This guest post was written by James Bennett, a senior economics major at Amherst College.
This past spring, my class and I studied general astrophysics, with a focus on planet formation and evolution. In my spare time, I enjoy traveling, fishing, and playing sports.
Title: A Neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models
Authors: Stefánsson, Guðmundur; Mahadevan, Suvrath; Miguel, Yamila; Robertson, Paul; Delamer, Megan; Kanodia, Shubham; Cañas, Caleb I.; Winn, Joshua N.; Ninan, Joe P.; Terrien, Ryan C.; Holcomb, Rae; Ford, Eric B.; Zawadzki, Brianna; Bowler, Brendan P.; Bender, Chad F.; Cochran, William D.; Diddams, Scott; Endl, Michael; Fredrick, Connor; Halverson, Samuel; Hearty, Fred; Hill, Gary J.; Lin, Andrea S. J.; Metcalf, Andrew J.; Monson, Andrew; Ramsey, Lawrence; Roy, Arpita; Schwab, Christian; Wright, Jason T.; Zeimann, Gregory
First Author’s Institution: University of Amsterdam
Status: Published in Science [closed access]
Terrestrial planets form from the clouds of gas and dust (protoplanetary disks) that surround newborn stars. Typically, the more massive the star, the more massive the quantities of dust in its disk, the more massive the planets that form. Consistent with this relationship, low-mass M dwarf (red dwarf) stars are theorized to have masses of dust particles in their protoplanetary disk sufficient only to birth compact, rocky, short-period orbit bodies of masses less than 5M (where M = one Earth mass) and potentially long orbit gas giants. As such, in today’s paper, discovery of the M dwarf star LHS 3154 and its accompanying planet LHS 3154 b with a close orbit and estimated mass of 13.2 Earth masses (M) is highly surprising and could imply a problem with current theories on planetary formation.
To begin, while the preliminary telescope measurements only provide a lower limit on the mass, Stefansson’s team is confident that LHS 3154b is a Neptune-mass planet and not a small companion star or brown dwarf because the motion of the star is not visibly affected by the presence of the companion, down to the measurement limits of the Gaia telescope. Having found this, the team compared the mass ratio of LHS 3154 b and its host star to other known M dwarf systems. Figure 1 below depicts this: plotted on the graph are exoplanets–planets outside of the solar system–orbiting M dwarf stars with masses less than 0.25M. The x-axis is the orbital period, and the y-axis is the mass ratio of the planet compared to the star. As expected, the mass ratio of LHS 3154 and LHS 3154 b is significantly higher than other similar systems. With a minimum expected mass of 13.2M (slightly less massive than Neptune), LHS 3154 b is difficult to explain with core accretion models, where dust accumulates due to gravitational attraction and forms terrestrial bodies, because there shouldn’t be enough dust particles in an M dwarf protoplanetary disk to enable the creation of such a massive planet. This is like the following situation baking a cake. Suppose a baker makes a cake with 2 kilograms of ingredients. After baking the cake, however, it weighs 10 kilograms. Like LHS 3154 b, a cake of this mass should not be possible given the mass of ingredients available pre-formation.
The sample is restricted to planet mass measurements (m) with an uncertainty smaller than 30% and host star masses less than 0.25 times the mass of the Sun. The circles represent transiting planets (planets crossing a star such that we can measure their radii using a telescope) with known masses. Triangles represent planets detected with the radial velocity technique, which calculates only a lower mass limit. Colors indicate the host star temperature. LHS 3154b is represented in red. Figure 2 in the paper.
To test possibilities for the creation of LHS 3154 b, the authors of today’s paper built a series of repeated simulations for protoplanetary disks with different combinations of surface area and dust particle masses. The results of the simulations are depicted in Figure 2 below, where the bottom row assumes a compact disk which would promote accretion due to a smaller space with high concentration of dust, and the right column assumes a greater mass of dust. The x-axis is orbital distance from the star, and the y-axis is the mass of the accompanying planet compared to LHS 3154 b. The first set of simulations assumed 0.8M of dust and a low nominal surface density distribution. These simulations did not produce any planets with the mass and close orbit of LHS 3154 b (shown by the lack of dots near the yellow triangle in panel A). The second set of simulations increased the disk dust mass to 8M and compacted the disk to a smaller surface area. These simulations produced a few planets with the qualities of LHS 3154 b (shown by the dots in the gray box near the yellow triangle in panel D). The larger amounts of solid material confined to a tighter surface area disk on average produced larger cores and thus more massive planets. Interestingly, a disk with these properties has not been observed around M dwarf stars before. However, simulations with a more compact disk without higher mass of dust particles did not tend to yield planets with the same properties as LHS 3154 b (shown by panels B and C).
Planet mass is shown as a function of orbital distance in astronomical units (au). Circles indicate results from simulated systems, 300 in each panel, colored by the disk dust mass in that simulation. LHS 3154b is indicated with an orange arrow in each panel, with its base indicating the minimum mass of 13.2M. Gray boxes denote the region and corresponding frequencies of short-orbit Neptune-mass planets formed in each simulation. (A) results with a the protoplanetary disk dust mass of 0.25 times the mass of the Sun and a disk of expected radius and surface area. (B) Same as in (A) but with dust mass of 8 times the mass of the Sun. (C) Same as in (A) but with a more compact protoplanetary disk. (D) Same as in (B) but with a more compact protoplanetary disk. Typical assumptions of planet formation as depicted in (A) are incapable of forming planets at massive as LHS 3154b. To form such a planet requires us to increase the mass of the dust on the protoplanetary disk (B), preferably on a more compact surface area disk (D). Figure 3 in the paper.
Ultimately, the results of this simulation imply that the formation of a planet like LHS 3154 b would require a protoplanetary disk with a substantially greater dust mass than those that are typically observed for M dwarfs. One potential source of observation bias could be that imperfect measurement systems of dust masses on protoplanetary disks fail to account for dust grains larger than a few millimeters and so underestimate dust masses. Another possibility is that these disks accrete material from surrounding parent molecular clouds. A third possibility suggests that, if planetary cores form fast enough (i.e. within 1 million years of the protostar birth), the protoplanetary disk is more massive than at later times, and so the formation of a massive rocky core could gravitationally attract gas particles to eventually develop a gas giant (gas giants are theorized to form as a rocky core attracts gas from the edge of a protoplanetary disk, leading to a gas envelop around a solid core). While these possibilities could explain the existence of LHS 3154 b and its host star, more theoretical and practical research will be needed to better understand the findings and whether they limit the effectiveness of core accretion models of planet formation
Astrobite edited by Sonja Panjkov
Featured image credit: NASA/JPL-Caltech
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