Planets may be present in a quadruple star system 150 light years from Earth, according to Spitzer Space Telescope observations.
The system, called HD 98800, consists of two pairs of stars in which the partners in each pair orbit one another closely. The pairs themselves travel around each other on a very elongated path.
A dusty disc was already known to surround one of the pairs of stars. Now, Elise Furlan of the University of California in Los Angeles, US, and colleagues have made more detailed observations of the disc with Spitzer.
The Spitzer observations suggest the disc is actually made of two concentric rings around the stellar pair (see image at right). One lies about twice as far as the Earth lies from the Sun, while other, which is denser, lies about six times the Earth-Sun distance.
The dust is not expected to be stable at the location of the inner ring. So the astronomers suspect it is being continuously replenished by dust from the outer ring, where the dust may be generated by collisions between comet- or asteroid-like objects.
The empty space between the two rings might have been cleared out by the gravity of a hypothetical planet orbiting there, the astronomers say. Alternatively, the gravity of the four stars could have cleared out the region, they say.
An observer watching from the surface of the planet could witness dual sunsets and sunrises involving the central pair of stars, Furlan says. The pair would appear as twin suns, both tinged orange because they are a bit less massive and cooler than the Sun. The other pair, being more distant, would merely look like bright stars.
The distance to the other pair varies over a period of several hundred years, making it appear brighter and dimmer with time. At its closest, its distance from the other pair is about 35 times the Earth-Sun distance, or slightly larger than Neptune's distance from the Sun.
"When it comes close, it most likely increases the frequency of collisions in that outer comet belt," Furlan told New Scientist.
Relatively massive stars are more often found with one or more partners than they are alone. Only a handful of quadruple systems with dusty discs are known, but since a close pair can masquerade as a single star, it is hard to know how common they really are. For example, the HD 98800 system was at first thought to merely be a double star system, but measurements of the spectrum of its light later revealed that each member was itself a close pair of stars.
Although planets would be unstable at some locations in a quadruple system like HD 98800, there are some spots where planets could survive. In HD 98800, planets would be stable between about 4.5 and 6 times the Earth-Sun distance from the binary possessing the dust rings, Furlan says.
She says it is possible, in theory, that life could arise on planets in such quadruple star systems, as long as the planets are on stable orbits.
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Journal reference: Astrophysical Journal (in press)
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