This exoplanet challenges our understanding of planet formation

The discovery of yet another exoplanet is no longer news. More than 4,000 planets around other stars have now been found since the detection of the first one in 1995. As astronomers long suspected, or at least hoped, it seems that planets are ubiquitous in stellar systems and there are probably more planets than stars in our galaxy.

But a new discovery of a large planet orbiting the small star GJ3512 is worth noting. The paper, published in Science, challenges our understanding of how planets form – and further blurs the line between small, cool stars known as brown dwarfs and planets.

The star itself is a red dwarf, about 30 light years away, with a luminosity less than 0.2% that of the sun. It has around 12% of the sun’s mass and 14% of its radius. Such cool, dim stars are in fact the most common stars in the galaxy, but only one in ten of the known exoplanets have been found to orbit red dwarfs.

This is likely to be a selection effect. Red dwarfs are so dim that it is hard to detect their planets with the “Doppler shift method”. This relies on detecting how the wavelength of the starlight gets periodically shifted (to blue or red) by a tiny amount as the unseen planet orbits, tugging the star to and fro. Several of the other planets that have been discovered orbiting red dwarf stars have instead been found by the transit method – looking at how a star’s light dims as a planet passes in front of it.