Monday, January 4, 2016

Forming the Five Small Rocky Worlds of Kepler-444

Kepler-444 is a planetary system hosting five sub-Earth-sized planets in orbit around a metal-poor Sun-like star. The five planets are in an extremely compact configuration, with the outermost planet orbiting at a distance of only 0.081 AU. For comparison, the average distance of Mercury from the Sun is 0.387 AU. The host star of the Kepler-444 planetary system is estimated to have 0.758 ± 0.043 times the mass and 0.752 ± 0.014 times the radius of the Sun.

Figure 1: Artist’s impression of a rocky planet.

Not far from Kepler-444, at a projected separation of 66 AU is a tightly-bound pair of red dwarf stars identified as Kepler-444BC. Both red dwarf stars are separated from each other by no more than ~0.3 AU and the red dwarf stars are estimated to have 0.29 ± 0.03 and 0.25 ± 0.03 times the Sun’s mass, respectively. If Kepler-444BC orbits Kepler-444 in a circular orbit at 66 AU, then the orbital motion of Kepler-444BC is expected to be 4.2 km/s. In terms of astrometric motion, this is 25 milli-arcseconds per year. An arcsecond is a unit of measurement for angular separation, whereby 60 arcseconds constitute an arcminute and 60 arcminutes constitute a degree.

However, measurements of the astrometric motion of Kepler-444BC indicate a value of only 1.0 ± 0.3 milli-arcseconds per year. This implies that either Kepler-444BC is near the furthest end of a highly eccentric orbit around Kepler-444, or the actual distance of Kepler-444BC from Kepler-444 is much further than the projected separation of 66 AU. Radial velocity measurements show that Kepler-444 is gravitationally “tugged” too strongly by Kepler-444BC and this makes the large separation scenario unlikely.

Kepler-444BC is in a highly eccentric orbit around Kepler-444. The orbital eccentricity is estimated to be 0.864 ± 0.023 and the orbital period is roughly 200 years. The highly eccentric orbit of Kepler-444BC brings it as close as ~5 AU to the planetary system around Kepler-444. Dynamical analysis indicates that the close passage of Kepler-444BC will not disrupt the planetary system around Kepler-444.

The orbit of Kepler-444BC is expected to be primordial, already in place before or during the epoch of planet formation. As a result, the protoplanetary disk around Kepler-444 is expected to have been truncated to within ~2AU due to the close passage of Kepler-444. Furthermore, Kepler-444 is a metal-poor star with a low abundance of heavy elements. These factors severely deplete the amount of solid material available for planet formation and can explain why the five planets around Kepler-444 are less massive than Earth.

All five planets around Kepler-444 have a total mass of only ~1.5 times the mass of Earth. Kepler-444 appears to be a hostile environment for the formation of planets given the low metallicity of its host star and the disrupting presence of the nearby Kepler-444BC. Yet the presence of five sub-Earth-sized planets seems to suggest that the formation process of such planets is quite robust.

Figure 2: The orbit of Kepler-444BC in the frame of the host star (black star) of the Kepler-444 planetary system. The best-fit orbit is shown in black, and 100 randomly drawn orbits from the analysis are shown in gray. Orbit locations that correspond to the range of the observation epochs are shown in red. Left: the orbit in plane of the sky, which is consistent with being seen edge on. Right: the same orbit shown in a top down view of the orbital plane. Kepler-444BC is currently close to the furthest end of its highly eccentric orbit, with almost no motion in the plane of the sky. Dupuy et al. (2015)

Dupuy et al. (2015), “Orbital Architectures of Planet-Hosting Binaries: I. Forming Five Small Planets in the Truncated Disk of Kepler-444A”, arXiv:1512.03428 [astro-ph.EP]