'Possible' and 'probable' alien planets.

'Possible' and Probable' alien planets

The possiblity of planets orbiting starsother than the sun interests a lot of astronomers -- if one may judge from the number of them who listened to a press conference on the evidence for low-mass companions to nearby starts at last week's meeting of the American Astronomical Society and the Canadian Astronomical Society in Vancouver, British Columbia. Bruce Campbell of the Dominion Astrophysical Observatory in Victoria, British Columbia, and Gordon Walker and Stephenson Yang of the University of British Columbia in Vancouver reported that a six-year survey of the motions of 15 stars yielded evidence of seven "possible" or "probable" companions with masses 1 to 10 times that of Jupiter. They did their work with the 3.6-meter Canada-France-Hawaii telescope on Mauna Kea on the island of Hawaii.

Previous results have claimed evidencefor dark companions that are generally much larger, particularly brown dwarfs, objects with masses between one-tenth and one-hundredth of the sun's mass (SN: 11/1/86, p.282). (Ten Jupiter massess equal one-hundredth of the sun's mass.) However, some of these claims have disappeared under further scrutiny, and unsuccessful searches for brown dwarfs have convinced a number of astronomers that, as Campbell says, "brown dwarfs may not exist at all." If so, the evidence for slight cyclic variations in the motions of the seven sun-like stars could point to planet-sized companions. Recognizing both the indirect nature of the evidence and the difficulties encountered by other such claims, Campbell declares: "Our results need to be confirmed."

Campbell, Walker and Yang find evidencefor such small companions by using a newly developed technique that is 50 to 100 times as sensitive as conventional methods. It can detect changes in stars' motions as small as 40 kilometers per hour. If a star has companions--of any size--the star and companions will orbit their common center of mass. If the companions are invisible, the star's orbital motion will betray their presence: Astronomers will see the orbital motion as a cyclic back-and-forth, either in the line of sight (radial motion) or across the sky (proper motion).

Campbell and his co-workers arestudying radial motion. Their technique is to take spectra of the star over a period of years. In the starlight are emission lines -- resonant frequencies of particular chemical elements. The wavelengths of these lines shift slightly as the star moves, to the red as it recedes from us and to the blue as it approaches. To detect the shifts, the astronomers compare the star's spectrum with that of a standard lamp.

In the conventional technique, observersfirst make a spectrum of the star, then turn the spectrograph on to the lamp and print its spectrum next to the star's. The switching back and forth introduces a lot of systematic errors that reduce the accuracy. Instead, Campbell, Walker and Yang pass the star's light through a tank of hydrogen fluoride, which happens to have the wanted emissions lines, on the way to the spectrograph. This imposes the reference spectrum on the star's spectrum in one act, reducing the errors considerably, the observers say.

The 15 stars surveyed were chosenbecause they are similar to the sun in major characteristics, Particularly mass, and do not have known stellar companions. However, this work found that one of them, Gamma Cephei, is in fact a binary star system and also has a third, much smaller object in the system. In this case the small object imposes a motion on the star that has a period about 2.7 years. Having followed that motion for more than a full period, the observers call this instance one of their "probables." The other probable is Epsilon Eridani. (Both stars are within 50 light-years of earth.) Five more are possible, whose motions have not been followed long enough for astronomers to be certain they are cyclic.

The work needs extension and confirmation,and this and other groups are at work on it.