Daniel Grether

How Dry is the Brown Dwarf Desert? Quantifying the Relative Number of Planets, Brown Dwarfs, and Stellar Companions around Nearby Sun-like Stars

Sun-like stars have stellar, brown dwarf, and planetary companions. To help constrain their formation and migration scenarios, we analyze the close companions (orbital period <5 yr) of nearby Sun-like stars. By using the same sample to extract the relative numbers of stellar, brown dwarf, and planetary companions, we verify the existence of a very dry brown dwarf desert and describe it quantitatively. With decreasing mass, the companion mass function drops by almost 2 orders of magnitude from 1 M☉ stellar companions to the brown dwarf desert and then rises by more than an order of magnitude from brown dwarfs to Jupiter-mass planets. The slopes of the planetary and stellar companion mass functions are of opposite sign and are incompatible at the 3 σ level, thus yielding a brown dwarf desert. The minimum number of companions per unit interval in log mass (the driest part of the desert) is at M = 31MJ. Approximately 16% of Sun-like stars have close (P < 5 yr) companions more massive than Jupiter: 11% ± 3% are stellar, <1% are brown dwarf, and 5% ± 2% are giant planets. The steep decline in the number of companions in the brown dwarf regime, compared to the initial mass function of individual stars and free-floating brown dwarfs, suggests either a different spectrum of gravitational fragmentation in the formation environment or post-formation migratory processes disinclined to leave brown dwarfs in close orbits.

What Fraction of Sun-like Stars Have Planets?

The radial velocities of ~1800 nearby Sun-like stars are currently being monitored by eight high-sensitivity Doppler exoplanet surveys. Approximately 90 of these stars have been found to host exoplanets massive enough to be detectable. Thus, at least ~5% of target stars possess planets. If we limit our analysis to target stars that have been monitored the longest (~15 years), ~11% possess planets. If we limit our analysis to stars monitored the longest and whose low surface activity allows the most precise velocity measurements, ~25% possess planets. By identifying trends of the exoplanet mass and period distributions in a subsample of exoplanets less biased by selection effects and linearly extrapolating these trends into regions of parameter space that have not yet been completely sampled, we find that at least ~9% of Sun-like stars have planets in the mass and orbital period ranges M sin i > 0.3MJup and P 0.1MJup and P < 60 years. Even this larger area of the log(mass)-log(period) plane is less than 20% of the area occupied by our planetary system, suggesting that this estimate is still a lower limit to the true fraction of Sun-like stars with planets, which may be as large as ~100%.

A Comprehensive Comparison of the Sun to Other Stars: Searching for Self-Selection Effects

If the origin of life and the evolution of observers on a planet is favored by atypical properties of a planet’s host star, we would expect our Sun to be atypical with respect to such properties. The Sun has been described by previous studies as both typical and atypical. In an effort to reduce this ambiguity and quantify how typical the Sun is, we identify 11 maximally independent properties that have plausible correlations with habitability and that have been observedby,orcanbederivedfrom,sufficientlylarge,currentlyavailable,andrepresentativestellarsurveys.Bycomparing solar values for the 11 properties to the resultant stellar distributions, we make the most comprehensive comparison of the Sun to other stars. The two most atypical properties of the Sun are its mass and orbit. The Sun is more massive than 95% � 2% of nearby stars, and its orbit around the Galaxy is less eccentric than 93% � 1% of FGK stars within 40 pc. Despite these apparently atypical properties, a � 2 analysis of the Sun’s values for 11 properties, taken together, yields a solar � 2 � ¼ 8:39 � 0:96. If a star is chosen at random, the probability that it will have a lower value (i.e., be more typical) than the Sun, with respect to the 11 properties analyzed here, is only 29% � 11%. These values quantify, and are consistent with, the idea that the Sun is a typical star. If we have sampled all reasonable properties associated with habitability, our result suggests that there are no special requirements for a star to host a planet with life.

THE METALLICITY OF STARS WITH CLOSE COMPANIONS

We examine the relationship between the frequency of close companions (stellar and planetary companions with orbital periods <5 yr) and the metallicity of their Sun-like (~FGK) hosts. We confirm and quantify a ~4 σ positive correlation between host metallicity and planetary companions. We find little or no dependence on spectral type or distance in this correlation. In contrast to the metallicity dependence of planetary companions, stellar companions tend to be more abundant around low-metallicity hosts. At the ~2 σ level, we find an anticorrelation between host metallicity and the presence of a stellar companion. After dividing our sample into FG and K subsamples, we find a negligible anticorrelation in the FG subsample and a ~3 σ anticorrelation in the K subsample. A kinematic analysis suggests that this anticorrelation is produced by a combination of low-metallicity, high-binarity thick-disk stars and higher metallicity, lower binarity thin-disk stars.

The observational case for Jupiter being a typical massive planet.

We identify a subsample of the recently detected extrasolar planets that is minimally affected by the selection effects of the Doppler detection method. With a simple analysis we quantify trends in the surface density of this subsample in the period-Msin(i) plane. A modest extrapolation of these trends puts Jupiter in the most densely occupied region of this parameter space, thus indicating that Jupiter is a typical massive planet rather than an outlier. Our analysis suggests that Jupiter is more typical than indicated by previous analyses. For example, instead of MJup mass exoplanets being twice as common as 2 MJup exoplanets, we find they are three times as common.

Erratum: “A Comprehensive Comparison of the Sun to Other Stars: Searching for Self-Selection Effects” (ApJ, 684, 691 [2008])

1. Appendix A3 should reference Figure 16 at the end, not the beginning. This section in full should read: ‘‘The Galactic orbital eccentricity (e) and the magnitude of the galactic orbital velocities with respect to the local standard of rest (vLSR) are strongly correlated (see Fig. 6). We selected e instead of vLSR because of its near independence of the maximum height above the galactic plane (Zmax); see Figure 16.’’ 2. The caption to Figure 6 should read ‘‘Left’’ and ‘‘Right’’ instead of ‘‘Top’’ and ‘‘Bottom’’. The Press sincerely regrets these errors. The Astrophysical Journal, 689:1457, 2008 December 20