Birth of a Compact Multi-Planet System

Osorio et al. (2016) present the discovery of a miniature protoplanetary disk around the star XZ Tau B. This discovery was made from observations using the Atacama Large Millimeter/Submillimeter Array (ALMA). XZ Tau B is a young red dwarf star estimated to be only ~4.6 million years old. It is located ~450 light years away, and it has ~1.2 times the radius and ~0.37 times the mass of the Sun. The large radius of the star indicates that it is still in the process of contracting to its final radius. The estimated effective temperature of XZ Tau B is 3550 K.

The protoplanetary disk around XZ Tau B extends out to ~3.4 AU from the host star. It also has a central cavity extending from the host star out to ~1.3 AU. This central cavity has been attributed to the presence of a compact system of newly-formed planets in orbit around the host star. Such a compact multi-planet system is consistent with observations by NASA's Kepler mission which has revealed that low-mass compact multi-planet systems are relatively common. In these planetary systems, the planets orbit within ~1 AU from their host star and have masses ranging from a fraction to a few times the mass of Earth.

The newly-formed planets hypothesised around XZ Tau B are likely to be super-Earths rather than sub-Earths. This is because the snowline around XZ Tau B is located ~0.5 AU from the host star, which places it well within the central cavity. The snowline is basically the distance from a star where temperatures start to become cool enough in the protoplanetary disk for volatiles such as water to condense into solid ice grains, thereby providing more planet-building material. Forming planets beyond the snowline increases the likelihood for more massive planets.

Reference:
Osorio et al. (2016), "A Dwarf Protoplanetary Disk around XZ Tau B", arXiv:1606.03118 [astro-ph.SR]