Astronomers search for exoplanets by measuring shifts in the pattern of a star's spectrum - the different wavelengths of radiation that it emits as light.
These "Doppler shifts" result from subtle changes in the star's velocity caused by the gravitational tugs of orbiting planets, but Doppler shifts of a star's absorption lines can also result from magnetic events like sunspots originating within the star itself -- giving false clues of a planet that does not actually exist.
"In the search for low-mass planets," said Suvrath Mahadevan, an assistant professor of astronomy and astrophysics at Penn State and a coauthor of a new research paper om the issue, "accounting for the subtle signature of a magnetics events in the star is as important as obtaining the highest possible Doppler precision."
Due to this discovery, a research team has found that some signals suspected to be coming from two planets orbiting the star at a distance where liquid water could potentially exist, actually are coming from events inside the star itself, not from so-called "Goldilocks planets" where conditions are just right for supporting life.
"This result is exciting because it explains, for the first time, all the previous and somewhat conflicting observations of the intriguing dwarf star Gliese 581, a faint star with less mass than our Sun that is just 20 light years from Earth," said lead author Paul Robertson, a postdoctoral fellow at Penn State. As a result of this research, the planets now confirmed to be orbiting this dwarf star total exactly three.
Older stars such as Gliese 581, an "M dwarf" star in the constellation Libra about one-third the mass of our Sun, have until now been considered highly attractive targets in the search for extraterrestrial life because they are generally less active and so are better targets for Doppler observations. "The new result from our research highlights a source of astrophysical noise even with old M dwarfs because the harmonics of the star's rotation can be in the same range as that of its habitable zone, raising the risk of false detections of nonexistent planets," Mahadevan said. "Higher-precision analysis for discovering Earth-like planets using spectrographs will be increasingly more necessary as next-generation spectrographs with the higher Doppler precision needed for detecting important subtle signatures come on line this decade -- like the Habitable Zone Planet Finder (HPF) that our team now is developing at Penn State." A description of this research is available on the Habitable Zone Planet Finder blog http://hpf.psu.edu.