The recently completed Herschel Space telescope mission discovered a vast dusty ring of material around the star Gliese 581, which also contains four relatively small planets. The planets have masses between 2 and 15 times that of the Earth, making the largest similar in mass to Neptune. The quantity of dust present is much higher than in our Solar System, implying that the lack of a massive Jupiter-like planet allows such debris discs to survive more easily.
To astronomers, a "potentially habitable" planet is one that could sustain life, not necessarily one where humans would thrive. Habitability depends on many factors, but having liquid water and an atmosphere are among the most important.
“We thought we would have to search vast distances to find an Earth-like planet. Now we realize another Earth is probably in our own backyard, waiting to be spotted,” said Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics (CfA).
Six percent of red-dwarf stars have habitable, Earth-sized planets, astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) have found. Red dwarfs are the most common stars in our galaxy; about 75 percent of the closest stars are red dwarfs. The closest Earth-like planet could be just 13 light-years away, Harvard astronomer and lead author Courtney Dressing calculated. Since red dwarf stars live much longer than Sun-like stars, this discovery raises the interesting possibility that life on such a planet would be much older and more evolved than life on 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. The cFa team culled the Kepler catalog of 158,000 target stars to identify all the red dwarfs, then reanalyzed those stars to calculate more accurate sizes and temperatures. They found that almost all of those stars were smaller and cooler than previously thought.
Locating nearby Earth-like worlds may require a dedicated small space telescope, or a large network of ground-based telescopes. Follow-up studies with instruments like the Giant Magellan Telescope and James Webb Space Telescope could tell us whether any warm, transiting planets have an atmosphere and further probe its chemistry.
Although Herschel cannot see the central star directly, the images can still be used to deduce the structure of the system. The planets orbit within a quarter of the Earth-Sun distance (called an astronomical unit, or "AU"), but the debris disc extends from approximately 25 to 60 AU.
In the image belwo, a sketch of the debris disc and planet orbits are shown, though it is not to scale - the planet orbits would actually be much, much smaller on this scale. The other blobs in the image, to the right, upper-left and lower-right of the image, are not part of the planetary system, but rather are background galaxies that Herschel also sees.
Gliese 581 is one of a number of stars observed by the DEBRIS Herschel observing programme. These low-mass M-type dwarf stars are the most common stars in the Galaxy. Of three such stars with known planets, Gleise 581 was the only one to show any sign of a debris disc, and also the only one without a massive, Jupiter-like planet orbiting it.
The only other M-type star known to possess a debris disc is AU Microscopii, which is much younger at only around 12 million years old. With an age of 2 billion years, Gleise 581 also shows that these debris discs are not always dispersed by the radiation emmanating from the central star.
This connection between the relatively low planet mass and the presence of a debris disc has also been seen in Sun-like stars such as 61 Virginis, which was also observed by the DEBRIS program.