Twin of Mars?

An exoplanet with the size of Mars: among those of which we have an estimate of the density, Kepler-138 b is by far the youngest. It is located in a planetary system 200 light-years away, towards the constellation of Lyra, and its mass was estimated from changes in transit times.
How many things can tell a short delay, only to know how to interpret? If someone, or something, do not show up on time for appointments, or does not respect the times, our reaction is more likely to be disappointment. A train leaving early, leaving us with the trolley on the tracks makes us justifiably fierce. And what about the worm that creeps in repeated delays without explanation? What unspeakable activities or presence betray? It was a number of discrepancies on the roadmap of Uranus to give rise to the French mathematician Urbain Le Verrier suspicions, in August of 1846, the existence of a secret planet. That was the planet that was actually observed two months later, for the first time, exactly where Le Verrier had predicted Neptune.


Credit: Danielle Futselaar, SETI Institute

Well, it is lining up advances and delays – the so-called TTV (transit timing variation), in the order of tens of minutes – of the three planets orbiting the star Kepler-138, a red dwarf 200 light-years away that Daniel Jontof-Hutter and colleagues were able to deduce the mass of the inner child and the three, the one marked with the letter ‘b’ (the two outer are ‘c’ and ‘d’). And the mass measurement for an exoplanet discovered by the transit method is a crucial step, fraught with consequences. Since the analysis of the transits allows obtaining directly a good estimate of the size of the planet, able to know – this time in an indirect way – the mass makes it possible to calculate the density. In the case of Kepler-138 b, we learn from the pages of Nature, being equal radius and mass, respectively, in 0522 and 0066 times that of the Earth, the density appears to be around 2.6 grams per cubic centimeter. He said otherwise, and even with all the uncertainties of the case (considerable, especially for the estimation of the mass), we are in front of a rocky planet.
Not bad, as a conclusion, taking into account that we started from the sun changes in transit times … a little how to guess, by the delays with which the husband comes home at night, not only that there is a mistress, but also of make assumptions about the size of the rival and of those who might look like. In the case of Kepler-138 b, the immediate comparison is with Mars – with its 3.94 grams per cubic centimeter density is, among the planets of our Solar System, to it as similar to mass and size. Then we must say, however, that the similarities end there: there on Kepler-b 138 a year lasts 10 days of ours, the average temperature – although only as a dim red dwarf – is one that we find in our oven when preparing a pizza and rotation is probably synchronous with the revolution (in the jargon, is tidally locked), so the planet always shows the same face to the star, as the Moon does to the Earth.
It is bad for life, in short, but excellent for the dreams of the astronomers. Of all the extrasolar planets which have a density estimate, Kepler-138 b is by far the smallest. This gives us hope that it is, therefore, the first of a long series of extrasolar planets with a size comparable to that of the Earth, for which, by measuring the variation of transit times, it is possible to obtain reliable estimates of the mass.

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