expect from the sunlight falling on them. 鈥淓xactly the same thing will happen in moon systems around extrasolar gas giants,鈥� Scharf says. His calculations have revealed that exomoons as large as Earth could experience at least 100 times as much heating as Io. Scharf thinks this combination of tidal heating and warmth from the parent star could keep exomoons cosy enough for water to remain liquid.
鈥淚 believe it could double the size of the habitable zone around a star,鈥� he says.
There could well be an abundance of such worlds. Astronomers have so far spotted more than 170 extrasolar planetary systems, and all of them contain gas giants up to 10 times the mass of Jupiter. If these planets are anything like the gas giants in our own solar system, they could each harbour several moons almost as large as Earth. That means the tally of exomoons in these planetary systems alone could run into thousands.
In April, Scharf published the results of a study of 74 gas giants orbiting more than 90 million kilometres out from their stars (). At this distance, which is almost as far out as Venus is from the sun, Scharf reasoned that any putative moons around the planet would be able to resist the gravitational force of their parent star and remain stable. Much closer and the star鈥檚 tidal forces would quickly rip the moon apart. By examining the mass of each planet and its proximity to its star, he calculated that many of the gas giants he studied harbour regions where tidal heating would make any moons warm enough for liquid water.
Assuming watery exomoons do exist, could life really arise on them? One problem would be the violent upheavals of the habitat. A moon experiencing 100 times the tidal heating of Io should have much more volcanic activity, and that could reduce a habitable world to nothing. Scharf sees no reason to worry, however. He points out that observations made by the Hubble Space Telescope and by NASA鈥檚 Voyager and Galileo probes show that the volcanic activity on Io is patchy. Some places are in severe turmoil, while others are relatively quiet. 鈥淭here may be quieter locations on giant exomoons where life could exist,鈥� Scharf says.
If many exomoons around gas giants are as large as Earth, their sheer size would boost the prospects for life. Only the gravitational pull of such a large body would be able to hold onto a thick, sheltering atmosphere. What鈥檚 more, a large moon is more likely to have a magnetic field. Just as Earth鈥檚 magnetic field deflects the energetic particles spewed out by the sun, so a moon鈥檚 magnetic field will protect life from damaging radiation. 鈥淏ig moons will be the safest places,鈥� says Scharf.
One day we might discover whether he is right. We can already detect planets around nearby stars if they pass in front of the parent star. Later this year, the French space agency CNES plans to launch the Corot telescope. It will simultaneously monitor 12,000 stars, looking for small dips in a star鈥檚 brightness when a planet passes in front of it. NASA aims to go one better in 2008 with the launch of Kepler, a space telescope that will look for such 鈥渢ransits鈥� in 100,000 stars at once.
The planet hunters might find even more than they hoped for. According to Jean Schneider of the Paris Observatory in France, massive moons tugging a gas giant first one way and then another would alter the precise timing of a transit. 鈥淚f the moon is as large as the Earth, it will also make its own detectable transit,鈥� he says. By observing such effects, Schneider believes that Corot should be able to detect moons with diameters 50 per cent bigger than the Earth as long as the planets orbit close to their parent stars.
In picturing habitats for life, our horizons may have been too limited. Scharf maintains that we should expand our search beyond looking for planets that match Earth. 鈥淚鈥檇 hate to miss spotting life because we abandon systems with no terrestrial planets.鈥�
