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Planet Detected in Habitable Zone of Nearby Star

by Louise Good

Orbit of the planet HIP 57050b superimposed on our solar system. The orbits are to scale, but the size of the Sun and the planets are not. The star HIP 57050 is much smaller than our Sun.

A team led by IfA astronomer Nader Haghighipour has discovered a planet with about the same mass as Saturn in the habitable zone of a star 36 light-years from Earth. Its detection is important because it "indicates that observational techniques are on the right track for finding habitable low-mass rocky planets similar to Earth," according to Haghighipour.

A planet is considered to be in the habitable zone if its temperature is just right for having liquid water. The approximate surface temperature of the above-mentioned newly discovered planet is about minus 42 degrees F (-44 C), which was calculated by considering the surface temperature of the star, the planet's size, the proportion of the star's light that the planet may reflect, and the time that it spends in close proximity to the star. This temperature is colder than the value one gets for Earth for the same kind of calculation. However, if the planet's atmosphere holds in heat sufficiently, then the surface temperature will certainly be warmer than minus 42 degrees.

The planet (HIP 57050b) circles its central star (HIP 57050) every 41 days. The star, which is classified as an M dwarf, has a radius approximately 0.4 times that of the Sun, and has a mass about one-third of the Sun's. "Because HIP 57050b is a giant planet, it is unlikely that it is habitable," Haghighipour notes.

In the quest for potentially habitable planets, the nearest stars are of special importance. We know their precise distances, as well as their masses, sizes, and temperatures, and they are the only stars for which follow-up by astrometry and direct imaging is possible. While the majority of the currently known extrasolar planets have been detected around larger nearby stars, more than 70 percent of the nearest stars are M dwarfs. As such, these stars have the special properties (distances and masses) that drive exoplanetary science, astrobiology, and the next generation of interferometry and direct imaging missions.

The low surface temperatures of M dwarfs place their habitable zones at distances that are approximately 10 to 20 percent of Earth's distance from the Sun. These distances correspond to orbital periods of 20-50 days, another advantage of M dwarfs as potential targets for detecting habitable planets--because of their small masses, these stars respond more noticeably to the gravitational forces of close-in planets.

Haghighipour adds that it is interesting to speculate about the possible presence of a moon around HIP 57050b. While it is not out of the question that HIP 57050b could harbor a moon, and that moon would thus be in the habitable zone of the parent star, an object with only one-fifth the mass of Mars is probably not a particularly good prospect for habitability from various standpoints. Furthermore, direct detection of such a moon would be extremely challenging.