View Full Version : “Monster Planet” Discovered, Makes Scientists Rethink Theories of Planetary Formation

2017-Nov-01, 07:10 PM
The discovery of a Jupiter-sized planet around a red dwarf star has caused astronomers to rethink some of their theories on planet formation.
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Jeff Root
2017-Nov-01, 07:42 PM
A large planet forming with a small star doesn't surprise me at all.
Binary stars form. Two stars roughly the same size. If the growth of
such a pair of stars is interrupted by stellar winds from a young and
energetic star nearby, the result is two undersized stars. Or a small
star and large planet. Or two planet-size bodies without any star.

-- Jeff, in Minneapolis

2017-Nov-01, 09:37 PM
What Jeff said.

Reality Check
2017-Nov-02, 12:08 AM
The paper is the discovery of the third transiting gas giant around an M-dwarf: "To date only two gas giant planets have been discovered transiting M-dwarfs - namely Kepler45b (Mp=0.505MJ, Rp=0.96RJ Johnson et al. 2012) and HATS-6b (Mp=0.32MJ, Rp=1.00RJ Hartman et al. 2015)." Section 4.1 is " Giant Planet Formation around M-dwarfs" and states "Planet formation theory suggests that giant planets should be rarer around M-dwarfs than around FGK stars (Kennedy & Kenyon 2008).".
The reference is Planet formation around stars of various masses: The snow line and the frequency of giant planets (https://arxiv.org/abs/0710.1065)

We use a semi-analytic circumstellar disk model that considers movement of the snow line through evolution of accretion and the central star to investigate how gas giant frequency changes with stellar mass. The snow line distance changes weakly with stellar mass; thus giant planets form over a wide range of spectral types. The probability that a given star has at least one gas giant increases linearly with stellar mass from 0.4 M_sun to 3 M_sun. Stars more massive than 3 M_sun evolve quickly to the main-sequence, which pushes the snow line to 10-15 AU before protoplanets form and limits the range of disk masses that form giant planet cores. If the frequency of gas giants around solar-mass stars is 6%, we predict occurrence rates of 1% for 0.4 M_sun stars and 10% for 1.5 M_sun stars. This result is largely insensitive to our assumed model parameters. Finally, the movement of the snow line as stars >2.5 M_sun move to the main-sequence may allow the ocean planets suggested by Leger et. al. to form without migration.
The host star of NGTS-1b (https://en.wikipedia.org/wiki/NGTS-1b) is a red dwarf with a mass of 0.617 solar masses giving a greater than 1% occurrence rate (assuming 6% around solar-mass stars) of a ~Jupiter mass gas giant.
Finding another gas giant around a M-dwarf should not have been a complete surprise or labeled as impossible in the press release.