orty-four light-years from Earth, in the constellation of Andromeda in
the northern sky, a new solar system of at least three planets has been
discovered around the star Upsilon Andromedae by two independent teams of
astronomers. The finding marks the first confirmed discovery of a
multi-planet system around a yellow-dwarf type star much like our own Sun,
lending hope that our solar system is not so unusual in the galaxy.
Extrasolar planets aren't an entirely new phenomenon. In 1992, a
three-planet system was discovered around the deadly neutron star (or
"pulsar") PSR 1257+12, which is more than 2,600 light-years from Earth. How
the planets got there is not at all clear, since the pulsar is the remnant
of a massive supernova explosion that ought to have vaporized them.
However, since no known lifeform could actually live on these
planets, the finding isn't much more than an astronomical curiosity.
Upsilon Andromedae's planets are a very different story. But finding them
wasn't easy.
The favorite method of today's planet hunters is doppler
analysis--careful measurement of the wavelengths of light a star gives off.
Like train whistles and hawk screeches, starlight changes "pitch" as its
source approaches or recedes. By measuring these changes over time,
astronomers can accurately compute the "wobble" of a star caused by the
gravity of any planets in orbit around it.
Unfortunately, this method finds the oddballs first: it favors massive
planets orbiting close to a parent star, such as Upsilon Andromedae's
innermost world, a behemoth about 75 percent as massive as Jupiter (.75
MJ), with an orbit less than one-eighth as large as Mercury's. This was the
first of the three planets to emerge from the data, back in 1996. The next
planet, discovered in early 1999, orbits a little further out than Mars and
weighs at least four times as much as Jupiter (4 MJ).
Crushing atmospheric pressure
Small or faraway planets require much longer observation times to
detect, so it'll probably be years or decades before anything resembling an
Earth-like planet can be detected. Interestingly, though, Upsilon
Andromedae's newest planet, a 2 MJ known to astronomers as Upsilon
Andromedae C, has an eccentric orbit that carries it from a Venus-like
distance in the "summer" of its 242-day year, to an Earth-like one at the
height of winter. Could the planet itself harbor life? Unlikely. As a
probable "gas giant" it would have crushing atmospheric pressure and
scorching temperatures--much worse than those of Venus--with no solid
surface for the enormous gravity to squish things against.
But in our solar system, most of the planets and all of
the gas giants have moons, and there's no reason to suspect Upsilon
Andromedae should be any different. Unfortunately, an atmosphere is
necessary for life, and most of our gas-giant moons are too small to retain
one. Still, on average, the mass of large gas-giant moons is roughly
proportional to the mass of the planets they orbit, so it's conceivable
that a 2 MJ planet could have one or more moons larger than Mercury, or
possibly even larger than Mars. As luck would have it, this is right inside
the range of masses that a temperate moon would require for its gravity to
hold onto life-giving O2, CO2, and H2O
molecules.
The key here is that the moon can't get too hot, since life as we know
it requires liquid water. If the oceans boil away, there isn't much hope for life. But Upsilon Andromedae puts out about four times as much energy as our own sun, and with UA-C swinging closer to its star than Venus does to the Sun at perihelion, how cool could its moons really be?
Between the boundaries
To answer that question, we have to take a look at orbits. Two factors
restrict the possible orbits of a moon: One is the Roche Limit, where tidal
stresses would tear the moon into Saturn-like rings. The other is the limit
of influence, past which the moon would drift away to become a new planet
of the parent star. Between these boundaries, UA-C could have moons with
orbits anywhere from a few hours to nearly a hundred days long, with
respectively about 25 percent and 0.3 percent of that time spent in
eclipse, hidden from UA's glare behind the darkened disc of the planet.
(Interestingly, planets massing anywhere from 0.2 to 4 MJ are thought to
have much more variation in density than in size. Saturn and Jupiter are an
excellent case in point.)
Another convenient property of moons (including Earth's) is that their
rotation periods are usually tidally locked with their orbit periods, so
they always turn the same face toward their host planet. Therefore, the
planetward faces of UA-C's speedy inner moons would never experience the
harsh noon of direct sunlight, only the morning and afternoon rays slanting
in around the edges of the giant planet overhead. At the height of summer,
these areas would receive about five times as much sunlight as the
equivalent latitudes on Earth, and in the winter they'd get about three
times as much, in cycles of night-morning-eclipse-afternoon-night that
would seem very short to us. The composition of the surface and atmosphere
would determine how much of this heat was actually retained, but
temperatures below the boiling point of water are entirely conceivable,
especially near the poles. If UA-C has an extensive ring system like
Saturn's, that might provide additional shade, cooling things off even
further.
Further out, where the shadow of the planet and rings are no defense
against the harsh sunlight, the chances of finding liquid water are more
remote, but still within the realm of possibility. For all we know, Upsilon
Andromedae's newest planet could harbor several steamy moons that
are friendly to life, and since the place is only 44 light-years away, we
may very well go there some day to find out.
Wil McCarthy is a rocket guidance engineer, robot designer, science fiction
author and occasional aquanaut. He has contributed to three interplanetary
spacecraft, five communication and weather satellites, a line of
landmine-clearing robots, and some other "really cool stuff" he can't tell
us about. His short fiction has graced the pages of Analog,
Asimov's, SF Age and other major markets, and his
novel-length works include Aggressor Six, the New York Times Notable
Bloom, and upcoming The Collapsium.
