Red Dwarfs and Habitable Planets

Astronomers searching for planets outside our solar system have found the Doppler method of observing light wavelengths to be one indirect, yet effective, strategy. Because a planet's gravity tugs on the star it orbits, the star wobbles (moves back and forth) slightly as the planet travels around it. This changes the wavelengths of the light the star emits, which is known as the Doppler effect. By observing the wavelength shifts, astronomers can infer the presence of a planet. Another common way of detecting planets is the transit method. Because a star becomes slightly dimmer when a planet passes in front of it, regular dips in a star's brightness can indicate the presence of a planet. Although a large number of planets around other stars have been discovered, many are not in the habitable zone—the region around a star where the temperatures could allow water to exist in liquid form—and thus cannot support life as we know it. But location is not the sole prerequisite for life; the planet must also be the right size. If it is too large, it would have a huge atmosphere and have no solid surface. If too small, its gravity would be too weak to retain a substantial atmosphere, so its water would escape into space. Nonetheless, scientists searching for habitable planets have become increasingly interested in the small, dim stars called red dwarfs.

Over 80 percent of stars in our galaxy (large group of stars) are red dwarfs.They usually have 10 to 50 percent of the mass of our Sun. But because they are so numerous and have extremely long lives, the habitability of planets in their midst is a critical question, scientists assumed that red-dwarf systems would make poor homes for life, for two reasons. First, their habitable zones are not only narrow—about a tenth as wide as the Sun's—but also so close in to their star—much closer than Mercury is to the Sun—that any planets orbiting there would be gravitationally locked in. That means one side of the planet always faces the star, just like one side of the Moon constantly faces Earth. You might expect the dayside to be scorching hot while the nightside remains in an eternal freeze, with a giant icecap gathering all of the planet's moisture. Second, many of these stars are terribly stormy, especially when they are young, frequently putting out strong flares of harmful radiation and fast-moving particles. Any life on planets in the close-in habitable zone might be destroyed.

However, scientists now think that some red-dwarf systems could contain habitable worlds after all. If a planet is massive enough to retain a substantial greenhouse atmosphere—an atmosphere with high levels of heat-trapping gases—wind circulation could keep the temperatures fairly mild all around the planet even if it is gravitationally locked. Incoming radiation from the star would simply turn oxygen in the planet's upper atmosphere into ozone, which in turn would shield the surface from harmful flares of radiation. Besides, some red dwarfs are less active than others. At least, these less active red dwarfs could permit life to develop on Earth-size rocky planets with moderately dense atmospheres located in their narrow, close-in habitable zones. Creatures on such a planet would be living in a world of contrasts, with the day lasting forever on one half and a permanent night on the other.

There is another reason that astronomers are hoping for habitable rocky planets around red dwarfs: they would be easier to detect than those orbiting Sun-like stars. A given planet's gravitational tug would have a bigger effect on a lower-mass star, rendering larger—thus easier to measure—wavelength shifts in Doppler observations. The transit method favors them immensely, too. First, habitable planets would be in tighter orbits around red dwarfs, thus increasing the likelihood that they will be seen in transit. Second, because red dwarfs are smaller, the relative dip in brightness when a planet passes in front would be bigger; thus a small, rocky planet's transit might be measurable even with a ground-based telescope, even though ground-based telescopes are subject to the interference of Earth's atmosphere.