A Statistical Analysis of The Extrasolar Planets and The Low-Mass Secondaries

TL;DR

This paper analyzes Hipparcos data to study the mass, period, and eccentricity distributions of exoplanets and low-mass stellar companions, revealing distinct populations and similarities in their orbital characteristics.

Contribution

It demonstrates that Hipparcos data are insufficient for orbit determination, establishes the mass distribution of planet candidates, and compares properties of planets and stellar secondaries.

Findings

Mass distribution of planet candidates is flat in log M up to 10 Jupiter masses.

Two populations—giant planets and stellar secondaries—are separated by the brown-dwarf desert.

Similar period and eccentricity distributions suggest common formation mechanisms.

Abstract

We show that the astrometric Hipparcos data of the stars hosting planet candidates are not accurate enough to yield statistically significant orbits. Therefore, the recent suggestion, based on the analysis of the Hipparcos data, that the orbits of the sample of planet candidates are not randomly oriented in space, is not supported by the data. Assuming random orientation, we derive the mass distribution of the planet candidates and show that it is flat in log M, up to about 10 Jupiter masses. Furthermore, the mass distribution of the planet candidates is well separated from the mass distribution of the low-mass companions by the 'brown-dwarf desert'. This indicates that we have here two distinct populations, one which we identify as the giant planets and the other as stellar secondaries. We compare the period and eccentricity distributions of the two populations and find them surprisingly similar. The period distributions between 10 and 1650 days are flat in log period, indicating a scale-free formation mechanism in both populations. We further show that the eccentricity distributions are similar - both have a density distribution peak at about 0.2-0.4, with some small differences on both ends of the eccentricity range. We present a toy model to mimic both distributions. We found a significant paucity of massive giant planets with short orbital periods. The low frequency of planets is noticeable for masses larger than about 1 Jupiter Mass and periods shorter than 30 days. We point out how, in principle, one can account for this paucity.