Centauri Dreams

I cannot live without good coffee, and that means fresh beans ground right before brewing, and either manual drip or French press extraction. Every morning after publishing Centauri Dreams I make a couple of cups and go out on the deck to rest my eyes and ponder the state of things before hitting the books for background research in the afternoon. Various thoughts about what to write next always come to me, but yesterday I mused about Enrico Fermi, the legendary Italian physicist who, among so much else, left us with a great unanswered question: Where are they? If it’s so easy for the universe to make intelligent species, why is SETI coming up so short?

Where are they indeed? The day was gorgeous, the air filled with birdsong, temperatures in the mid-60s and a mild breeze. What better setting to be immersed in, thinking about where life emerges and when? I imagined Fermi sitting across from me with a cup of my Costa Rica Tres Rios in his hand, wondering what he might say about the fuss his question has caused over the years. I can almost hear him saying, “Look, it wasn’t serious. It was just a throw-away comment over lunch. I didn’t even think about it.” And then I imagine him gazing out over the hillside and wrinkling his brow. “But you know, it really is an interesting question, isn’t it? I mean, really!”

Image: Enrico Fermi, with no coffee in sight. Credit: AIP Emilio Segrè Visual Archives.

Alpha Centauri: The Age of Things

Obviously I’m putting words in the man’s mouth, but that’s the thing about the Fermi paradox: It keeps coming around. And in one respect it seems particularly disturbing. If the Sun is in the vicinity of stars that are far older than it is, that would give planets around those stars far more time to produce their own living species and far more time for intelligence and technology to emerge. We can think about these things in terms of Alpha Centauri, the subject of these last few posts, because based on recent studies, these stars are much older than our Sun.

How old? To answer the question, the go-to people are Patrick Eggenberger (Observatoire de Genève) and colleagues, who in 2004 produced a paper on the matter that ran in Astronomy & Astrophysics (citation below). A wide range of ages has been posited for these stars over the years, ranging from as little as 2.71 billion years up to well over 6 billion — the paper runs through the previous analyses — but Eggenberger and team go to work with the latest astrometric, photometric, spectroscopic and especially asteroseismic data to reach a strong conclusion: “The global parameters of the ? Cen system are now ?rmly constrained to an age of t = 6.52 ± 0.30 Gyr.”

6.52 billion years, plus or minus 300 million. Now we can think about the Sun’s age, thought to be about 4.57 billion years, and you can see that Alpha Centauri A and B have a 2 billion year jump on us. So does Proxima Centauri, because as we saw yesterday, Greg Laughlin and Jeremy Wertheimer made a convincing case that Proxima is indeed bound to Centauri A and B, and thus probably originated in the same molecular cloud that produced its companions.

I think I’m going to pour Dr. Fermi another cup of coffee about now, because that 2 billion years provides ample time for interesting things to develop given an astrobiologically friendly planet. Long-time Centauri Dreams readers will also know that Charles Lineweaver (Australian National University) has studied the galactic habitable zone and the distribution of stars in the Milky Way by age, finding that 75 percent of the stars in an annular region between 7 and 9 kiloparsecs from galactic center, where life should be possible, are older than the Sun.

Alpha Centauri, using the age estimates of Eggenberger and colleagues, turns out to be fairly average, for Lineweaver says “…the average age of Earths around Sun-like stars is 6.4 ± 0.9 billion years.” He thus thinks that planets around other stars in the galactic habitable zone should be, on average, 1.8 billion years older than our planet, about the same difference as between our Sun and Alpha Centauri. And this is only an average. Milan ?irkovi? (Astronomical Observatory of Belgrade) notes that there should be inhabited worlds in our galaxy as much as 3 billion years older than our own. So we have on our own doorstep (in astronomical terms) a triple star system that dramatically points to the time frames life has had available to develop civilizations.

Proxima’s Deadly Flares

At this point Dr. Fermi might well take me to task (at least, the imaginary Dr. Fermi who is not only still with us, but completely up to speed on red dwarf studies). I think he would point out that Proxima Centauri is an active flare star with loads of coronal X-ray emission, not exactly a hospitable place for life. We can imagine a calmer, much older Proxima Centauri eventually settling down into a benign middle age, but imagining that also makes us realize that while Alpha Centauri A and B may have provided the opportunity for intelligent life to develop long ago, Proxima may be the most marginal of the Centauri possibilities as of now.

Red dwarfs live, depending on their mass, for trillions of years, so we shouldn’t despair about the future — and we can always ponder whether any kind of adaptive mechanism might rescue astrobiology even in so hostile a place. But as a home for human colonists of the first interstellar mission, any conceivable planet of Proxima Centauri gives way to what we hope to find around Centauri A or B, a rocky world in a habitable zone we might be able to survive within.

Two Charles Lineweaver papers are in play here, the first being “An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect,” Icarus 151 (2001), pp. 307-313 (full-text). The second is “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way,” Science Vol. 303, No. 5654 (January, 2004), pp. 59-62 (abstract). The Eggenberger paper is “Analysis of alpha Centauri AB including seismic constraints,” Astronomy & Astrophysics Volume 417, Number 1 (April I 2004), pp. 235-246 (abstract).

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 ? 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).

Let’s start the week with a reminder about Debra Fischer’s work on Alpha Centauri, which we talked about last week. There are several ongoing efforts to monitor Centauri A and B for planets and, given the scrutiny the duo have received for the past several years, we should be getting close to learning whether there are rocky worlds in this system or not. Fischer’s continuing work at Cerro Tololo involves 20 nights of observing time that her grant money can’t cover. Private donations are the key — please check the Planetary Society’s donation page to help if you can.

While interest in the Alpha Centauri system is high, the small red dwarf component of that system has been getting relatively little press lately. But I don’t want to neglect Proxima Centauri, which as far as we know is the closest star to the Earth (some 4.218 light years away, compared to Centauri A and B’s 4.39 light years). From a planet around Centauri B, it would be hard to know that Proxima (also known as Alpha Centauri C) was even associated with the primary stars. It’s fully 15,000 AU out (about 400 times Pluto’s distance from the Sun), and only a check on its large proper motion would reveal its true nature. The photo below makes this point as well as anything. Proxima Centauri is not a star that immediately grabs the attention.

Image: Proxima Centauri (shown by the arrow) in relation to Centauri A and B. The latter appear as a single bright object at upper left. Credit: European Southern Observatory.

In his book Alpha Centauri: The Nearest Star (New York: William Morrow & Company, 1976), Isaac Asimov imagined what Proxima Centauri would look like from a planet around either of the binary stars. He worked out that Proxima’s magnitude would be a fairly dim 3.7, making it a naked eye object but not a particularly noticeable one. Its proper motion would be about 1 second of arc per year. As always, Asimov writes entertainingly:

Neither its brightness nor its proper motion would attract much attention, and stargazers might look at the sky forever and not suspect this dim star of belonging to their own system. The only giveaway would come when astronomers decided to make a routine check of the parallaxes of the various visible stars in the sky. After a month or so, they would begin to get a hint of an extraordinarily large parallax and in the end they would measure one of 20 seconds of arc, which would be so much higher than that of any other star that they would at once suspect it of being a member of their own system.

When Asimov wrote this, it was generally accepted that Proxima Centauri was gravitationally bound to Centauri A and B, but that finding has come in for serious review in the years since. In 1993, Robert Matthews and Gerard Gilmore (Cambridge University) took a hard look at the kinematic data and found that Proxima Centauri was on the borderline for an object in a bound orbit around Centauri A and B. But a later study by Jeremy Wertheimer and Gregory Laughlin (UCSC) firmed up the case using data from the European Space Agency’s Hipparcos satellite and concluded that Proxima was indeed bound to the system and not just passing in the night.

All this can have big implications. To understand why, read what Laughlin says on his systemic site about potential planets around Centauri A and B:

At first glance, one expects that the Alpha Centauri planets will be very dry. The period of the AB binary pair is only 79 years. The orbital eccentricity, e=0.52, indicates that the stars come within 11.2 AU of each other at close approach. Only refractory materials such as silicates and metals would have been able to condense in the protoplanetary disks around Alpha Centauri A and B. To reach the water, you need to go out to the circumbinary disk that would have surrounded both stars. With only A and B present, there’s no clear mechanism for delivering water to the parched systems of terrestrial planets.

But now let’s assume that Proxima Centauri is indeed bound to this system in a million-year orbit. Given our age estimates of these stars, that would mean Proxima has orbited Centauri A and B roughly 6500 times. Its presence, note Laughlin and Wertheimer, introduces a mechanism for dislodging comets from outer orbits and pushing them into the inner system(s), allowing for the water they might otherwise lack. Even in terms of astrobiology, then, Proxima Centauri may play a role in making planets around Centauri A and B interesting, not to mention what it offers up in its own right.

In thinking about planets around Proxima itself, we might start with metallicity, a major factor in planet formation. Centauri A and B have higher levels of metallicity than the Sun, but calculating red dwarf metallicities is tricky, for reasons I want to get into tomorrow, when we’ll look more closely at Proxima Centauri as a possible home for planets. We’re learning more about this intriguing star all the time, and any planets there would play into our thinking with regard to future interstellar probes, something to keep in mind as tomorrow’s discussion of Proxima proceeds.

The Matthews and Gilmore paper is “Is Proxima really in orbit about Alpha CEN A/B?,” Monthly Notices of the Royal Astronomical Society Vol. 261, No. 2 (1993), p. L5-L7 (abstract). The Wertheimer and Laughlin paper is “Are Proxima and Alpha Centauri Gravitationally Bound?” The Astronomical Journal 132:1995-1997 (2006), available online.