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Lobbying for Alpha Cen

August 29th, 2009

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.

  1. coolstar
    August 30th, 2009 at 05:03 | #1

    Care to elaborate on how we can be so sure there were not substantial quantities of water in the Alpha Cen system? I’m pretty mystified as to how we can know this?

  2. August 30th, 2009 at 10:26 | #2

    The problem with Alpha Centauri and water is that the snow line around each of the components is rather close to the region where the orbits become unstable. This suggests the water reserves in the circumstellar discs were quite small.

  3. August 31st, 2009 at 00:04 | #3

    What is the minimum altitude you need to do RV measurements? Would a more southerly scope give you more opportunities? You mentioned previously that a 1-2m scope would be sufficient, due to the brightness of the star. Are there any 1m scopes at any of the Antarctic bases that you could fly a spectrometer down to?

  4. greg
    August 31st, 2009 at 01:30 | #4

    Hi Coolstar,

    Regarding water in the Alpha Cen system, I’m in full agreement with Andy’s comment.

    For Alpha Cen, yes, you need to be in the Southern hemisphere — Antarctica would be a fine location, although the usual observing sites in Chile, Southern Africa and Australia are also all close to ideal, and are likely more cost-effective. The season on Alpha Cen, for example, at CTIO is close to ten months.


  5. coolstar
    August 31st, 2009 at 03:22 | #5

    Thanks Andy and Greg: that really should have occurred to me. I need to look up the orbital separations, I guess. I thought the Marcy group was also doing the Alpha Cen system from Chili but have I mis-remembered that?

  6. greg
    August 31st, 2009 at 04:34 | #6

    Hi Coolstar,

    Debra Fischer (with Geoff Marcy as a collaborator) is indeed running a competing program on Alpha Cen from the southern hemisphere. From what I hear, it’s progressing quite well — I think it’s important though, for a discovery of this importance, to have two competing teams.

    You can read the details in both the Seed article, as well as in the google translation of the FAZ article.


  7. August 31st, 2009 at 06:31 | #7

    How much faster could they get a detection if the competing teams pooled their data and collaborated?

  8. bruce01
    August 31st, 2009 at 13:08 | #8

    My experience using the console with two or more data sets usually results in a poorer fit than if all the data came from one instrument. This indicates to me that there are more instrumental corrections, besides a single rv bias, that need to be incorporated into the fit. The more precise the individual rv observations become, the more important systematic instrumental effects become. So, whether pooling data helps depends upon how well the systematic effects can be eliminated from the rv observations.

  9. August 31st, 2009 at 21:31 | #9

    Talking of the console, how’s the work on the backend part of the site going?

  10. pete j
    September 11th, 2009 at 20:37 | #10

    A closing comment on Alpha Cen – and the question of water, perhaps there is an AlphaCen Oort cloud, which delivers comets (therefore water) to any inner terrestrials around A and B. If I’m not mistaken this mechanism is likely not to be as efficient as the delivery in our Solar System.

    The studies of Wiegert & Holman (1997) reveal that a single planet on a circular orbit is unstable over much of the region of the central AlphaCentauri Binary. However, there are zones in which such a planet could be stable over million year time scales. These zones are located both far from the binary (a >=70au) and near to (a <=3au) the primary and the secondary. Therefore the delivery (additional water) could only be from this region.

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