In Search Of Habitable Moons

January 16, 1997

University Park, Pa. -- Recent identification of Jupiter-like planets around distant stars has raised hopes of extraterrestrial life outside our solar system, but not on the gas giants themselves.

"While gas giants probably will not support life, the moons orbiting these planets might meet the requirements necessary to sustain life," says Darren Williams, graduate student in astronomy and astrophysics at Penn State.

In today's (Jan. 16) issue of the journal Nature, Williams, Dr. James F. Kasting, professor of geosciences, and Dr. Richard Wade, associate professor of astronomy and astrophysics, outline these requirements.

"First, the gas giant must orbit its star within the habitable zone -- the zone around a star where the solar flux allows liquid water to exist," Williams says. "If the orbit is too distant, water freezes. If it is too close, high temperatures cause the hydrogen in water to be lost to space. "

The researchers examined the known gas giants to see if they fell into their star's habitable zone.

"Only 16 Cyg Bb and 47 Uma B come near to being in the habitable zone," says Williams. "Also, moons around gas giants must be able to sustain an atmosphere for billions of years and must also be close enough to their planet to have a stable orbit."

A moon's mass, the ionizing radiation it receives, the solar flux and the magnetic effects of the gas giant all play a part in trying to remove the atmosphere.

If a moon is too small, heating will cause the molecules of oxygen and nitrogen in the atmosphere to attain escape velocity -- the speed at which the moon's gravity will no longer hold them -- and disappear into space. To retain oxygen and nitrogen, the moon must be at least .07 the size of the Earth. But stellar heating is not the only consideration. When ionized atomic nitrogen recombines with electrons, it may also be lost to space. A moon must be at least .12 the mass of Earth to keep from losing appreciable amounts of nitrogen by this process.

Another way to lose atmosphere is through the action of the gas giant's magnetosphere -- the area in which the planet's magnetic field operates. Moons in the magnetosphere lose atmosphere because of bombardment by trapped energetic charged particles. A planet with its own magnetic field is protected from this effect, but, until recently, it was thought small bodies, like moons, did not have magnetic fields.

"The Galileo spacecraft's recent identification of a magnetosphere around Ganymede, which is only .03 the mass of Earth, suggests that some moons may not be affected by their planets magnetosphere," says Williams. "We also know that Saturn's moon Titan travels in and out of the magnetosphere, but still has a dense nitrogen atmosphere. This may not be the problem it was once thought."

To retain an atmosphere, moons must first form an atmosphere. Moons around extra solar gas giants might have received their water through bombardment by icy comets or carbonaceous asteroids, but research in our own solar system suggests that moons orbiting Jupiter-size planets have trouble retaining volatiles from comets.

If, however, moons originated in the outer part of stellar nebula, they may have incorporated large amounts of water. These moons may have so much water that when in the habitable zone, they are oceanic with little dry land. Between these watery moons and those devoid of water are inner moons like Jupiter's Europa which have a good balance of rock and water and are most likely to be Earth-like.

In the long term, habitable moons must also be able to compensate for the increasing brightness of their suns through time. An increase in carbon dioxide, from volcanic activity, can cause greenhouse warming which compensates for a fainter sun. As the moon ages -- and the star becomes brighter -- rock weathering continues to remove carbon dioxide from the atmosphere, but a decrease in geologic activity reduces the amounts of carbon dioxide replaced by geologic activity which, in turn, decreases greenhouse warming. Normally, for a planet to retain internal heat and remain geologically active for 4.5 billion years, it must be at least .23 the mass of Earth or just over twice the mass of Mars. This would be a large, planet-sized moon.

Moons close enough to gas-giants, however, may be warmed by tidal heating -- the gravitation pull on the moon of the gas giant. These moons would support tectonic activity or at least individual volcanoes.

Williams is not the first to suggest moons of gas giants as likely locations for extraterrestrial life. In the popular film "Return of the Jedi," the Ewoks race through a terrestrial-looking landscape on the Forest Moon of Endor, in pursuit of the minions of Darth Vader, while the planet orbits a gas giant similar to Jupiter.

The planets 47 Uma B and 16 Cyg Bb are not perfect subjects for habitable moons. 47 Uma B lies just outside the habitable zone and 16 Cyg Bb has an orbit that is so eccentric it traverses the entire habitable zone dipping inside and outside the acceptable orbit.

While these are not perfect, there seems to be sufficient flexibility and variety of factors to suggest that given a large enough gas giant with large enough moons, life could evolve and persist.

Mr. Williams may be reached at (814) 863-7947 or
Dr. Kasting may be reached at (814) 865-3207 or
Dr. Wade may be reached at (814) 865-0418 or

Penn State

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