MOA-2013-BLG-605Lb: a Neptune Analog Around an M Dwarf

The First Cold Neptune Analog Exoplanet: MOA-2013-BLG-605Lb

Authors:

Sumi et al

Abstract:

We present the discovery of the first Neptune analog exoplanet, MOA-2013-BLG-605Lb. This planet has a mass similar to that of Neptune or a super-Earth and it orbits at 9∼14 times the expected position of the snow-line, asnow, which is similar to Neptune's separation of 11asnow from the Sun. The planet/host-star mass ratio is q=(3.6±0.7)×10−4 and the projected separation normalized by the Einstein radius is s=2.39±0.05. There are three degenerate physical solutions and two of these are due to a new type of degeneracy in the microlensing parallax parameters, which we designate "the wide degeneracy". The three models have (i) a Neptune-mass planet with a mass of Mp=21+6−7Mearth orbiting a low-mass M-dwarf with a mass of Mh=0.19+0.05−0.06M⊙, (ii) a mini-Neptune with Mp=7.9+1.8−1.2Mearth orbiting a brown dwarf host with Mh=0.068+0.019−0.011M⊙ and (iii) a super-Earth with Mp=3.2+0.5−0.3Mearth orbiting a low-mass brown dwarf host with Mh=0.025+0.005−0.004M⊙. The 3-D planet-host separations are 4.6+4.7−1.2 AU, 2.1+1.0−0.2 AU and 0.94+0.67−0.02 AU, which are 8.9+10.5−1.4, 12+7−1 or 14+11−1 times larger than asnow for these models, respectively. The Keck AO observation confirm that the lens is faint. This discovery suggests that Neptune-like planets orbiting at ∼11asnow are quite common. They may be as common as planets at ∼3asnow, where microlensing is most sensitive, so processes similar to the one that formed Uranus and Neptune in our own Solar System may be quite common in other solar systems.

The Differences Between Ice and Gas Giant Formation

Separating gas-giant and ice-giant planets by halting pebble accretion

Authors:

Lambrechts et al

Abstract:

In the Solar System giant planets come in two flavours: 'gas giants' (Jupiter and Saturn) with massive gas envelopes and 'ice giants' (Uranus and Neptune) with much thinner envelopes around their cores. It is poorly understood how these two classes of planets formed. High solid accretion rates, necessary to form the cores of giant planets within the life-time of protoplanetary discs, heat the envelope and prevent rapid gas contraction onto the core, unless accretion is halted. We find that, in fact, accretion of pebbles (~ cm-sized particles) is self-limiting: when a core becomes massive enough it carves a gap in the pebble disc. This halt in pebble accretion subsequently triggers the rapid collapse of the super-critical gas envelope. As opposed to gas giants, ice giants do not reach this threshold mass and can only bind low-mass envelopes that are highly enriched by water vapour from sublimated icy pebbles. This offers an explanation for the compositional difference between gas giants and ice giants in the Solar System. Furthermore, as opposed to planetesimal-driven accretion scenarios, our model allows core formation and envelope attraction within disc life-times, provided that solids in protoplanetary discs are predominantly in pebbles. Our results imply that the outer regions of planetary systems, where the mass required to halt pebble accretion is large, are dominated by ice giants and that gas-giant exoplanets in wide orbits are enriched by more than 50 Earth masses of solids.