Space found for Earth-like planets

Analysis of all 79 star systems known to have transiting exoplanets has revealed that only two could definitely not support life as we know it, according to astronomers at the Open University whose work will appear in an upcoming edition of Monthly Notices of the Royal Astronomical Society.

Most of the transiting planets (planets that move in front of their star and block some of its light as they orbit) known thus far are hot jupiters, which are gas giants close to their star. Professor Barrie Jones and Dr Nick Sleep of the Open University wondered how these ‘hot jupiters’ would affect Earth-like planets in the habitable zone, which is defined as the region where the surface temperature of a terrestrial planet is suitable for liquid water, essential for life as we know it, to exist on the surface of a planet. To this end they ran computer simulations that assessed whether the gravitational reach of a hot jupiter would penetrate into the habitable zone. “We wanted to see whether an Earth-like planet could exist in the habitable zone without being ejected from its orbit by a nearby giant planet,” Jones tells Astronomy Now.

An artist’s impression of a hot jupiter exoplanet. Image: G Bacon (STScI/AVL).

Another consideration was the evolution of the habitable zone, which moves outwards as the star ages and grows warmer, which led to Jones and Sleep developing two scenarios, one in which a system is habitable now but may not be in the future if the habitable zone moves out, and the other in which planets can sustain habitability for at least 1.7 billion years. This would incorporate the first 0.7 billion years of the planet’s life during its heavy bombardment from comets and asteroids, and then another billion years to give conditions for life a chance to flourish. They found only two systems, HAT-P-13 and HD 80606, that supported neither scenario.

“In one case (HAT-P-13) the outermost of the two giant planets was too close to the habitable zone and would have ejected the Earth-like planet from its orbit,” says Jones. In the second case of HD 80606, a 3.4 Jupiter-mass planet’s eccentric orbit would ride roughshod over any other worlds, destabilising them.

Another 28 systems display stable orbits for potential Earth-like planets in the habitable zone today, but in the past the habitable zone such that the giant planet’s would have destabilised the orbits and life on any planets there would become unsustainable.

However, we shouldn’t forget that the architecture of the vast majority of these planetary systems is nothing like our Solar System, points out Jones. Hot jupiters are not born close around their star, but move inwards from their birthplace further away, and it is possible that their migration (particularly if it is via the Kozai mechanism, see here for more), could disrupt the formation of Earth-like planets. If this is the case, then it could mean that none of the 79 planetary systems studied could support habitable worlds.

Astronomers however remain optimistic, and computer models indicate that disc-driven migration in particular can leave room for Earth-like planets to form after the migration has taken place. All astronomers have to do now is find these worlds. NASA’s Kepler spacecraft will hopefully turn up dozens of Earth-like planets in habitable zones, although the wider their orbit, the less chance we have of seeing them transit unless their orbital plane is aligned almost exactly with us. However, if only a hot jupiter is found around any given star, follow-up radial velocity measurements with the likes of instruments such as the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph at the European Southern Observatory could find the telltale ‘wobble’ of the star caused by unseen planets. Although Jones’ and Sleep’s work does not mean these planets are definitely there, it implies there is a good chance that there are many habitable Earth-like planets out there for us to find.