by ruprecht147 » Thu Dec 08, 2011 9:12 pm
A preprint of the discovery paper for Kepler-22b, by William J. Borucki and colleagues, is now available at archive.org. This paper demonstrates that Kepler-22 is a very interesting planetary system, but it also reveals that the hype we’ve been hearing about a “second Earth” or an “Earth twin” is largely or entirely a lot of media blather.
The star, Kepler-22, is a G5 dwarf similar to our Sun in mass, radius, and effective temperature. However, its metallicity (proportion of heavy elements to hydrogen) is only -0.29, whereas the metallicity of our Sun is 0. That means that the protoplanetary nebula where the Kepler-22 system formed most likely had a much smaller quantity of rocks and metals than the nebula in which the Solar System formed. Such an environment is not conducive to the formation either of gas giants like Jupiter or of massive rocky planets (i.e., scaled-up versions of Earth or Venus).
The radius and orbital period of Kepler-22b are well determined, so we know its size (2.38 times the radius of Earth) and its approximate temperature. However, as the discovery team notes, this information places virtually no constraints on the composition and internal structure of Kepler-22b.
According to the widely used models of Jonathan Fortney and colleagues (“Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation,” 2007), if Kepler-22b had exactly the same structure and composition as the Earth, it would be more than 32 times as massive as Earth. In other words, it would be equivalent to the combined masses of Uranus and Neptune, except that this mass would be squeezed into a much smaller and denser volume. Because the parent star’s low metallicity rules out such a composition, we can safely conclude that Kepler-22b is not a rocky Super Earth, and thus not our big sister in the Galactic family of planets.
Again, according to Fortney’s models, a more physically plausible composition for a planet of 2.38 Earth radii would include at least 50% ice (along with rocks & metals) and yield a planet mass in the vicinity of 6 to 10 Earth masses. Kepler-22b would then be a scaled-up (and warmed-up) version of Ganymede or Titan, with the potential for a very deep global ocean surrounding an even deeper layer of ice. After the landmark study of A. Leger and colleagues (2004), such objects are typically called Ocean Planets. Whether such a planet would permit the genesis of living organisms (or make a congenial home for our descendants in the far future, a la Costner) is anybody’s guess, but the result would not be an Earthlike environment.
A fascinating and very recent study by Leslie Rogers and colleagues (“Formation and Structure of Low-Density Exo-Neptunes,” 2011) demonstrates that planets with structures similar to Neptune and Uranus – rock/ice cores with deep hydrogen atmospheres – can be formed with radii as small as 2 times Earth. At the orbital distance of Kepler-22b, such a Mini-Neptune could retain its hydrogen envelope for billions of years, and yet be only a few times heavier than Earth. As far as I understand, the atmospheric pressure implied by such an envelope would not permit the survival of liquid water, but others with a better knowledge of these things may wish to chime in.
In any event, it is far more likely that Kepler-22b is a Mini-Neptune, as theorized by Rogers et al., or an Ocean Planet, as theorized by Leger et al., than it is a sibling to our beautiful home. But the Kepler mission still has a long way to go, and we can be sure that many more fascinating planets will be revealed on this page over the next few years.
A preprint of the discovery paper for Kepler-22b, by William J. Borucki and colleagues, is now available at archive.org. This paper demonstrates that Kepler-22 is a very interesting planetary system, but it also reveals that the hype we’ve been hearing about a “second Earth” or an “Earth twin” is largely or entirely a lot of media blather.
The star, Kepler-22, is a G5 dwarf similar to our Sun in mass, radius, and effective temperature. However, its metallicity (proportion of heavy elements to hydrogen) is only -0.29, whereas the metallicity of our Sun is 0. That means that the protoplanetary nebula where the Kepler-22 system formed most likely had a much smaller quantity of rocks and metals than the nebula in which the Solar System formed. Such an environment is not conducive to the formation either of gas giants like Jupiter or of massive rocky planets (i.e., scaled-up versions of Earth or Venus).
The radius and orbital period of Kepler-22b are well determined, so we know its size (2.38 times the radius of Earth) and its approximate temperature. However, as the discovery team notes, this information places virtually no constraints on the composition and internal structure of Kepler-22b.
According to the widely used models of Jonathan Fortney and colleagues (“Planetary Radii across Five Orders of Magnitude in Mass and Stellar Insolation,” 2007), if Kepler-22b had exactly the same structure and composition as the Earth, it would be more than 32 times as massive as Earth. In other words, it would be equivalent to the combined masses of Uranus and Neptune, except that this mass would be squeezed into a much smaller and denser volume. Because the parent star’s low metallicity rules out such a composition, we can safely conclude that Kepler-22b is not a rocky Super Earth, and thus not our big sister in the Galactic family of planets.
Again, according to Fortney’s models, a more physically plausible composition for a planet of 2.38 Earth radii would include at least 50% ice (along with rocks & metals) and yield a planet mass in the vicinity of 6 to 10 Earth masses. Kepler-22b would then be a scaled-up (and warmed-up) version of Ganymede or Titan, with the potential for a very deep global ocean surrounding an even deeper layer of ice. After the landmark study of A. Leger and colleagues (2004), such objects are typically called Ocean Planets. Whether such a planet would permit the genesis of living organisms (or make a congenial home for our descendants in the far future, a la Costner) is anybody’s guess, but the result would not be an Earthlike environment.
A fascinating and very recent study by Leslie Rogers and colleagues (“Formation and Structure of Low-Density Exo-Neptunes,” 2011) demonstrates that planets with structures similar to Neptune and Uranus – rock/ice cores with deep hydrogen atmospheres – can be formed with radii as small as 2 times Earth. At the orbital distance of Kepler-22b, such a Mini-Neptune could retain its hydrogen envelope for billions of years, and yet be only a few times heavier than Earth. As far as I understand, the atmospheric pressure implied by such an envelope would not permit the survival of liquid water, but others with a better knowledge of these things may wish to chime in.
In any event, it is far more likely that Kepler-22b is a Mini-Neptune, as theorized by Rogers et al., or an Ocean Planet, as theorized by Leger et al., than it is a sibling to our beautiful home. But the Kepler mission still has a long way to go, and we can be sure that many more fascinating planets will be revealed on this page over the next few years.