Sunday 22 January 2012

Could there be a habitable planet in the Kepler-16 system?

The Kepler-16 system was the first stellar system discovered in which a planet orbited a binary star. The discovery was made by the Kepler Space Telescope, and reported in September 2011 in a paper in the journal Science by a team lead by Lawrence Doyle of the Carl Sagan Centre for the Study of Life in the Universe and the SETI Institute. The planet was dubbed Kepler-16 (AB)b, implying that the body orbits both stares in the system (Kepler-16 A and Kepler-16 B), and although this is slightly confusing (two bodies in the system have the designation B/b), it seems to have gained acceptance, with later discoveries of planets in binary star systems named in the same way.

A model of the Kepler-16 system, showing the position of the two stars (A & B) and the planet (Kepler-16b). For scale the comparative positions of Earth and Mercury in our own solar system are shown. Image from NASA.

On 11 January 2012 Billy Quarles, Zdzislaw Musielak and Manfred Cuntz, all of the Department of Physics at the University of Texas at Arlington, published a paper on the arXiv database at the Cornell University Library, in which they discuss the possibility of their being a habitable planet in the Kepler-16 system.

Kepler 16 is located in the Cygnus Constellation, 196 light years from the Earth. It comprises a K-type orange dwarf star, with a mass of roughly 69% of the Sun's, orbited by an M-type red dwarf star with a mass of roughly 20% of the Sun's, at a distance of 0.224 AU (24.4% of the distance between the Earth and the Sun) every 41 days. Orbiting both these stars is a Saturn-like planet, with a mass 33.3% of that of Jupiter, which is 0.705 AU from the primary star (70.5% of the distance between Earth and the Sun) and takes 229 days to complete a revolution.

An artist's impression of Kepler-16 (AB)b. Image from NASA/JPL-Caltech.

Quarles et al. considered that an Earth-sized planet could exist in six different positions within the Kepler-16 system. It could orbit Kepler-16 A, It could orbit Kepler-16 B, it could orbit Kepler-16 A and B closer than Kepler-16 (AB)b, it could orbit Kepler-16 (AB)b as a moon, it could share an orbit with Kepler-16 (AB) as a Trojan object in either the preceding or trailing Lagrangian Points (the Lagrangian Points are 60° ahead and behind a body in its orbit where its gravity is neutralized by that of the body it orbits; an object in the Lagrangian Points will be in a stable orbit and will remain there) and it could orbit Kepler-16 A and B outside the orbit of Kepler-16 (AB)b.

In order to be habitable a planet needs to lie within the Habitable Zone (sometimes called the Goldilocks Zone) of it's star, roughly speaking the zone in which water will remain a liquid (which, as far as we can tell, is the single most important factor in making a planet habitable). In our own solar system the Earth sits rather neatly in the Habitable Zone, but scientists have recently realized that a planet with a denser or more CO₂ rich atmosphere than the Earth could be father away and still retain liquid water due to the warming greenhouse effect (the reverse does not hold, on a planet with a thiner atmosphere closer to the sun, water would sublimate directly from ice to vapor, making the planet uninhabitable to life as we understand it). This Extended Habitable Zone is thought to extend to 2 AU in our solar system, twice as far from the Sun as the Earth, and 25% beyond the orbit of Mars.

Although the Kepler-16 system has two stars, their combined mass is only 89% of that of the sun, and they are both considerably cooler. This means that the Habitable Zone is closer to the centre of the system than in ours, estimated by Quarles et al. to extend from 0.36 to 0.71 AU from the centre of the system. They estimate the Extended Habitable Zone to extend a bit further, to about 1.02 AU. Interestingly the planet, Kepler-16 (AB)b lies on the outer limit of the standard Habitable Zone, but this is purely co-incidental.

Quarles et al. calculated that a planet orbiting either Kepler-16 A or Kepler-16 B on their own would be inside the habitable zone, too hot to retain liquid water, and therefore unlikely to host life. This scenario was not considered further.

A planet orbiting both stars inside the orbit of Kepler-16 (AB)b could be in the habitable zone, but Quarles et al.'s model calculated that such a planet would be severely effected by the gravity of the larger planet, and almost certainly slung into an eccentric orbit further out in the system within a thousand years.

Model showing the fate of a planet orbiting Kepler-16 A (red) & B (blue) within the orbit of Kepler-16 (AB)b. The planet (black dots) starts within the Habitable Zone (dark grey area), but is thrown further out in the system by the gravity of the larger planet (Planet b, pink dots), ending up in an eccentric orbit that takes it outside of the Extended Habitable Zone (light grey area) for part of the year, and making it unsuitable for life. The scale bars are in AU. From Quarles et al. (2012).

A planet in an orbit outside the orbit of Kepler-16 (AB)b could be in a stable orbit inside the Extended Habitable Zone. Such a planet could host life, but would not be particularly Earth-like, since it would need to be larger to retain a denser atmosphere, or have a more CO₂ rich atmosphere to support a stronger greenhouse effect (actually another greenhouse gas, such as methane, would also do the job).

A planet (black dots) orbiting Kepler-16 A (red) and B (blue) outside the orbit of Kepler-16 (AB)b (pink) would be in a stable orbit, but outside of the standard Habitable Zone (dark grey area). It would still be within the Extended Habitable Zone, so a sufficiently large planet, or one with a strong greenhouse effect, could still sustain life. The scale bars are in AU. From Quarles et al. (2012).

A an Earth-sized moon orbiting Kepler-16 (AB)b would also be potentially habitable. Such an object would be at first surprising to us, we do not think of moons as being large habitable objects. However several moons in our own system are larger than the smaller planets, and one, Titan, retains a substantial atmosphere. An Earth-sized moon orbiting a Saturn-sized planet would actually be smaller in comparison to the planet than our moon is to us. In addition, one possible outcome of the scenario in which a planet orbits inside the orbits of Kepler (AB)b is that this planet would not be thrown into the outer system, but instead captured by the planet as a moon, making this scenario more likely still.

An Earth-sized moon (black dots) orbiting Kepler (AB)b (pink) would be on the border between the system's Habitable Zone and Extended Habitable Zone, and could well support life. The scale bars are in AU. From Quarles et al. (2012).

Possibly the most intriguing scenario is the one in which a planet shares an orbit with Kepler-16 (AB)b as a Trojan object. Like a moon of Kepler-16 (AB)b such a planet would be on the boundary of the Habitable Zone and the Extended Habitable Zone, and could well host life. Such an outcome could also come about through the influence of Kepler-16 (AB)b on a planet in an inner orbit. Strangely Quarles et al. calculate that such a body would not remain in one of the Lagrangian Points, as it would be influenced by the gravity of Kepler-16 B, instead it would progress (move back-and-forth) within the orbit. Such a planet would still be habitable, but would have a fairly odd seasonal cycle.

An Earth-sized planet (black) sharing an orbit with Kepler-16 (AB)b would not remain at a Lagrangian Point, but would instead move back-and forth within the orbit due to the gravitational influence of Kepler-16 (B) (blue). It would remain on the boundary between the Habitable Zone (dark grey) and the Extended Habitable Zone (light grey), and so could harbor life, but would have an unusual seasonal cycle. The scale bars are in AU. From Quarles et al. (2012).