Smallest terrestrial exoplanet so far: 1.4 Earth masses
Following up a 2008 discovery of an exoplanet massing about 3 Earths, astronomers have refined that estimate to 1.4 Earths… the smallest planet discovered so far around a normal star. The planet, MOA-2007-BLG-192-L b, was originally thought to orbit a brown dwarf, but the newer data has refined that estimate upwards to a red dwarf, making it an actual star instead of a gigantic sub-fusion gas giant.
One and a half Earths is small, and clearly terrestrial. The diameter of such a world – as long as it’s composed of the expected silicate and light element components – would be about the same as Earth, with only slightly stronger surface gravity. If the planet has a lower density then Earth – say it’s made mostly of ices with only a small silicate mantle, for example – it would be somewhat larger than Earth and probably have a surface gravitational pull equivalent to our own world. No one knows that much about the world, yet. But either way, it’s Earth-sized for all intents and purposes.
This is a strong step forward. This discovery was made using gravitational microlensing techniques that take advantage of a closer star positioned between the red dwarf MOA-2007-BLG-192-L a and our Sun. The intermediate star’s gravity well warps light from the more distant star, and if the stars are right – as they say – the gravitational lensing effect helps to improve our observations of the distant object. Also it’s easier to detect terrestrial planets around red dwarf stars, because the signal to noise ratio is better than for larger stars.
Red dwarfs might or might not be likely abodes of life…. they’re low energy stars, but they make up for that by their stability and ridiculously long lifespans. A planet huddling close enough to keep liquid water on its surface would enjoy the bioevolutionary benefits of potentially trillions of years of stellar quietude… far longer than most planets’ interior radiogenic heat would last. As long as the star hosts enough metals to make rocky planets that can sustain interesting chemistry and tectonics, life is just as likely there as any other terrestrial world circling a brighter, shorter-lived sun. Perhaps more likely, because a red giant primary would be more likely to offer a steady, monotonous supply of light and heat for a very long time.