We’ve identified over 200 planets around other stars, but in many ways we know little about other solar systems. The problem is in extrapolating from our knowledge of one or two planets to an entire planetary system, much of which we cannot detect. Can we expect to find gas giants mixed with small terrestrial worlds around most Sun-like stars? And what about the smaller and far fainter red dwarfs? Clearly, the job of characterizing not just planets but entire systems is going to occupy astronomers for many a decade.
A new paper from the California & Carnegie team takes helpful steps in that direction. New planet finds are always fascinating, and the team does have four of them, the highlight being the pair orbiting the Sun-like star HIP 14810. Greg Laughlin (UC-SC) writes about that system on the systemic site, noting this:
The fact that the orbit is clearly non-circular would be strong evidence for the presence of planet c, even if there weren’t enough data to detect c directly. If planet b was the only significant planet in the system, its orbit would have circularized via tidal dissipation on a timescale that is less than the age of the star.
I mention Laughlin’s comment because it bears on where we are in the planet hunting process. The radial-velocity measurements that have found most known exoplanets depend on collecting data about how a star wobbles as it is gravitationally influenced by the planet(s) around it. Clearly, larger planets close in to their primary star are going to be the first seen with this method because they affect the star more clearly in short time frames.
But as we collect data over longer and longer periods, we begin to see more subtle signatures in the data that can point to other planets. Gas giants in outer system orbits eventually get teased out of the noise, and most intriguingly of all, we start to look for smaller worlds that may be terrestrial in nature. The California & Carnegie team points out that the mass distribution of planets is thought to increase steeply as we move into the range of lower masses. And that means that there are probably a large number of planets in systems already identified that are accompanied by other worlds.
So here are some conclusions from the study: The number of known multiple-planet systems is 22, and the number of exoplanet-bearing stars with trends in the radial velocity data that point to additional planets is also 22. The overall number of nearby stars bearing planets is 152. From the paper:
This means that 30% of known exoplanet systems show signiﬁcant evidence of multiplicity. Considering that the mass distribution of planets increases steeply toward lower masses…, our incompleteness must be considerable between 1.0 and 0.1 Jupiter-masses. Thus, the actual occurrence of multiple planets among stars having one known planet must be considerably greater than 30%.
And backing out to a wider view:
From an anthropocentric perspective, this frequency of multiplicity suggests that in some respects, the Solar System is not such an aberration. Our Sun has 4 giant planets, and it appears that such multiplicity is not uncommon, although circular orbits are.
The news keeps getting more interesting the more time elapses, for as data accumulates, the planet hunt for a given star becomes that much more precise. The California & Carnegie team points out that its search is only now becoming sensitive to planets like Jupiter in 12-year orbits. Finding a true analog to Saturn would demand another 15 years of observation. It’s hard to be patient when the ultimate goal for some of us is a terrestrial-sized world in a star’s habitable zone, but every passing day yields that much more information that can tell us what other solar systems really look like.
The paper, which will run in The Astrophysical Journal in February, is Wright et al., “Four New Exoplanets, and Hints of Additional Substellar Companions to Exoplanet Host Stars,” now available online in preprint form.