The Proxima Centauri Planet Hunt

by Paul Gilster on April 24, 2012

Although we haven’t yet found any planets around Proxima Centauri, it would be a tremendous spur to our dreams of future exploration if one turned up in the habitable zone there. That would give us three potential targets within 4.3 light years, with Centauri A and B conceivably the home to interesting worlds of their own. And the issue we started to look at yesterday — whether Proxima Centauri is actually part of the Alpha Centauri system or merely passing through the neighborhood — has a bearing on the planet question, not only in terms of how it might affect the two primary stars, but also because it would tell us something about Proxima’s composition.

A Gravitationally Bound System

Greg Laughlin makes this case in the systemic post I referred to yesterday. It was Laughlin and Jeremy Wertheimer (UCSC) who used data from ESA’s Hipparcos mission to conclude that Proxima was indeed bound to Centauri A and B. Here I want to quote the conclusion of the duo’s paper on the matter, which takes us into some interesting ground indeed:

The availability of Hipparcos data has provided us with the ability to implement a significant improvement over previous studies of the α Cen system. Our results indicate that it is quite likely that Proxima Cen is gravitationally bound to the Cen A-B pair, thus suggesting that they formed together within the same birth aggregate and that the three stars have the same ages and metallicities. As future observations bring increased accuracy to the kinematic measurements, it will likely become more obvious that Proxima Cen is bound to the Cen A-B binary and that Proxima Cen is currently near the apastron of an eccentric orbit…

First of all, the idea of Proxima Centauri as part of a triple star system makes sense given Proxima’s small relative velocity with respect to Centauri A and B (0.53 ± 0.14 km s-1). The star lies close by the binary pair at 15000 plus or minus 700 AU [and note that I incorrectly posted this distance yesterday, though the error is now corrected to reflect these figures]. Laughlin and Wertheimer calculate that the likelihood of this configuration occurring by chance is less than 10-6, adding that “ …based on this incredibly improbable arrangement it has been suspected that the stars constitute a bound triple system ever since Proxima’s discovery…”

Note too the implication that Proxima Centauri was born out of the same molecular cloud as its close neighbors, which would imply that all three have the same age and metallicity. Now that’s interesting. We know a lot about Centauri A and B, and in particular we know that both stars are more rich in metals than the Sun, with recent work by Jeff Valenti and Debra Fischer indicating a metallicity of about 150 percent of the Sun’s, and this may be a low-ball estimate, given other recent work on the matter. The link between metals — elements higher than hydrogen and helium — and planets is well established for gas giants and may well extend to small, rocky worlds, as we saw the other day in Falguni Suthar and Christopher McKay’s paper on habitable zones in elliptical galaxies. A metal-rich Proxima Centauri would boost its chances for planets.

Image: Light takes only 4.22 years to reach us from Proxima Centauri. This small red star, captured in the center of the above image, is so faint that it was only discovered in 1915 and is visible only through a telescope. Recent research is revealing much about its composition and the likelihood of planets there. Credit & Copyright: David Malin, UK Schmidt Telescope, DSS, AAO.

If we thought Proxima were not bound by Centauri A and B, we’d have a problem, because as Laughlin noted on systemic, it’s tricky to figure out the metallicity of a solitary red dwarf:

Metallicities for red dwarf stars are notoriously difficult to determine. Low-mass red dwarfs are cool enough so that molecules such as titanium oxide, water, and carbon monoxide are able to form in the stellar atmospheres. The presence of molecules leads to a huge number of lines in the spectra, which destroys the ability to fix a continuum level, and makes abundance determinations very difficult.

But if you have a red dwarf within a multiple star system, you can use the metallicity of the more massive primary star(s) to infer the metallicity of the red dwarf, a method that has produced a metallicity calibration for red dwarfs that thus far has proven useful. All of this means that if Proxima Centauri is indeed bound to Centauri A and B, then its metallicity is on the same order as theirs. When Xavier Bonfils (Observatoire de Grenoble) and colleagues went to work on red dwarf metallicity in 2008, they examined 20 red dwarfs whose metallicity could be estimated in this way. Of those 20 stars, reports Laughlin, only five were higher than the Sun in metallicity, and only one star, GL 324, proved to be as rich in metals as Proxima Centauri.

Proxima Planets: What We Can Exclude

From the standpoint of metals, then, we’d expect the Alpha Centauri stars should have the materials needed for the formation of rocky planets, but what have we observed around Proxima Centauri? The most recent work from Michael Endl (UT-Austin) and Martin Kürster (Max-Planck-Institut für Astronomie) works with seven years of high precision radial velocity data using the UVES spectrograph at the European Southern Observatory. These observations went from March of 2000 to March of 2007 and found no planet of Neptune mass or above exists there out to about 1 AU from the star.

Let’s pause for a moment. These are radial velocity studies about a star the orientation of whose planetary system — if one exists — is unknown to us. If there are planets here, we could be looking at the system from almost any angle, so that any mass calculations have a lot of play in them. Consider a large planet at a high angle to the line of sight. A gas giant like this could cause a radial velocity signature roughly similar to a much smaller planet in a system where the orbital plane was on the line of sight. In other words, the radial velocity technique is insensitive to the reflex velocity in the plane of the sky, giving us only a lower limit to a planet’s mass.

All that is by way of saying that a planet larger than Neptune could conceivably still be there around Proxima Centauri, but the odds do not favor it. We also learn from Endl and Kürster that no super-Earths have been detected larger than about 8.5 Earth masses in orbits with a period of less than 100 days. As for the habitable zone of this star — thought to be 0.022 to 0.054 AU, which corresponds to an orbital period ranging from 3.6 to 13.8 days — we can rule out super-Earths of 2-3 Earth masses in circular orbits. Here we pause again: The authors stress that their mass limits apply only to planets in circular orbits. Planets above these mass thresholds could still exist on eccentric orbits around this star.

So no planets yet around Proxima Centauri, and we’re beginning to rule out entire categories of planet here. We also have the possibility of smaller worlds in interesting orbits. The encouraging thing is that the radial velocity work on Proxima is getting better and better, and the authors see us closing in on planets of Earth size:

With the results from this paper we demonstrate that the discovery of m sin i ≈ 1 M⊕ [one Earth mass] is within our grasp. Since sensitivity is a function of RV [radial velocity] precision, number of measurements and sampling, adding more points to the existing data string in a pseudo-random fashion, will allow us to improve the detection sensitivity over time.

On the broader question of M-dwarf planetary systems in general, Endl and Kürster note the ambiguity inherent in radial velocity studies in terms of mass. One way we can supplement our red dwarf studies is to find low-mass planets inside the habitable zone of such stars, where their close orbits make them more likely to transit the star than planets on wider orbits. Moreover, such transits will be more detectable — we say they have a greater ‘transit depth’ than for other types of star, referring to the change in brightness as the planet transits the star. As we discover transiting super-Earths in close orbits around M-dwarfs, then, we’ll be able to put them on a mass/radius diagram that will help us understand what’s happening around other such stars.

But we’re still not through with Proxima Centauri and its nearby companions. Just how old are these stars? More on that tomorrow, when I’ll consider the question in terms of what’s around us in the stellar neighborhood.

The paper is Endl and Kürster, “Toward Detection of Terrestrial Planets in the Habitable Zone of Our Closest Neighbor: Proxima Centauri,” Astronomy and Astrophysics, Volume 488, Issue 3, 2008, pp.1149-1153 (abstract).

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henk April 24, 2012 at 11:06

if Proxima Centauri have such a high Metallicity, than je chance is very high that it must have planets or doen’t it work that way ?

i do not really now the age of alpha centauri, but here is a interesting paper that say it is between 5.6 to 5.9 billion years old
http://arxiv.org/abs/astro-ph/0611733

Interstellar Bill April 24, 2012 at 12:29

Three cheers from the vacuum homesteaders for high stellar metallicity, especially because the concentration of economically vital high-Z elements is even more sensitive to metallicity than that of iron or silicon.

Rafal April 24, 2012 at 13:01

A question to more astronomy-wise people out there. It seems that discovery of Proxima Centauri wasn’t such an easy thing, which is surprising since this is the closest star to our system. Is it possible then that they might be some other red or brown dwarfs even closer to us that we haven’t discovered yet? Say at 2 ly. Can astronomers exclude such a possibility?

Paul Gilster April 24, 2012 at 13:07

Rafal writes:

A question to more astronomy-wise people out there. It seems that discovery of Proxima Centauri wasn’t such an easy thing, which is surprising since this is the closest star to our system. Is it possible then that they might be some other red or brown dwarfs even closer to us that we haven’t discovered yet? Say at 2 ly. Can astronomers exclude such a possibility?

This is one of the questions we hope the WISE mission can resolve — the data analysis continues. I think we can exclude a red dwarf closer than Alpha Centauri — we should have found it by now. But the brown dwarf question is still open.

Michael April 24, 2012 at 14:18

I once seen an article that Alpha Centuari maybe a relative of our Sun created in the same dust cloud and therefore should have a similar age -I think it was based on the iron ratio, see linkhttp://www.adrianberry.net/alpha.htm

Paul Gilster April 24, 2012 at 14:57

Check in tomorrow, though, when I’ll be looking at a more recent paper that makes the case for a much older Alpha Centauri system.

FrankH April 24, 2012 at 15:11

Paul,
One huge problem with Proxima is that it is one of the most active M dwarf flare stars, with frequent, short flares, on par with Solar flares. The flares mean that Proxima’s HZ is a technicality; a planet in the HZ will be bathed in bursts of full spectrum (X-rays to infrared) radiation every few days. What isn’t irradiated will be heated or broiled.

Paul Gilster April 24, 2012 at 15:59

Proxima’s flare activity is indeed a problem, and provokes some thoughts in tomorrow’s post, which I’ve just written and will have up in the morning.

Duane April 24, 2012 at 17:44

I’m in the screenwriting program here at UCLA, and I’m writing a script about humankind’s first interstellar colonizing mission. The planet? A large, rocky world tidally locked to a (relatively nearby) red dwarf primary that has a flare issue. I’m calling it the “anti-Star Trek” (not that ST wasn’t awesome).

One of the planet’s elements is a permanently rotating cyclone directly under the primary star, fueled by the constant heat.

And the people? Well, they have their problems, with their new home and each other.

If it ever gets made, I’ll let you know!

Paul Gilster April 24, 2012 at 19:53

Interesting! It does sound like Proxima might be a model for you. Keep us posted and good luck with the screenplay. You’re in a great school to be doing screenwriting, that’s for sure. The trick may be to figure out how the colonizing idea works given the extent of the flare problem, and it sounds like a film I’m going to want to see. Be sure to check out some of the climate models for red dwarfs, especially Joshi and Haberle’s latest, “Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone,” found here:

http://arxiv.org/abs/1110.4525

and also check Segura et al., “The Effect of a Strong Stellar Flare on the Atmospheric Chemistry of an Earth-like Planet Orbiting an M Dwarf”:

http://arxiv.org/abs/1006.0022

jkittle April 24, 2012 at 20:52

One life form’s deadly poison can be a feast for another. Think of the great die off when the earth’s atmosphere became oxidizing! ( somewhere out there may be a civilization contemplating earth and saying= no life there – poisonous oxygen rich atmosphere!
Frequent flares create new compounds high in the atmosphere that are out of chemical equilibrium , that can rain down onto the surface ( literally) These energy rich compounds can feed living micro-organisms, perhaps in combination with compounds from geochemical processes
Chemolithotrophs are very common on earth. They eat ( sulfur and or iron bearing)rocks or compounds like nitrates.they live on weathering rocks or deep in the crust

Eniac April 24, 2012 at 22:29

I would have to admit that the visible and IR part of the flares, which could well pass through the atmosphere, could be a problem for surface dwellers. FrankH puts it well with “broil”. There is, however, the deep sea, the far side, and cloud cover to consider, all of which seem to leave plenty of room for habitability. If not for us, at least creatures more adapted to the circumstances.

Scott G. April 24, 2012 at 23:44

Michael’s comment: “I once seen an article that Alpha Centuari maybe a relative of our Sun created in the same dust cloud…” reminded me of an interesting article from Scientific American (published 2009) that suggested that it was possible to actually find the Sun’s real siblings. I managed to locate two links to that and a related article. See: The Long-Lost Siblings of the Sun (preview) and Seeking Out the Sun’s Long-Lost Siblings. Very interesting topic.

bill April 25, 2012 at 2:40

Consider a large planet at a high angle to the line of sight. A gas giant like this could cause a radial velocity signature roughly similar to a much smaller planet in a system where the orbital plane was on the line of sight. In other words, the radial velocity technique is insensitive to the reflex velocity in the plane of the sky, giving us only a lower limit to a planet’s mass.

reflex velocity ?

Paul Gilster April 25, 2012 at 8:38

Stellar reflex motion is what Doppler methods are measuring. Daniel Benest talks about the ‘Doppler reflex velocity method’ as in this: “If a planet has a circular orbit, the analyzed stellar spectrum yields a sinusoidal oscillation of the stellar reflex motion.” This is in Topics in Gravitational Dynamics — I think I’m using the term correctly here but checked Benest to be sure. Let me know if I’m off-base.

ljk April 25, 2012 at 10:57

Duane said on April 24, 2012 at 17:44:

“I’m in the screenwriting program here at UCLA, and I’m writing a script about humankind’s first interstellar colonizing mission. The planet? A large, rocky world tidally locked to a (relatively nearby) red dwarf primary that has a flare issue. I’m calling it the “anti-Star Trek” (not that ST wasn’t awesome).

“One of the planet’s elements is a permanently rotating cyclone directly under the primary star, fueled by the constant heat.

“And the people? Well, they have their problems, with their new home and each other.”

Duane – you want to be really “anti-Star Trek”? Then don’t use people.

If you conduct a search in this blog, you will see a number of articles discussing the merits of using artificial intelligence and robotics to explore the galaxy.

Star Trek and other related types of science fiction television series and films used humans not because of course they gave actors some employment and a paycheck, but they also came from an era when humans were still better than computers and machines in general.

While we still do not have AI that is equivalent to a human brain’s style of functioning, this will likely change by the time we are able to send real missions to the stars. It has to, otherwise the mission success will be very problematic with communication delay times of years between the ship and home in case there are problems. Solutions and repairs will have to be done on the spot.

I do not want to overwhelm you with this idea, but start by looking at the Web site Orion’s Arm to see one example of how an interstellar society might work that does not involve humans who look and act just like we do now or a few decades ago. And read up on Hugo de Garis.

Perhaps you can be the one to get SF out of this paradigm. It may actually make a difference because most people are still focused on having a human crew as the vital parts in a starship. Good luck!

bill April 25, 2012 at 14:54

Paul,

“Stellar reflex motion is what Doppler methods are measuring. Daniel Benest talks about the ‘Doppler reflex velocity method’ as in this: “If a planet has a circular orbit, the analyzed stellar spectrum yields a sinusoidal oscillation of the stellar reflex motion.” This is in Topics in Gravitational Dynamics — I think I’m using the term correctly here but checked Benest to be sure. Let me know if I’m off-base.”

Looked up the definition on the Internet, and I’ll be darned if there isn’t a word exactly as you described it. Just goes to show that a research scientist will make up any word he or she can think of to make them stand out.

Duane April 25, 2012 at 18:27

Perhaps “anti-ST” was too strong a term. Kirk & co. land on various alien worlds, taking no more precaution than making sure their phasers are drawn. I’m updating this a bit with a world that isn’t necessarily better or worse than Earth; it’s just different.
“Solutions and repairs will have to be done on the spot.” Yep. In spades. In fact, I open with just that scenario.
I’m using people because my overarching theme is something Harlan Ellison once said in one of his SF film reviews. Paraphrasing: “We are the children of Creation and we deserve to take our place under the lights of many suns.”

Nathan Currier April 25, 2012 at 19:15

Duane,

There’s another habitability issue to consider with red dwarf planets: Tidal heating. Tidal heating varies as 1/distance^3, so it can be severe in planets that would need to huddle up to a very dim red dwarf like Proxima. Habitable-zone orbits would need to be very circular for any star less than 1/3 of a solar mass, or your potential Earth would end up as a super-Io. There was a paper on this recently; I forget where.

ljk April 26, 2012 at 8:50

Duane said on April 25, 2012 at 18:27:

“I’m using people because my overarching theme is something Harlan Ellison once said in one of his SF film reviews. Paraphrasing: “We are the children of Creation and we deserve to take our place under the lights of many suns.”

Ellison should have amended that to “We are ONE of the children of Creation.” Things will get pretty interesting if other intelligences also think they are the special children of existence and that their deities have made the Universe just for them.

Do the science fiction community a big favor: If you are going to have people colonizing an alien planet (and you really need to read the comments on the recent post about Worldships regarding actual humans traversing the stars), do not end up being like Earth 2 or Terra Nova. Those characters were boring and the stories predictable despite the exotic locations that were so full of promise for interesting plots. And worst of all, they tried to hammer home an environmental message. I am not against that, just when it is done so obviously.

And don’t try to become Lost 2, either. NBC recently failed miserably at that. :^)

You CAN do a science fiction story without baseline humans. It is possible, not to mention much more likely for interstellar travel. Break the paradigm. The Hunger Games just introduced the Dystopian Society to the masses; sure it is SF 101 but it is a start.

Understood, Duane. Just be sure that

stephen April 26, 2012 at 19:22

Maybe Proxima did form at a different time, in a different dust cloud and was later captured by ACen A-B. Is that too unlikely to consider?
Has a study been done of the possibility of other single or multiple stars gravitationally capturing other stars? What would we expect to see if that had happened? Thanks.

Daniel Alman April 26, 2012 at 22:30

ljk: “Things will get pretty interesting if other intelligences also think they are the special children of existence and that their deities have made the Universe just for them.”

Well it seems many ruthless and cut throat societies often claim that they have been directly blessed by some sort of deity:
The Egyptians
The Greek city states
The Roman empire
Various Islamic Kingdoms
Aztecs
Incas
The Spanish empire
The British empire
The US of America
The Boer republics
Nazi Germany
Imperial Japan

It seems that the most ruthless and cutthroat and arrogant societies often wield large amounts of power(relatively speaking) and brutally conquer most if not all regional competition. How do you expect humanity gained a foothold on the shores of civilization. I would expect any alien intelligence that have successfully colonized and maintained large amounts of infrastructure in many star systems to be ruthless and willing to exploit everything and anything in its path.

If you think about it. Our willingness to exploit resources in the moon and in Antarctica that may not even exist shows that we MIGHT become a great space faring sometime in the future.

Rob Henry April 28, 2012 at 18:36

If Proxima Centauri is such a notorious flare star, that makes if even better for Duane’s plot. Exploring a planet that may be as hot as Mercury’s equator averages at its sub solar point, as cold as Titan on its night side, and very Earth-like near the rim of its terminator would be very interesting and challenging.

Duane, may I suggest infusing your plot with the mystery of science, where there is not always a clear answer. If I were writing it I would base at least one new phenomena their on our *long delayed radio echoes*.

barry w.bruington January 10, 2013 at 14:29

I look up at the night sky and my mind races with curiosities. How many intelligent beings are there in our galaxy of hundreds of billions of other suns, and then there are the billions of other galaxies. With these enormous numbers life must be common and even intelligent life has to be there to. I am also saddened, I think about what my species is doing to save itself from the extintion that must come if we stay here on our world, I think we are capable of so much more than we are now. Yes we are a young species yet the way we are going now we dont have thousands of years to become a true spacefaring species. Our time is short, our numbers are increasing at a rate that is not sustainable for another 200 years. Unless we come up with some now source of energy say fusion then our extiction may not be that far off. It saddens me that there are still humans who believe in gods and devils, heavens and hells, we have religions whose beliefs are so utterly rediculous that I wonder if those who believe in them can truely be called intelligent. We know there are many ways our species can go extinct and we do nothing to decrease these odds, can you call a species such as this truely intelligent ?

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