Saturday, July 5, 2014

Cold Earth-Size Planet Circling a Red Dwarf

Figure 1: Artist’s impression of a planet in a binary star system.

An international team of astronomers has reported the discovery of a planet with twice the mass of Earth circling one of two stars in a binary star system. The discovery was published in the July 4 issue of the journal Science. Both stars in the binary system are red dwarfs that are much cooler and fainter than the Sun. The planet, identified as OGLE-2013-BLG-0341Bb, was detected using a technique known as gravitational microlensing. In 2013, the two red dwarfs happen to pass almost directly in front of a more distant background star. As a result, the gravity of the two red dwarfs acted as a “lens”, magnifying light from the background star. The gravitational microlensing event is represented by a light-curve depicting the brightening of the background star. Based on the light-curve, the presence of the planet was determined from a small “dip” in the light-curve several days before the main magnification event. Additionally, the planet’s presence was also inferred from its distortion of the main magnification event.

Figure 2: Light-curve of the gravitational microlensing event as recorded by a number of observatories. (Top) Light-curve features “C to F” are induced by the binary star system. (Bottom Left) A low-amplitude early “bump” caused by the background star passing moderately close to the companion star ~300 days earlier. (Bottom Right) A small “dip” due to the presence of the planet. Source: A. Gould et al. (2014).

Figure 3: An artist’s rendering shows OGLE-2013-BLG-0341Bb (far right) orbiting one of two stars (right) of a binary star system. Image credit: Cheongho Han, Chungbuk National University, Republic of Korea.

From analysis of the light-curve, the planet’s host star is estimated to be about 0.10 to 0.16 times the Sun’s mass, while the slightly brighter companion star is about 0.11 to 0.19 times the Sun’s mass. The two red dwarf stars are spaced 10 to 15 AU apart, comparable to the distance between Saturn and the Sun. As for OGLE-2013-BLG-0341Bb, the planet is at a projected distance of ~0.8 AU from its host star, similar to the Earth-Sun distance. Since the host star is ~400 times less luminous than the Sun, the temperature on OGLE-2013-BLG-0341Bb is estimated to be less than 60 K. This is even colder than the temperatures on Jupiter’s icy moon Europa.

The discovery of OGLE-2013-BLG-0341Bb is evidence that Earth-mass planets can form in Earth-like orbits, even in a binary system where the two stars are relatively close to one another. Although OGLE-2013-BLG-0341Bb is quite certainly a frozen world, binary star systems like the one it is in are quite abundant. If its host star were more like the Sun, OGLE-2013-BLG-0341Bb, at ~0.8 AU from its host star, would be perfectly in the habitable zone where temperatures are just right for liquid water to exist. It would be great if more observations of OGLE-2013-BLG-0341Bb could be done. However, such a gravitational microlensing event is non-repetitive and all data obtained thus far from this single event will be all there is. OGLE-2013-BLG-0341Bb is also estimated to lie ~3,000 light-years from Earth and such a large distance means follow-up observations are not possible in the foreseeable future.

A. Gould et al., “A terrestrial planet in a ~1-AU orbit around one member of a ∼15-AU binary”, Science 4 July 2014: 345 (6192), 46-49.