To a biologist, the ingredients needed to form life include water, heat and organic chemicals. But some in the astrophysics and astronomy community argue that life, at least advanced life, may require an additional component: a Jupiter-sized planet in the solar neighborhood.

“A long-period Jupiter may be a prerequisite for advanced life,” said Dr. Alan Boss, a researcher in planetary formation. Boss, who works at the Carnegie Institution of Washington, is a member of the NASA Astrobiology Institute (NAI).

In our own solar system, Jupiter, with its enormous gravitational field, plays an important protective role. By deflecting comets and asteroids that might otherwise hit Earth, Jupiter has helped to create a more stable environment for life to evolve here. It’s generally believed that a massive impact was responsible 65 million years ago for wiping out dinosaurs on Earth. If not for Jupiter, it’s possible that many other such impacts would have occurred throughout Earth’s history, preventing advanced life from ever gaining a foothold.

Jupiter is significant not only for its size but also for its location in our solar system, far from the Sun. Because it orbits at slightly more than 5 AU (astronomical units – the distance between the Earth and the Sun is 1 AU), there is plenty of room in the inner part of our Solar System to accommodate a range of smaller planets.

Within the inner solar system there exists a region, known as the habitable zone, where liquid water, and therefore life, can potentially exist on a planet’s surface. Without liquid water, life as we know it is not possible. The habitable zone around our Sun stretches roughly from the orbit of Venus to the orbit of Mars. Venus is generally believed to be too hot to support life. Earth, it appears, is just right. And the jury is still out on Mars.

Understanding the role that Jupiter plays in our own Solar System helps astronomers focus their search for habitable planets around other stars. “If,” Boss explains, “a Jupiter-mass planet on a stable, circular orbit [around another star at] around 4 to 5 AU was found, without any evidence for other gas giant planets with shorter period orbits, such a discovery would be like a neon light in the cosmos pointed toward that star, saying ‘Look here!’. That star would be a prime target for looking for a habitable, Earth-like planet.”

But to date, no such planetary systems have been found orbiting distant stars. That’s due in large part to the technique used by astronomers to search for extrasolar planets. The technique that has been used to locate most of the 59 known extrasolar planets is called radial velocity or Doppler spectroscopy. It is based not on observing a distant planet directly, but by observing the effect that the planet’s gravity has on the motion of the star it orbits.

As a giant planet moves around a star, its gravity pulls the star first one way, then the other. “Strictly speaking, Copernicus had it wrong,” said Boss. “Planets don’t move around their stars; they actually move around the center of mass of the planetary system, and so does the star.” This motion of the star is detectable from Earth as a minute periodic shift in the color of the star’s light.

When a clear pattern emerges in this color shift over a number of orbits of the planet around the star, astronomers are confident that they have detected a giant extrasolar planet. “We can infer the presence of planets indirectly by observing the wobble of a star in space caused by its motion around the center of the system,” said Boss. By studying this wobble pattern in detail, they can determine a minimum mass for the planet, its distance from the star and the shape of its orbit.

To date, however, nearly all of the giant planets found have been much closer to their stars than Jupiter is to the Sun. None of the extrasolar Jupiters discovered so far orbits with a period large enough to encourage the formation of a habitable Earth-mass planet. Astronomers believe that this is probably an effect of the radial velocity search technique, not necessarily an indication of what’s actually out there. Because closer-in planets orbit their stars more frequently, it takes less time for an Earthbound observer to see a pattern emerge in its star’s wobble than it would for a planet farther out, with a longer orbital period.

One shouldn’t conclude, however, that Solar-System-like configurations are rare – indeed, such systems could still be quite commonplace. We just haven’t found them yet. Jupiter, for example, takes 12 years to orbit the Sun. To firmly identify a similar-sized planet at a similar distance from another star would require a minimum of 24 years, or two full orbits.

Boss points out that “several planet search programs have been in action since 1987. Their accuracy has increased significantly in the last five years, so we can expect that long-period Jupiters will be found by these programs in the coming years – it is just a matter of a few more years before astronomers should start to find them. So stay tuned!”

What Next?

Future projects for the discovery of extrasolar worlds include NASA’s Space Interferometry Mission, due to be launched in 2005. This space-based telescope will be better able to detect the motions of distant stars. In 2011, NASA hopes to launch the Terrestrial Planet Finder, which would search for light reflecting off of distant planets, including planets as small as Earth. This space-based telescope would be able to also determine a planet’s temperature and the composition of its atmosphere.