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Scientists love outliers. Outliers are nature\u2019s way of telling us what its boundaries are and where its limits lie. Rather than being upset when an outlier disrupts their understanding, scientists feed on the curiosity that outliers inspire.<\/p>\n
It\u2019s true in the case of a new discovery of a massive planet orbiting a small star. That goes against our understanding of how planets form, meaning our planet-formation model needs an update.<\/p>\n
<\/p>\n In a paper published in Science, researchers announced the discovery of a Neptune-mass exoplanet orbiting a low-mass star. The star is LHS 3154, an M-dwarf, or red dwarf star. It\u2019s only 0.11 times as massive as the Sun, which is a normal mass for a red dwarf.<\/p>\n But what\u2019s surprising is the size of the planet orbiting the star. The planet is called LHS 3154b, and it\u2019s a monster compared to most planets orbiting red dwarfs. It has at least 13.2 Earth masses. That places it in the same range as Neptune, which has 17 Earth masses. LHS 3154b is also in a very close orbit, taking only 3.7 days to orbit the star.<\/p>\n \u201cThis discovery really drives home the point of just how little we know about the universe.\u201d<\/p>\n Suvrath Mahadevan, Penn State University<\/cite><\/p><\/blockquote>\n<\/figure>\n The new paper is \u201cA Neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models.\u201d The lead author is Gudmundur Stefansson, NASA Sagan Fellow in Astrophysics at Princeton University. Stefansson was a graduate student at Penn State while working on this discovery. <\/p>\n \u201cThis discovery really drives home the point of just how little we know about the universe,\u201d said Suvrath Mahadevan, a Professor of Astronomy and Astrophysics at Penn State and co-author of the paper. \u201cWe wouldn\u2019t expect a planet this heavy around such a low-mass star to exist.\u201d<\/p>\n Why is this discovery surprising? It\u2019s all about stars and their protoplanetary disks. <\/p>\n When a star forms, it starts as a protostar in the center of a solar nebula. As the star forms, a rotating disk of gas and dust called a protoplanetary disk forms around the star. Dense knots form in the disk, and this is how planets and planetesimals form. It\u2019s a detailed process and one we don\u2019t entirely understand. But what scientists do know, or thought they knew, is that the more mass there is in the disk, the more massive the planets that can form. And the mass in the disk scales steeply with the mass of the star.<\/p>\n It looks like this: massive star = massive disk = massive planets. Naturally, we consider the obverse to be true, too. Small star = small disk = small planets. But LHS 3154b and its star don\u2019t conform to this. There simply shouldn\u2019t have been enough mass in the protoplanetary disk for the planet to form.<\/p>\n \u201cThe planet-forming disk around the low-mass star LHS 3154 is not expected to have enough solid mass to make this planet,\u201d Mahadevan said. \u201cBut it\u2019s out there, so now we need to reexamine our understanding of how planets and stars form.\u201d<\/p>\n It took a special instrument to spot the massive planet, and Mahadevan led the team of scientists that built it. It\u2019s called the Habitable Zone Planet Finder or HPF, a spectrograph built at Penn State. HPF is designed to detect planets orbiting cool stars that might have liquid surface water. Small planets can be very difficult to detect around large, bright stars like our Sun because the Sun\u2019s light overpowers everything else.<\/p>\n \n\n