Scorpius Star is Twinned with Sun

By Ken Croswell

Published in New Scientist (May 3, 1997, page 18).

If you want to boost your chances of finding extraterrestrial intelligence, point your telescope at a star in the constellation of Scorpius, say astronomers in Brazil. They have found that this star resembles our Sun more closely than any other investigated.

Although the Sun is often dismissed as just an average star, it actually outshines most of its peers. It is also one of only 4 percent of stars in our Galaxy known as G-type main-sequence stars--yellow stars that burn hydrogen into helium at their centres. Sunlike stars are the obvious targets in the search for life elsewhere.

Now Gustavo Porto de Mello of the Federal University in Rio de Janeiro and Licio da Silva of the National Observatory have found that the yellow star 18 Scorpii is a virtual carbon copy of the Sun. The star lies 46 light-years from Earth and is dimly visible to the naked eye.

The astronomers used telescopes in Chile and Brazil to obtain high-resolution spectra of several Sunlike stars, including 18 Scorpii. Using a precise distance to the star measured by the Hipparcos satellite launched in 1989, they determined very accurately how much light the star emits.

After analysing the data, the researchers conclude that 18 Scorpii "surpasses all previously claimed solar twins in likeness to the Sun." As far as they can tell, the star's mass, temperature, colour, surface gravity, rotation speed, surface activity, and iron abundance all match the Sun's. Only the luminosity and age differ modestly: 18 Scorpii emits 5 percent more light than the Sun and it is slightly older.

The Sun's most famous near-twin is Alpha Centauri A, a yellow star just 4.35 light-years away. But compared with 18 Scorpii, Alpha Centauri A is a distant cousin: it emits 52 percent more light than the Sun and has 60 percent more iron. Furthermore, Alpha Centauri A has two companion stars, whereas 18 Scorpii and the Sun are single. Single stars such as 18 Scorpii offer the best prospects for having planets with stable orbits.

"We recommend that it be considered for strong priority in the ongoing planet searching programs as well as in SETI surveys," say the astronomers. They will publish their work in the June 10, 1997, issue of Astrophysical Journal Letters.

Ed Guinan of Villanova University in Pennsylvania says that observations of 18 Scorpii could shed some light on climate change: "Solar twins can tell us about the Sun, which is the main driver of Earth's climate."

Ken Croswell is an astronomer in Berkeley, California, and author of The Alchemy of the Heavens and Planet Quest.

"An engaging account of the continuing discovery of our Galaxy."--New York Times Book Review. See all reviews of The Alchemy of the Heavens here.

"An excellent introduction to a branch of astronomy that may eventually help to establish the presence or absence of life elsewhere in the universe...Fascinating and worthwhile."--New York Times Book Review. See all reviews of Planet Quest here.