Tuesday, November 13, 2007

Alien Solar Systems

Last week a team of scientists in California announced the discovery of the fifth planet orbiting a distant star similar to our sun. The star is known as "55 Cancri", and is located 41 light years away in the constellation Cancer. This star now holds the record for hosting the largest known planetary system outside of our own. Only one other star (called mu Arae) is known to host four planets, while triple-planet systems have been documented around another half-dozen stars.

Since 1995, more than 220 planets have been discovered in orbit around other stars. The overwhelming majority of these planets have been found by their gravitational influence on the star that they orbit. We tend to think of a planet as simply moving around its star -- but even small planets exert a gravitational tug on the star that causes it to wobble around slightly in space. Astronomers can measure this wobble, essentially by passing the light from the star through a prism to spread it out into all of its colors (think of the famous Pink Floyd album cover) and then carefully measuring the positions of dark lines that appear where different chemical elements like hydrogen and helium absorb light near the surface of the star. The color corresponding to the position of these dark lines moves around as the star wobbles -- basically for the same reason that the pitch of an ambulance siren sounds different when it is moving towards you or speeding away (the effect is known as the Doppler shift).

The team has been monitoring 55 Cancri for more than 18 years. Since the wobbles caused by the five planets are all happening at the same time, and with different orbital periods, it took a long time to isolate the wobbles caused by each planet individually. The outermost planet in the system is about four times the size of Jupiter, and it takes 14 years to orbit the star. The other previously known planets are similar in size to Neptune, Jupiter, and Saturn, but they orbit the star relatively close in -- circling every 3, 15, and 44 days (all much faster than Mercury orbits the Sun). The newly discovered planet falls somewhere in between, orbiting the star in about 260 days. Since 55 Cancri is a little fainter than the Sun, this places the new planet inside an Earth-like "habitable zone" where liquid water can theoretically exist. Although the planet is roughly the size of Saturn, it could have large Earth-like moons where water and potentially life could survive (Saturn's largest moon "Titan" is bigger than the planet Mercury).

It's only a question of time before astronomers discover more and larger planetary systems around distant stars. The longest running surveys have been operating for less than 20 years, so even in the best case we would not be able to detect Saturn, Uranus, or Neptune if we were watching the Sun from a distance. As the technology improves we should also begin finding smaller planets, closer to the size of the Earth. In the future, people might wonder what all the fuss was about over this little five-planet system around 55 Cancri.