The Worlds of Red Dwarfs
A red dwarf is a small and relatively cool star on the main sequence. Red dwarfs range in mass from a low of 0.075 solar masses (the upper limit for a brown dwarf) to about 50% of the Sun and have a surface temperature of less than 4,000 K. So they are not very impressive but there are a lot of them. Using publicly available data from NASA's Kepler space telescope, astronomers at the Harvard-Smithsonian Center for Astrophysics estimate that six percent of red dwarf stars in the galaxy have Earth-size planets in the habitable zone, the range of distances from a star where the surface temperature of an orbiting planet might be suitable for liquid water.
Red dwarfs are by far the most common type of star in the Milky Way galaxy, at least in the neighborhood of the Sun, but due to their low luminosity, individual red dwarfs cannot easily be observed. From Earth, not one is visible to the naked eye. Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are twenty of the next thirty nearest.
"We don't know if life could exist on a planet orbiting a red dwarf, but the findings pique my curiosity and leave me wondering if the cosmic cradles of life are more diverse than we humans have imagined," said Natalie Batalha, Kepler mission scientist at NASA's Ames Research Center in Moffett Field, Calif.
The research team analyzed 95 planet candidates in the Kepler catalog orbiting 64 red dwarf stars. Most of these candidates aren't the right size or temperature to be considered Earth-like, as defined by the size relative to Earth and the distance from the host star. However, three candidates are both temperate and smaller than twice the size of Earth.
Red dwarf stars are smaller, cooler and fainter than our sun. An average red dwarf is only one-third as large and one-thousandth as bright as the sun. Consequently, the not-too-hot or not-too-cold habitable zone would be much closer to a cooler star than it is to the sun. Indeed such a world would lie in what to Earth as a highly compact solar system.
"This close-in habitable zone around cooler stars makes planets more vulnerable to the effects of stellar flares and gravitational interactions, complicating our understanding of their likely habitability," said Victoria Meadows, professor at the University of Washington, Seattle, and principal investigator with the NASA Astrobiology Institute. "But, if the planets predicted by this study are indeed found very nearby, then it will make it easier for us to make the challenging observations needed to learn more about them, including whether or not they can or do support life."
The three planetary candidates highlighted in this study are Kepler Object of Interest (KOI) 1422.02, which is 90 percent the size of Earth in a 20-day orbit; KOI-2626.01, 1.4 times the size of Earth in a 38-day orbit; and KOI-854.01, 1.7 times the size of Earth in a 56-day orbit.
Located between 300 and 600 light-years away, the three candidates orbit stars with temperatures ranging from 5,660 to 5,840 degrees Fahrenheit (3,400 to 3,500 degrees Kelvin). By comparison, the temperature of the sun is nearly 5,800 degrees Kelvin (9,980 degrees Fahrenheit).
At least four and possibly up to six extrasolar planets were discovered orbiting the red dwarf Gliese 581 between 2005—2010. One planet has about the mass of Neptune, or sixteen Earth masses. It orbits just 6 million kilometers (0.04 AU) from its star, and so is estimated to have a surface temperature of 150 °C, despite the dimness of the star. In 2006, an even smaller extrasolar planet (only 5.5 times the mass of Earth) was found orbiting the red dwarf OGLE-2005-BLG-390L; it lies 390 million km (2.6 AU) from the star and its surface temperature is −220 °C (56 K).
In 2007, a new, potentially habitable extrasolar planet, Gliese 581 c, was found, orbiting Gliese 581. If the minimum mass estimated by its discoverers (a team led by Stephane Udry), namely 5.36 times that of the Earth, is correct, it is the smallest extrasolar planet revolving around a main-sequence star discovered to date and since then Gliese 581 d, which is also potentially habitable, was discovered.
For further information see Red Dwarf Earths.
Planet image by D. Aguilar/Harvard-Smithsonian Center for Astrophysics via NASA.