New Alpha Centauri planets quest, thanks to TPS ! Options

[JRehling]

Interesting point, Reed. What I'd noticed is that the Kepler target star list was explicitly seeded with known eclipsing binaries (thus, performing targeted science of such stars) but that number came out to only 600 of the ~200K observed stars. So, as an absolute count, that's a lot of binaries, but as a proportional count, it's less than 1%.

But the citation you name (from last week!) highlights the number of unknown binaries, and indeed, it's an open question as to how many stars are binary. Alpha Centauri is on the short side of separation distance. Where the periods are quite long, we might expect planetary systems that more closely resemble single stars, and greater difficulty determining if the pair of stars is actually bound or not.

[vikingmars]

I just received from TPS my "Certificate of Mission Participation" and I discovered that my 50 USD offered 15 minutes of observation time to search for exoplanets in the Alpha Centauri system... I'm proud of being member of TPS and now it's time to donate : maybe one of you will offer the minutes when a crucial observation will be made also. Thanks in advance for your support and thanks to TPS to have this mission available to all !

[JRehling]

Another tidbit about Alpha Centauri which I didn't notice in the papers (though I admit I didn't read them all). Both -A and -B have significantly higher metallicity than the Sun. As has been recorded in the literature, and as I'm seeing more and more in the Kepler data, that is a significant factor in favor of planet formation. Were it not for the relatively close minimum distance, there would be little reason to doubt some planets around one or both stars. I think there's an excellent chance of terrestrial-sized planets in this system. Whether or not we can detect them and do follow-up science is a tougher prospect, but it's definitely an exciting notion.

[marsophile]

Would it be feasible to observe occultations of more distant stars by planets around Alpha Centauri A or B? Or gravitational lens effects?

[Hungry4info]

Maybe. The idea has been thought of before for VB 10.

Searching for Planets During Predicted Mesolensing Events: I. Theory, and the Case of VB 10
http://arxiv.org/abs/1202.5316

Searching for Planets During Predicted Mesolensing Events: II. PLAN-IT: An Observing Program and its Application to VB 10
http://arxiv.org/abs/1202.5314

[JRehling]

The first sentence of Cassan, et al, 2012 is "Gravitational microlensing is very rare: fewer than one star per million undergoes a microlensing effect at any time." So, that's very bleak. Success depends upon observing a massive number of targets and finding a few successes. The papers just mentioned involve a cherry-picked star which is known to be favorable. Picking a desired star and hoping for microlensing is almost certainly a lost cause.

The transiting method is also blind to most planets, but it's not that bleak. Detecting a planet at 1 AU out works out, geometrically, more than 0.1% of the time. However, we can see perfectly well that the Alpha Centauri stars are not aligned favorably, so we have zero probability of a transit for any planet whose plane is near that which contains the stars' mutual orbit.

The radial velocity / Doppler method, on the other hand, is least affected by geometrical luck, but is also less favorable for small planets. (Ie, Mars has only 1/3000th Jupiter's mass, which determines its detectibility by the RV method, but 1/400th its cross section, which is relevant for the transiting method.)

So the RV method is our best bet for Alpha Centauri. But if we had systems for observing close-in planets directly by their reflected light, Alpha Centauri would be a highly favorable target given its proximity. So far, only distant planets (much further than the Alpha Centauri inter-star distance) have been imaged directly. Giants have been observed by their reflected light in Kepler light curves, but such planets are also good targets for RV.

[ngunn]

So far, only distant planets (much further than the Alpha Centauri inter-star distance) have been imaged directly.

And not by reflected starlight, only by their thermal IR. The one object imaged by reflected starlight is much brighter than any planet could be. It's postulated to be a satellite-forming disc on the scale of Jupiter's gallilean system. So directly imaging smallish, cool, non-transiting planets remains a very distant prospect. I don't think next generation telescopes will do it. We have to work with how these worlds influence the light we receive from their parent star.

[JRehling]

We have a report of Alpha Centauri Bb, with publication by the HARPS team in Nature tomorrow. Earth-sized, but orbiting with a 3.2 day period.

Obviously a fantastic discovery, if it stands. This would mean that whenever we have the means for follow-up science we'll have at least one planet that is much closer than we are likely to find around any other star.

[Explorer1]

Yep, I was pretty much ecstatic when I read it this morning. It looks pretty clear that anywhere a planet can form, it will. It also puts in perspective how much observation work will be needed to find something at a less broiling orbit.

[Seryddwr]

In 1989, Astronomy magazine ran a feature on the possibility of life in the Alpha Centauri system, with diagrams showing the 'Goldilocks zone' etc. - a thought that fuelled my young mind! Pity the discovered planet's so close to the star, but at least it validates the computer modelling done back then, which showed (IIRC) that orbits stable over cosmic timescales were possible. Kudos to all involved

[belleraphon1]

I’ve known there are several teams searching Alpha Centauri and have been waiting for such an announcement!!!!

Wo Hooo!!!!

An Earth-mass planet orbiting α Centauri B
Xavier Dumusque,1, 2 Francesco Pepe,1 Christophe Lovis,1 Damien Ségransan,1 Johannes Sahlmann,1 Willy Benz,3 François Bouchy,1, 4 Michel Mayor,1 Didier Queloz,1 Nuno Santos2, 5 & Stéphane Udry1
I am so excited I can barely type...

Wo Hooo!!!!

An Earth-mass planet orbiting α Centauri B
Xavier Dumusque,1, 2 Francesco Pepe,1 Christophe Lovis,1 Damien Ségransan,1 Johannes Sahlmann,1 Willy Benz,3 François Bouchy,1, 4 Michel Mayor,1 Didier Queloz,1 Nuno Santos2, 5 & Stéphane Udry1

“Exoplanets down to the size of Earth have been found, but not in the habitable zone—that is, at a distance from the parent star at which water, if present, would be liquid. There are planets in the habitable zone of stars cooler than our Sun, but for reasons such as tidal locking and strong stellar activity, they are unlikely to harbour water–carbon life as we know it. The detection of a habitable Earth-mass planet orbiting a star similar to our Sun is extremely difficult, because such a signal is overwhelmed by stellar perturbations. Here we report the detection of an Earth-mass planet orbiting our neighbour star α Centauri B, a member of the closest stellar system to the Sun. The planet has an orbital period of 3.236 days and is about 0.04 astronomical units from the star (one astronomical unit is the Earth–Sun distance).”

Alpha Centauri system holds the closest stars to Earth… just 4.4 light years away.

http://www.nature.com/news/the-exoplanet-next-door-1.11605

http://www.sciencenews.org/view/generic/id...alien_next_door

http://www.centauri-dreams.org/?p=25109

http://oklo.org/2012/10/16/alpha-centauri-b-b/

[belleraphon1]

News release and journal paper...

http://www.eso.org/public/news/eso1241/

http://www.eso.org/public/archives/release...41/eso1241a.pdf

[Holder of the Tw...]

From the "Exoplanet next door" article at the Nature site:

" Ralph McNutt, a planetary scientist at Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, has received NASA study funding to design an ‘innovative interstellar explorer’. Even when launched by one of the most powerful rockets on Earth, boosted by a gravitational slingshot around Jupiter, and further accelerated by a radioisotope thruster, that probe would take about 28,000 years to reach α Centauri."

Doesn't say whether they are taking the star system's own motion into account, which most people don't. The Toliman (Alpha Centauri) system will be at its closest to us at just about that time, 3.0 light years away.

Plus, it won't be in the constellation Centaurus anymore as viewed from Earth, so we'll be needing a new designation (Alpha Hydrae?).

[JRehling]

Well, planets don't end up everywhere they can, because our solar system has no planet interior to Mercury, whereas a lot of stars (and now we know, Alpha Centauri do.

Alpha Centauri's mutual approach of 18 AU obviously doesn't preclude the existence of planets. I might guess that it impaired formation of solid bodies in the ice belt which would go on to be the seeds of gas giants. The fact that an earth-sized planet was discovered and not a jovian first or concurrently suggests that at least there are no jovians in the inner system, and maybe no jovians at all.

Given that some systems have planets in very short periods (<20d) and others don't, it's intriguing to consider whether there is a negative relationship, that inner planets are a sign that planets will not be found at longer periods (~50-500d). For jovians, we could get some data on this. For terrestrial planets, we're still waiting to find any at longer periods, so obviously we'll have to wait longer to find out about the relationship between occurrence at different periods. But the specific query here is: Does Alpha Centauri Bb mean that the star is more or less likely also to have planets in the habitable zone?

[Hungry4info]

We also knew from RV data that there were no Jovian planets around either star to begin with. In the paper, they state that they have enough data to detect a 4 Earth-mass planet in the habitable zone of Alf Cen B if it were there. Since the habitable zone is about as far as you can get from the star and remain in a stable orbit, I take this to mean there are no planets with an m sin i > 4 M_E around the star (obviously you can rule out lighter and lighter planets as you get closer to the star).