Ever more refined radial velocity searches for exoplanets are reaching into the domain of lower and lower mass targets. It’s natural enough that we’re most interested in planets of Earth mass and even smaller, but as a new paper on the work of the European Southern Observatory’s HARPS instrument reminds us, one of the great values of this work is that we’re getting a broad view of how exoplanets form and evolve in their systems, no matter what their size. Characterizing not just planets but entire systems is becoming a profitable investigation.

But small worlds continue to fascinate us, particularly in the hopes of finding possible abodes for life. HARPS’ involvement in the hunt now includes an intense campaign to monitor ten stars that are relatively near our Sun, all of them slowly rotating and quiet solar-type stars. Mounted on ESO’s 3.6-meter instrument at La Silla Observatory in Chile, HARPS (High Accuracy Radial Velocity Planet Searcher) has produced more than 100 exoplanet candidates in its first eight years of operation, including not just Neptune-mass planets but super-Earths and intriguing systems like Gliese 581, with two possibly rocky planets near the habitable zone.

Moreover, from the system-wide point of view, the system around HD 10180 includes seven low-mass planets including the 1.5 Earth mass HD 10180 b. So when HARPS talks, we listen, and I want to quote this from the paper at the outset (internal references omitted for brevity):

… a recent investigation of the HARPS high-precision sample has shown that about 1/3 of all sample stars exhibit RV variations indicating the presence of super-Earths or ice giants… Indeed, planet formation models… show that only a small fraction (of the order of 10%) of all existing embryos will be able to grow and become giant planets. Hence, we expect that the majority of solar-type stars will be surrounded by low-mass planets.

Good news for small planets! If this is the case, we would expect that even a small sample like the current ten solar-type stars now under intense investigation by HARPS will turn up several Earth-like planets (i.e., rocky worlds in the inner system), and the new paper does not let us down. Three of the host stars involved in this program have already produced detections; these are HD 20794, HD 85512 and HD 192310. There are no giant planets here but study of the three stars has thus far yielded six low-mass planets, including three super-Earths around HD 20794 (82 Eridani), with semi-major axes of the planetary orbits measured as 0.12AU, 0.20AU and 0.35AU. The semi-major axis measures the radius of an orbit taken at the orbit’s two most distant points.

No habitable zone planets here, though, with even the furthermost planet reaching likely equilibrium temperatures of 388 K, which works out to about 115 degrees Celsius. Remember that equilibrium temperature is not the same thing as temperature at the surface. The equilibrium temperature of the Earth without an atmosphere is 255 K ( -18 degrees Celsius), but adding in the various effects of our atmosphere we come to an average of 288 K (15 degrees Celsius), so it’s clear how careful we have to be with these numbers, given how little we know about the planets in question. The surface temperature of a planet with a dense atmosphere will depend upon our atmospheric models.

That issue applies to the system around HD 85512 as well, which is described as the most stable of the stars in the HARPS sample. This star is found to have a possible super-Earth in an interesting orbit indeed, with a semi-major axis of 0.26 AU and a computed equilibrium temperature of 298 K, one that could place this potentially rocky world within the inner edge of the habitable zone. As my friend Ronald Botterweg reminds me in one of the comments to an earlier post, this equilibrium temperature is not far from that of southern France about now, but again, that has to be adjusted for atmospheric effects (for a paper analyzing different atmospheric models for this planet, see Kaltenegger et al., linked to at the end of this post).

In fact, let me go ahead and quote from the Kaltenegger paper, which calls HD 85512 b “…with Gl 581 d, the best candidate for habitability known to date.”:

We focus our analysis on HD 85512 b. We show the influence of the measurement uncertainties on its location in the Habitable Zone as well as its potential habitability. We find that HD 85512 b could be potentially habitable if the planet exhibits more than 50% cloud coverage. A planetary albedo of 0.48 +/- 0.05 for a circular orbit, and an albedo of 0.52 for e=0.11 is needed to keep the equilibrium temperature below 270K and the planet potentially habitable.

And this:

If clouds were increasing the albedo of HD 85512 b, its surface could remain cool enough to allow for liquid water if present. HD 85512 b is a planet on the edge of habitability.

But back to the original HARPS paper. HD 192310 has been under investigation for several seasons following the earlier discovery of a Neptune-mass planet there. HARPS confirms that earlier discovery and adds another possibly Neptune-class world, the two semi-major axes being 0.32 AU and 1.18 AU. According to the paper, we’re again bracketing the habitable zone, with equilibrium temperatures on the order of 355 K and 185 K — possibly at the very inner and outer edges of the habitable zone, respectively.

So far, then, three of the ten stars observed in this program have yielded low-mass planets. From the paper:

Although statistics is poor over only ten targets, it is interesting to note that this 30% value was already announced by Lovis et al. (2009) who based their analysis on the larger (< 200 stars) HARPS high-precision program. Theoretical works by Mordasini et al. (2009) actually forecasted that the frequency of small Neptunes and super-Earths on short and intermediated orbits would be considerably higher than that of Saturns and Jupiters. The recent amazing discoveries made by the KEPLER satellite using the transit technique further strengthen this fact. Borucki et al. (2011) report that the probability of finding low-mass planets is considerably higher than for Jupiter or Saturn-mass planets. Furthermore, when summing up the frequency of finding a planet of any mass, they end up with a probability of about 30%, again in perfect agreement with the results of Lovis et al. (2009).

All good news for finding Earth-class worlds as we push the radial velocity method into this mass range. It’s interesting, too, to look at what this paper has to say about Alpha Centauri, Centauri B being one of the ten targets on the HARPS list for the study. As the work continues, the researchers have to contend with the bright magnitude of the Centauri stars, which “may result in poorer RV precision due to incomplete light scrambling across the spectrograph’s entrance slit.” Another major issue: Alpha Centauri B is a member of a triple star system, which means the radial velocity analysis must include a complete and precise orbital model. All of this is tricky but a thorough reading of the paper yields the conviction that HARPS is up to the task.

Tau Ceti is also a member of the list — this is one of the two stars from the original Project Ozma that Frank Drake made famous back in 1960 (the other being Epsilon Eridani). Tau Ceti as yet shows no planetary signatures, and again I’m going to turn to Centauri Dreams regular Ronald Botterweg, who has been in the thick of our ongoing exoplanet discussions for many years. Ronald analyzed the metallicity of the ten stars in the HARPS sample and found that eight of them have lower metallicity than the Sun (seven, in fact, have considerably lower metallicity than Sol). Which leads Ronald to quote a recent Greg Laughlin post on systemic:

“First, among host stars with masses similar to the Sun that harbor giant planets, there’s a strong preference for metal-rich stars. This is the classic planet-stellar metallicity effect. Second, among low-mass stars, there’s a dearth of giant planet candidates. This is the known giant planet-stellar mass effect.”

Interesting stuff, and I’m pleased at the way readers here have been digging into these papers, which not only alerts me to new work but points to issues I might otherwise have missed. Solar-type stars of low metallicity are places where we find few giant planets, the latter seeming to favor high-metallicity stars of solar size and larger. Meanwhile, the relatively high metallicity content of Centauri B, which might lead us to expect a gas giant, is presumably offset by its position as a close binary. We’ll now wait with great interest to see how the HARPS work continues on the vital and fascinating question of smaller worlds in the Alpha Centauri system. With two other teams also on the case, I suspect we won’t have to wait too much longer before we learn something definitive about the planetary situation around our nearest neighbor.

The paper is Pepe et al., “The HARPS search for Earth-like planets in the habitable zone: I — Very low-mass planets around HD20794, HD85512 and HD192310,” accepted by Astronomy & Astrophysics (preprint). See also Kaltenegger et al., “A Habitable Planet around HD 85512?” submitted to Astronomy & Astrophysics (preprint).