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Kinds of Super-Earths

As we search for alien life on Earths and Super-Earths out there, we would focus on planets that are potentially habitable. Two types of Super-Earths would likely end up on our search list: dry, rocky planets and water worlds.

Even though 71% of the Earth's surface is covered by ocean water, it would appear to be a dry, rocky planet when measured from afar. The enormous amount of water contained in our oceans is only 0.02% of the total mass of the Earth. We calculate the density of distant planets from our measurements of their mass and radius, and the small amount of water present on Earth is not detectable by a simple measurement of the bulk mean density of our planet. Therefore, the dry, rocky Super-Earths are planets that would most resemble Earth in our initial search database. The water worlds, as detected by mean density, will have global oceans. We need to evaluate whether a global ocean would be a suitable environment for the origins of a biosphere.

Here is a short portrait gallery of Super-Earths we have discovered and measured to-date, which are likely to end up on our search list. Earth is included for size comparison. These are artist renditions, not photographs of the planets.

Of these, Kepler-22b is most likely to be a water world - it appears to have too large of a size for its mass to be rocky. We are still exploring the properties of the others. Among them Kepler-62e appears to be in the most comfortable orbit inside the habitable zone of its star, not near the outer or inner borders of the zone. Let’s visit some of these worlds and describe them to the best of our current knowledge.

The star Kepler-62 is known to have 5 planets, all transiting, with two of them in its habitable zone. Because Kepler-62 is a star smaller and cooler than the Sun, its habitable zone is closer in, and the orbital periods of the potentially habitable planets are correspondingly shorter than that of Earth. Exoplanet Kepler-62e orbits in 122 days and is near the warm inner edge of the habitable zone. Kepler-62f orbits in 267 days and is near the colder outer edge. Both exoplanets are super-Earths with 60% and 40% larger size than Earth, respectively. They are likely to be solid planets in bulk, with significant water content, but until we measure their masses we will not know for sure. Kepler-62e would be a warm, humid and cloudy place with an endless ocean (no continents or islands composed of rocks); Kepler-62f could be mostly covered with ice, unless there is a significant amount of greenhouse gases in its atmosphere.

In April 2014 the Kepler team announced another potentially habitable exoplanet - Kepler-186f. This is the first Earth-size exoplanet in a habitable zone, with a size of 1.11 +/- 0.14 Earth radius. The star is a very small M-dwarf, less than half the radius of the Sun.

Exoplanet Kepler-186f is near the outer cold edge of the habitable zone; it receives only one third of the illumination and heat that Earth receives from our Sun. Kepler-186f might be rocky, given it small size, but we still have no estimate of the planet mass so we can't be sure. The amount of greenhouse gasses in its atmosphere would have to be very significant in order to keep it from being completely frozen.

Kepler-186f has an orbital period of 130 days. Its four known companion planets, all transiting and Super-Earth size, have orbital periods between 3.9 and 22.4 days. Super-Earths are common in proximity to stars smaller than the Sun, both planetary systems Kepler-62 and Kepler-186 are very rich in them.

Currently, there are only a few super Earths with measured masses whose densities indicate a similar composition to that of Earth. These rocky Super Earths, however, are very close to their stars and are very hot. The record holder is Kepler-78b, at an 8.5-hour orbit, hotter than 2,500 K on its surface - nothing Earth-like when it comes to its surface conditions. But with a mass of about 1.7 Earth mass and a size of 1.12 Earth radius, Kepler-78b is the closest analog to Earth in terms of bulk interior composition. Other similar super-Earths are Kepler-10b, CoRoT-7b, and Kepler-36b.

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