Thursday, April 18, 2013

The most exciting candidates for habitable exoplanets yet

An international team of scientists analyzing data from NASA's Kepler mission, which includes Lisa Kaltenegger from the Max Planck Institute for Astronomy, has announced the discovery of the first small, potentially rocky Kepler planets, orbiting in the Habitable Zone of their star. The planets, called Kepler-62e and Kepler-62f, orbit a star that is slightly smaller and cooler than our own Sun. Going by their radii, these planets should be rocky, making them our best candidates for habitable planets out there yet. Analysis by Dr. Kaltenegger indicates that both planets lie in their host star's habitable zone – where liquid water is possible, the necessary precondition for life as we know it.

Figure 1: Comparison of the planetary system around the star Kepler-62 with our own Solar System. The relative size of the planetary orbits (top and bottom) is to scale. The planets (center) are also to scale, relative to each other. The habitable zone – the zone around the star that allows for liquid water on the surface of a planet orbiting at that distance – is shown in green. Kepler-62e and Kepler-62f are the best candidates yet for habitable planets: solid planets orbiting their host star in the habitable zone.Credit: NASA [Larger version for download]

Figure 2: Masses and sizes for selected planets. The curves show the mass-radius-relation (average density) for different types of planets: The blue line indicates the loci of planets made mostly (75%) of water, the black line that of planets like our Earth that consist almost exclusively of rock (represented here by the mineral Enstatite, MgSiO3, a member of the pyroxite silicate mineral series that makes up most of the Earth's mantle), and so on. The measured radii of Kepler-62e and Kepler-62f plus an estimate of their mass places them in a region (blue areas) where it is highly probable for them to be earth-like planets, that is: planets with a solid (if possibly covered in water) surface. Kepler-11f, on the other hand, is a Mini-Neptune, showing clearly that a comparatively low mass does not necessarily make for a solid planet.Credit: L. Kaltenegger (MPIA)  [Larger version for download]

Figure 3: The habitable zone (in which liquid water on a planet's surface can exist) for different types of stars. The inner planets of our Solar System are shown on top, with Earth and Mars in the habitable zone. Kepler-62 is a notably cooler star, and Kepler-62e and -62f are in its habitable zone. For Kepler-69c, another planet announced today by NASA, the error bars for the star's radiation are such that it could possibly in the habitable zone as well. Kepler-22b, the smalles planet found in a habitable zone before the recent discoveries, is very likely a Mini-Neptune, and not a solid planet. In what is denoted the empirical habitable zone, liquid water can exist on the surface of a planet if that planet has sufficient cloud cover. In the narrow habitable zone, liquid water can exist on the surface even without the presence of a cloud cover. Credit: L. Kaltenegger (MPIA).  [Larger version for download]

The detection of two planetary systems, around the stars Kepler-62 and Kepler-69, was announced at a press conference at NASA's Ames Research Center in Moffett Field, California, today, Thursday, April 18, at 11 am PDT (8 pm Central European time). Panelists included NASA's director of astrophysics, Paul Hertz, Kepler science principal investigator William Borucki, and Lisa Kaltenegger from the Max Planck Institute for Astronomy in Heidelberg. Kaltenegger, who also holds a position at the Harvard-Smithsonian Center for Astrophysics in Boston, is not a part of the Kepler team, but has been in charge of estimating the potential life-friendliness of the Kepler-62 system.

Kepler-62 in the constellation Lyra, at a distance of 1200 light-years from Earth, is a star slightly smaller and cooler than our own Sun (spectral type K2V, estimated mass 0.7 solar masses, estimated radius 0.63 solar radii). The stellar system is oriented in such a way that, from the point of view of an observer on Earth, its planets periodically pass in front of their host star, blocking a tiny fraction of its light. This is how the Kepler space telescope, which is capable of particularly precise measurements of changes in brightness, detected the telltale signs of five planets that orbit Kepler-62. They are now designated Kepler-62b through Kepler-62f.

One overarching theme of exoplanet research is the search for planets that might be capable of supporting life – a key step towards actually detecting life on other planets. This is where Kepler-62e and Kepler-62f become really interesting. Lisa Kaltenegger, an expert on the atmospheres of exoplanets and the scientist who undertook the habitability analysis for the Kepler-62 system, explains:

»I first heard about this exciting discovery from William Borucki at a conference in October 2012, when the Kepler team asked me to examine whether or not Kepler-62e and Kepler-62f could indeed fall into their star's life-friendly, habitable zone. As it turned out, they do – and they are special because they are the smallest we have yet found inside the habitable zone of a star.«

Kepler-62e has a radius 1.61 times that of the Earth, Kepler-62f 1.41 times. Previously, the smallest planet with known radius inside a habitable zone was Kepler-22b, with a radius of 2.4 times that of the Earth.

»All of the other interesting planets in the Habitable Zone were until now discovered by what is known as the radial velocity method. This method gives you a lower limit for the planet's mass, but no information about its radius. This makes it difficult to assess whether or not a planet is rocky, like the Earth. A small radius (< 2 Earth radii), on the other hand, is a strong indicator that a planet around is indeed rocky – unless we are talking about a planet around a very young star.«

That these planets are indeed rocky, that is, have a solid surface like the Earth, as opposed to gas planets such as Jupiter or Neptune, is a key aspect of the discovery. The most interesting confirmed candidates for habitable planets so far (GJ 667Cc, Gl 581d, HD 85512b, and Gl 163c) were all detected with the radial velocity method, which can only yield a lower limit for a planet's mass. The planet's true mass is likely to lie within a factor 2 of this lower limit. For the previous candidates, this translates to a range of masses that allows for rocky planets as well as miniature versions of Neptune. One planet that is known to be a mini-Neptune, and provides a cautionary example, is Kepler-11f, with a mass around 2.3 times that of the Earth, but 2.6 times the Earth's radius.
»Statements about a planet's habitability always depend on assumptions. Let us assume that the planets Kepler-62e and -62f are indeed rocky, as their radius would indicate. Let us further assume that they have water and their atmospheric composition is similar to that of Earth, dominated by nitrogen, and containing water and carbon dioxide. In that case, both planets could have liquid water on their surface: Kepler-62f gets less radiation energy from its host star than the Earth from the Sun and therefore needs more greenhouse gases, for Instance more carbon dioxide, than Earth to remain unfrozen. Kepler-62e is closer to its star, and needs an increased cloud cover – sufficient to reflect some of the star's radiation – to allow for liquid water on it's surface.«

Whether or not a planet could harbor life is judged by the possibility of liquid water existing on the planet – one of the preconditions for life as we know it. The astronomical criterion of habitability goes one step further and demands the possibility of water on the planet's surface – necessary for future remote observations to be able to detect whatever life might exist on the planet.

Habitability does not imply that the planet will be the spitting image of Earth. Notably, for planets with larger radii than Earth's, as in the case of these two new planets, the same relative chemical make-up would likely result in a water world, covered by a deep global ocean.

Habitability analysis does not prove that the planet in question is indeed habitable, only whether or not, given suitable atmospheric conditions, it could be. Proof positive will be left to future telescope, larger than those currently available, which will be able to analyze (spectroscopically) the "chemical fingerprints" of planetary atmospheres. An important part of the work of Kaltenegger and her collaborators involves model calculations that predict those chemical fingerprints for specific kinds of planets, such as Kepler-62e and Kepler-62f.

Eventually, such observations could even show chemical signatures characteristic of life on another world. Until these direct tests of habitability become available, we can only estimate planetary habitability on the basis of all available data– and on that score, the two newly discovered planets lead the field.

Kaltenegger concludes:
»What makes Kepler-62e and Kepler-62f so exciting is a combination of two factors: we know their radius, which indicates that these are indeed rocky planets, and they orbit their star in the habitable zone. That makes them our best candidates for habitable planets out there yet. And it's been very exciting for me to play a part in this milestone Kepler discovery.«

Background information The discovery paper for the Kepler-62 b system will be published as W. J. Borucki, E. Agol, F. Fressin, L. Kaltenegger et al., "Kepler-62: A five-planet system with planets of 1.4 and 1.6 Earth-radii in the Habitable Zone" in the journal Science.

Contact information

Dr. Lisa Kaltenegger (co-author)
Max Planck Institute for Astronomy
Heidelberg, Germany
Phone: (+49|0) 6221 – 528 446

[For contact data starting Thursday, 18 April, contact M. Pössel, below]

Markus Pössel (public information officer)
Max Planck Institute for Astronomy
Heidelberg, Germany
Phone: (+49|0) 6221 – 528 261