New Search for Centauri Planets Begins

by Paul Gilster on December 2, 2009

To the two ongoing hunts for planets around the Alpha Centauri stars we can now add a third. John Hearnshaw (University of Canterbury, Christchurch) reports in a recent post on Cosmic Diary that the university’s Mt. John Observatory has begun a program to search for Earth-mass planets around Centauri A and B. Although the observatory is heavily invested in microlensing technologies (working with the Microlensing Observations in Astrophysics collaboration), the new efforts will put radial velocity methods to work using the Hercules spectrograph.

The program is a joint effort with Stuart Barnes at the Anglo-Australian Observatory and Mike Endl at the University of Texas (Austin). And as Hearnshaw notes, the problem is a formidable one, given that an Earth-mass planet in the habitable zone around Centauri A creates a ‘wobble’ of only 10 cm/s (slightly larger for the less massive Centauri B). Yet the observatory is banking on Hearnshaw’s statement that 30,000 spectra of Centauri A or B over three years can detect a habitable zone ‘Earth.’

The habitable zone around Centauri A should be found at about 1.2 AU, while 0.75 AU is calculated for Centauri B. What else do we know about the primary Centauri stars? Earlier work has demonstrated that no gas giants as massive as Jupiter can exist there — Doppler studies would have found them by now. But the case for Earth-mass planets remains open. Hearnshaw notes the more positive findings that have emerged in the past decade or so:

We are encouraged to undertake these key observations for several reasons. First, theoretical studies by Javiera Guedes et al. in 2008 showed that Earth-mass planets are likely to have formed in the alpha Centauri system. In their simulations, planets of mass 1 to 2 Earth masses always form in the habitable zone around alpha Cen B, no matter what the initial conditions. What’s more, Paul Holman and Matt Wiegert found that stable orbits are possible in this binary provided they are within about 3 A.U. of either star. The orbits are almost certain to be coplanar with the binary star orbit, which has a semi-major axis of 23 A.U. What is more, the binary orbit is tilted at 79 degrees to the line of sight, so any putative planetary orbits are likely to be at that same favourable angle for detecting Doppler shifts (if the angle is small, the orbits would be close to face on and no Doppler shifts are then detectable).

We’ve looked at the studies mentioned above repeatedly in these pages (run a search for past articles), but we’ve also noted recent work by Philippen Thébault (Stockholm Observatory), Francesco Marzari (University of Padova) and Hans Scholl (Observatoire de la Côte d’Azur), who question whether the accretion process of planetary formation would allow such planets to form. The heartening thing is that this dispute is likely to be settled one way or the other within a few short years. Mt. John Observatory already has several thousand spectra in its new campaign and intends to intensify the search, an effort Hearnshaw calls “a realistic target within our grasp.”

And note this:

One other fortunate circumstance makes this the ideal programme for Mt John Observatory. Being the world’s southernmost optical observatory (at 44ºS), we are able to observe alpha Centauri for 12 months of the year, even in November and December. The star is circumpolar and passes the southern horizon at lower culmination at an altitude of some 15 degrees, when it is still readily observable. No other observatory can see alpha Centauri all year, and a periodic gap in the data every year can be disastrous when trying to detect periodic signals which may well have around a one-year period. We therefore plan to press ahead with the alpha Centauri campaign during 2010 and 2011, with the hope of making the historic discovery of an Earth-like analogue orbiting our nearest star, at just 4.3 light years distance.

mt_john_observatory_1

Image: The night sky above Mt. John Observatory. Note the Southern Cross (just above and to the right of the dome), with Alpha and Beta Centauri the two stars to its left. Alpha Centauri (the leftmost bright star) is a triple system made up of Centauri A, Centauri B and Proxima Centauri. Credit: Fraser Gunn.

We should know something, and relatively soon. Debra Fischer’s work at the Cerro Tololo Inter-American Observatory (Chile) is ongoing, as is that of Michel Mayor and Stéphane Udry using the High Accuracy Radial velocity Planet Searcher (HARPS) at the European Southern Observatory facilities at nearby La Silla. The betting here has been that while the Mayor team may be the first to find a larger world, an Earth-mass planet is likely to be claimed by Fischer, whose resources, unlike HARPS, are totally committed to the Centauri search. Now we add this interesting New Zealand campaign as excitement grows that whatever Centauri planets there be may soon be discovered.

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{ 20 comments }

Paul Titze December 2, 2009 at 10:38

Good to also see the KIWIs getting into the space program:

http://tvnz.co.nz/technology-news/second-time-lucky-nz-rocket-launch-3207087/video

Jeff Milam December 2, 2009 at 11:59

This is great news! My pick axe and plow are packed and I’m ready to go.

Kenneth December 2, 2009 at 12:06

Ken,

The real question is if we find something of interest around Alpha Centauri in the next few years what does the follow-up program look like to comprehensively study our nearest neighbor? There is SIM and perhaps TESS on the Space side, but the entire community starting with Webster Cash and the various Observatory’s around the World should now be hard at work defining a combined International Space and Terrestrial Observatory program in the $1 Billion range to really look at the entire Alpha Centauri System in exquisite detail including Mars size planets and even potential Habitable Moons. Add another $1 Billion to that program to survey everything out to 5 light years from Earth including searching for Brown Dwarfs and undiscovered Asteroids and Comets that may intercept Earth over rhe next millenium and there should be a ground swell of political support and funding from many nations starting with the U.S.. The key is to do the sound Systems Engineering upfront and put together a highly cost effective and balanced program based on near term (2012/2013) technology that can be fully deployed and operational by 2020.

Jim Gagnon December 2, 2009 at 14:25

While the search for planets around Alpha Centauri is science in its truest form, it’s not likely to reveal a fully formed solar system as we have here around Sol. From what I understand, the lithium levels in Alpha Centauri are so high that according to some it’s unlikely planets coalesced around either Alpha Centauri A or B. Our nearest neighbor in space is most likely barren.

yeti December 2, 2009 at 14:48

the thing about those simulations that say the chance of terrestrials in the HZ is high assumes a protoplanetary disc of a big size. We know from spitzer data that binary systems with similar separation to centauri A/B are the worst kind of binaries for substantial discs.

Its still possible to get planets but its ashame the centauri system isnt alot tighter or alot wider seperation. Dont want to be the prophet of doom but I’ll be preparing for the worst and hoping for the best on this one.

tacitus December 2, 2009 at 16:36

A negative result, while it would be disappointing, is a valid (and valuable) scientific finding nonetheless.

Enzo December 2, 2009 at 16:47

There’s an interesting interview with Fisher here :
http://marketsaw.blogspot.com/2009/10/eyes-on-alpha-centauri-hunt-for-pandora.html

Enzo December 2, 2009 at 16:51

I forgot to ask : Fisher has been observing for one year. Not enough to publish, but is it enough to strongly suspect the presence or absence of the planet ?
Either way, she’ tight lipped.

Thomas December 2, 2009 at 18:33

@Jim Gagnon

Where did you get that information about Alf Cen’s lithium abundance? Greg Laughlin over at the oklo.org blog writes that Alpha Centauri is reasonably lithium-poor.

http://oklo.org/2009/11/22/lithium-induced-speculations/

Carl December 2, 2009 at 21:52

Wonderful that another observatory is on this. Bagging terrestrial planets in the next system would be a tremendous feather in an astronomer’s cap…

I had read that Percival Lowell’s search for Planet X was really to help his studies of Mars. Discovering the new planet would have been a boon to his prestige.

Debra Fischer’s efforts are interesting, because of the tight focus of her study, just like many of the notable astronomers of old. Whatever the outcome, the data from all three searches will certainly add to our knowledge of the binary in the Centauri system.

Jim Gagnon December 2, 2009 at 22:17

@Thomas

http://adsabs.harvard.edu/abs/1984A&A…140..427S

This article states lithium depletion of 1.28 for Alpha Centauri A and .7 for B. The article you sent places AC A at almost 1.5. The metallicity of each star is approximately 50% greater than Sol, and from what I understand there can’t be a stable orbit around each star unless it’s less than 1.1AU. Also, even though it’s been searched for there has never been any evidence of an Oort cloud around Alpha Centauri. All radial velocity or star transit methods have failed to reveal any indication of planets.

The evidence at hand precludes gas and ice giants of significant size. AC A’s lithium content serves as a weak indicator that Earth like rocky bodies are not present. AC B’s lithium content does indicate that it could have a small number of rocky bodies near it, at least at some point in its history.

So, I stand corrected: there could be a rump system present at Alpha Centauri B, though bodies orbiting Alpha Centauri A are unlikely.

keith December 3, 2009 at 14:51

If the age of CenA is a billion years older than the sun, AND it is more massive, how come it’s luminosity is not much more? After all, the Earth is meant to be fried in the next 700 million years?

andy December 3, 2009 at 15:19

Alpha Centauri B’s lithium content is probably a non-issue… if you take a read through the paper about the lithium/planets anticorrelation, it is noted that the effect holds only in a very narrow range of effective temperatures: roughly 5600-5900 K. Alpha Centauri B falls outside this range.

Predicting no planets around Alpha Centauri A based on lithium may be premature: whether the high-lithium systems are planetless or exhibit very different architectures (e.g. no gas giants) is an issue. The known planetary systems consisting of super-Earths without gas giants seem to be located around stars too cool to fall into the lithium anticorrelation range.

Ronald December 3, 2009 at 20:24

keith: I think your point is quite essential, but I think its luminosity is roughly in line with expectations for a star of that mass and age:
Alph Cen A is considered a G2V star, but has 10% greater mass than our sun, and is estimated from about 0.5 tot 1.5 gy older than the sun. For such a massive planet that does not bode well: and indeed it is already over 50% brighter thsn the sun.

I would rather put my money on Alph Cen B, a calm and stable K0/K1 of slightly less than solar luminosity.

Ronald December 3, 2009 at 20:25

Of course when I wrote “for such a massive *planet* that does not bode well”, I meant “for such a massive *star *…”.

Ronald December 4, 2009 at 9:00

I must have been tired last night, another blunderous errorin my post:
“I would rather put my money on Alph Cen B, a calm and stable K0/K1 of *slightly less than solar luminosity*.”

Of course that should have been: “slightly less than half of solar luminosity” (it is about 46 %).

spaceman December 4, 2009 at 11:48

So, if I am understanding the literature correctly, the question is not really whether or not there was enough stuff (metals) to form centauri planets, but rather was the stuff able to form into planets in the first place. Is this a valid assessment of the debate?

Administrator December 4, 2009 at 16:09

spaceman, that’s certainly a major issue. The recent work by Philippen Thébault’s team makes the case that the process of planetesimal formation would be all but impossible around the Centauri stars. We should know fairly soon whether that idea holds water.

spaceman December 4, 2009 at 17:14

Speaking of the Thebault et al paper, have any recent papers arisen directly challenging or at least addressing the pessimistic conclusion of this work? Also, if not planets, what would have become of the proto-planetary stuff (metals) that once existed around the centauri stars? Would there be a giant asteroid belt of Ceres-sized objects in lieu of planets? Administrator, what are you betting on, do you think the Thebault idea will end up holding water? Thanks.

andy December 4, 2009 at 18:29

Speaking of the Thebault et al paper, have any recent papers arisen directly challenging or at least addressing the pessimistic conclusion of this work?

Yes. For example this paper, which argues that a small relative inclination between the plane of the circumstellar disc and the binary orbit can make accretion more favourable than the coplanar case.

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