systemic

I’m nostalgic for ’97, when the discovery of a new extrasolar planet was literally front-page news. What’s now cliche was then fully viable poetic sweep. Epicurus and his multitude of worlds. Bruno burning at the stake. In that frame of mind, it’s fascinating to go back and read John Noble Wilford’s extended New York Times piece, written at the moment when the number of known extrasolar planets equaled the number of planets in our own solar system.

Some of the hyperbole still seems fresh, especially with regard to the frequency and diversity of planetary systems:

And the discoveries may be only beginning. One recent study suggested that planets might be lurking around half the Milky Way’s stars. Astronomers have already seen enough to suspect that their definition of planets may have to be broadened considerably to encompass the new reality. As soon as they can detect several planets around a single star, they are almost resigned to finding that the Sun’s family, previously their only example, is anything but typical among planetary systems.

At the recent Porto conference, the Geneva team not only reiterated their claims regarding the frequency of low-mass planets, but actually upped their yield predictions. According to a contact who heard Stephane Udry’s talk, the latest indication from HARPS is that between 38% (at the low end) and 58% (at the high end) of nearby solar-type stars harbor at least one readily detectable M<50 Earth-mass planet. This is quite extraordinary, especially given the fact that were the HARPS GTO survey located 10 parsecs away and observing the Sun, our own solar system (largely in the guise of Jupiter’s decade-long 12-m/s wobble)  would not yet be eliciting any particular cause for remark.

It also looks like planets beyond the snowline are quite common. In yesterday’s astro-ph listing, there’s a nice microlensing detection of a cold Neptune-like planet orbiting a ~0.65 solar mass star with a semi-major axis of at least 3 AU. The microlensing detections to date indicate that Neptune-mass objects are at least three times as common as Jupiter mass objects when orbital periods are greater than five years or so. Microlensing detections are an extremely cost-effective way to build up the statistics of the galactic planetary census during belt-tightening times. Much of the work is done for free by small telescope observers.

Yet another dispatch pointing toward a profusion of planets comes from an article posted last week on astro-ph by Brendan Bowler of the IfA in Hawaii. Work that he’s done with John Johnson and collaborators indicates that the frequency of true gas giant planets orbiting intermediate-mass stars (former A-type stars like Sirius that are now in the process of crossing the Hertzsprung gap) is a hefty 26% within ~3 AU.

An embarrassment of riches? Certainly, the outsize planetary frequency means that the cutting-edge of the planet-detection effort will be shifting toward the Sun’s nearest stellar neighbors, as these are the stars that offer by far the best opportunities for follow-up with space-based assets such as HST, Spitzer, JWST et al.

As competition for ground-based large-telescope RV confirmation of run-of-the-mill planet transit candidates orbiting dim stars heats up, the threshold magnitude (at a given bandpass) at which stars become largely too faint to bother with will grow increasingly bright. We’re talking twelve. Maybe nine. Pont et al., in their discovery paper for OGLE-TR-182b refer to this threshold as the “Twilight Zone” of transit surveys:

The confirmation follow-up process for OGLE-TR-182 necessitated more than ten hours of FLAMES/VLT time for the radial velocity orbit, plus a comparable amount of FORS/VLT time for the transit lightcurve. In addition, several unsuccessful attempts were made to recover the transit timing in 2007 with the OGLE telescope, and 7 hours of UVES/VLT were devoted to measuring the spectroscopic parameters of the primary. This represents a very large amount of observational resources, and can be considered near the upper limit of what can reasonably be invested to identify a transiting planet.

As is usually the case, there’s been little or no shortage of interesting developments in the field of extrasolar planets. The biggest recent news has been the announcement at the Barcelona conference of a definitive mass for the ultra-short period transiting planet CoRoT-7b. It weighs in at a mere 4.8 Earth Masses (copy of the Queloz et al. preprint here).

Recall that CoRoT-7b caused quite a stir earlier this year with its weird properties. The planet’s year is a fleeting twenty hours and twenty nine minutes, and it induces a tiny transit depth of 0.03%. Unfortunately, the parent star presents a less-than-ideal target for high-precision radial velocity work. It has spots that come and go, and its stellar activity produces frustratingly noisy Doppler measurements. As a result, at the time of CoRoT-7b’s initial announcement, there was no definitive measurement of the planet’s mass.

That’s changed, however, with an unprecedentedly all-out deployment of the HARPS spectrograph. From the Queloz et al. preprint:

A total of 106 measurements between 30 and 60 minute exposure time each were obtained over 4 months, and with sometimes 3 measurements being taken on the same night.

Now in my notoriously biased opinion, such observational enthusiasm is perhaps best reserved for stars such as Alpha Cen B, but a fair argument can be made that the massive investment of time did pay off. Remarkably, the radial velocity data set shows that there are two short-period planets in the CoRoT-7 system. The outer companion, which doesn’t transit, has a period of 3.7 days and at least eight Earth masses. Most dramatically, by combining the mass and radius measurements of CoRoT-7b, one arrives at a density of 5.5 grams per cubic centimeter, essentially identical to that of the Earth, suggesting that the planet is largely composed of refractory materials. (I hesitate to apply the term “rocky” to the CoRoT-7c landscape for the same reason that I’d refrain from describing the Amazon Delta as “icy”.)

In a very real sense, the HARPS campaign on CoRoT-7b has given us our last first look at a fundamentally new category of planet — that is, a world lying in the factor-of-fourteen mass gap spanned by Earth and Uranus. And, from exo-political point of view, the stakes surrounding this discovery were very high. The first density measurement of a planet in this category could just as easily have been made by teams combining high-precision Doppler measurements with either (1) Warm Spitzer, (2) ground-based photometry, (3) Kepler, (4) MOST, (5) HST, or (6) CoRoT. So I can imagine that there was a certain impetus underlying the scheduling of that huge block of HARPS time.

The discovery could, however, still be waiting to be made. Despite all the effort with HARPs, there remains a hefty 70% error on the density determination. This means that there’s a ~16% chance that CoRoT-7b is actually less dense than Neptune.

I’ll go out on a limb: CoRoT-7b’s density will turn out to be anomalously high. More than 90% of “super Earths” will turn out to be “sub-Neptunes” as far as their density is concerned.

Philippe Thebault sent me a link to an article on the Alpha Centauri planet search published earlier this month in the Frankfurter Allgemeine Zeitung. The text is in German, but the Google translator does a passable job of getting the gist across.

I got my first inkling of the Geneva Planet Search’s Alpha Centauri campaign through Lee Billings’ article in Seed Magazine. (See this post). In the Frankfurter Allgemeine article, Francesco Pepe gives further details — Alpha Cen B is one out of ten stars that are receiving special scrutiny for terrestrial planets at HARPS. They are getting one observation every two weeks, meaning that the star is being hit roughly one out of every two of their planet search nights:

“Allerdings müssen wir uns Harps mit anderen Gruppen teilen”, sagt er. Zudem ist Alpha Centauri B nur einer von zehn Sternen, die sie auf erdähnliche Planeten absuchen wollen. “Aber alle zwei Wochen schauen wir damit auf Alpha Centauri, und das Gerät ist sehr effizient.”

This quote implies that my speculations regarding the Geneva team’s data collection rate on Alpha Cen B were somewhat overheated. Instead of getting 100 ultra-high-precision HARPS velocities per year, it looks like a more realistic estimate of their current rate is 25 velocities per year. Since signal-to-noise increases as the root of the number of observations, this means that the minimum mass threshold for Alpha Cen Bb at any given time is approximately doubled relative to my estimates at the beginning of the Summer. Instead of arriving at 2.5 Earth masses in the habitable zone a bit more than a year from now, they’ll be at roughly 5 Earth masses.

Now nobody likes backseat drivers. As the saying goes, “theorists know the way, but they can’t drive”, and theorists have had a particularly dismal record in predicting nearly everything exoplanetary.

But nevertheless, I’m urging a factor-of-four increase to that data rate on Alpha Cen B. I would advocate two fully p-mode averaged velocities per night, 50 nights per year. I know that because Alpha Cen B is so bright, the duty cycle isn’t great. I know that there are a whole panoply of other interesting systems calling for time. It is indeed a gamble, but from the big-picture point of view, there’s a hugely nonlinear payoff in finding a potentially habitable planet around Alpha Centauri in comparison to any other star.

During the next few months, it’s inevitable that one of the numerous Super-Earths that have been turning up in the radial velocity surveys will be announced to be observable in transit (see, e.g. this post). When that occurs, we’ll effectively have had our last first look at a truly new category of planet — the logarithmic mass interval between Earth an Uranus is currently by far the largest among the 70-odd planets that have accurately determined radii. My own guess is that the emerging population of super-Earths will be better described as a population of sub-Neptunes. That is, the transit depths will indicate compositions that are largely water.

So if 5-Earth mass planets turn out to be primarily water-based rather than rock-based, it’s (in my mind) an argument in favor of cranking up the data rate on Alpha Cen B. There were no structurally substantial quantities of water in the Alpha Cen planet-forming environment. If we’re seeing sub-Neptunes rather than super-Earths in the HD 40307, Gliese 581, et al. systems, then the odds are heightened that any planets orbiting Alpha Cen B are less than 2 Earth masses. There’s no payoff in tuning your Alpha Cen B strategy for sub-Neptunes. Finding truly terrestrial-mass planets will require paying full freight.

In the early nineteenth century, the detection of stellar parallax was a problem fully equivalent in both scientific excitement and prestige to the modern-day detection of the first potentially habitable extrasolar planet. I think it’s worth noting that the prize of discovery of the first stellar parallax went not to the eminently capable (but overly cautious and slow-moving) observer who accumulated data on the best star in the sky, but rather to an observer who focused on a rather obscure star in the constellation Cygnus.

Here’s a link to the article, “Thomas Henderson and Alpha Centauri” by Brian Warner of the University of Cape Town.

Woke up this morning to the startling news that the Geneva team has added an Msin(i)=1.9 Earth mass planet to the Gliese 581 system! The preprint (Mayor et al. 2009) is available from exoplanet.eu, and will appear in Astronomy and Astrophysics. With a radial velocity half-amplitude, K=1.85 m/s, Gl 581e is the lowest-mass planet detected to date.

“The orbital period of the new planet “e” is quite close to pi days. i would mark down a score of -1 for competing planet hunters, whose signals-to-noise are accumulating in proportion to the root of the number of measurements.” said Greg Laughlin, an astronomer at the University of California, Santa Cruz.

In addition to the detection of the new 1.9 Earth-mass planet in the system, the period of Gliese 581d has been revised (to great habitability fanfare) from 84 days to 66 days. Indeed, the new, shorter period raises the habitability value of Gliese 581d from about 0.5 cents to nearly one penny.

As often happens, a strong hint of the new planet was lurking unnoticed in the previously published radial velocity data, and it’s especially interesting to look at the details in this particular case to see how the period revision came about. Let’s work with the 50 radial velocities published by Udry et al. 2007.

The two strongest periodicities in the system come from planets b and c. Removing these planets with the assumption of circular orbits leaves a residuals periodogram that has its strongest peak at 84 days:

The 66 day periodicity is lagging in second place with 66% of the power. Nevertheless, both periodicities provide significant improvement to the fit. An 84-day planet has K=2.67 m/s, and leaves an RMS of 1.43 m/s to the three planet fit:

A best fit 66-day planet has a slightly higher K=2.77 m/s, but leaves an RMS of 1.72 m/s. The chi-square is also higher: 5.10 as compared to 3.65. In the 2007 data, the 84-d planet thus looked quite secure. With hindsight, though, one notices that the phase coverage in the 66-day fit is better than for the 84-day fit. As more data was obtained, it became clear that the 84-day period was an alias of the true 66-day periodicity. Fair enough — RVs are expensive to obtain, and revisions of this sort are an inevitable product of progress.

In the residuals to the fit with the 84-day planet, planet e is present, but it’s masked by a spurious periodicity at 3.45 days,

whereas in the residuals to the fit with the 66-day planet, planet-e is in the #1 spot — not yet significant, but certainly more tantalizing…