Proxima Centauri: Looking at the Nearest Star

by Paul Gilster on April 23, 2012

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.

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{ 18 comments }

jkittle April 23, 2012 at 9:58

So it sounds like Proxima is the “nemesis” of this system… e very million years or so nudging comets out of orbit to fall toward the closer binary- what wild orbits they would have!
just imagine a the view of a comet from a planet in this binary system anyway- tails changing direction visible on “dark nights ” and disappearing on “bright nights”
Note as well the possibility that Proxima is not really that stable an orbit. Over a 1 billion year time frame other stars are bound to pass by and interact with its orbit. We ( earthlings) are supposed to have another star pass within 1 light year of our sun during the next million years. by comparison- that might be enough in a system like this to unbound the red dwarf.

henk April 23, 2012 at 10:00

if Proxima Centauri is 6.5 billion years old. Does that mean that alpha centauri A and B are just as old ?

What is the true age of alpha centauri ?

can Proxima Centauri also not send killer asteroids to the planets by alpha centauri A and B ?
let we first find those planet’s inside that habitable zone, and than just check if those planet’s have water.

Paul Gilster April 23, 2012 at 10:36

henk, I’ll be discussing Proxima’s age in tomorrow’s post, in relation to Centauri A and B. It’s an interesting issue, and has ramifications for the planet hunt. More tomorrow.

Interstellar Bill April 23, 2012 at 12:03

The main question for vacuum homesteaders:
is there enough matter to build a Type II civilization?
They would prefer asteroids to planets,
but will be thankful there’s any matter at all.

Imagine getting to a star system and finding only dust.
Harvesting stellar wind would require a planetary scale apparatus,
so there has to be some matter there to start with.

Greg April 23, 2012 at 12:10

Paul, What is the Proxima Centauri’s direction of travel in comparison to Sol’s? are they moving closer or are they traveling away from one another, meaning they could have been closer at one time or will be in the future.

Paul Gilster April 23, 2012 at 13:11

Assuming Proxima Centauri is indeed bound to Centauri A and B, the system as a whole is moving closer to us — the last figure I saw on this is 25.1 ± 0.3 km/s, so that roughly 28,000 years from now, the distance will close to about 3.26 light years, which is about as close as it will get. More on this tomorrow as well.

ljk April 23, 2012 at 13:59

Here is what intelligent life will be doing with Proxima Centauri about ten thousand years from now, according to Orion’s Arm:

http://www.orionsarm.com/eg-article/4ce52fdf5e9de

And here is Alpha Centauri A and B for good measure:

http://www.orionsarm.com/eg-article/487146f744c8e

andy April 23, 2012 at 14:48

Radial velocity studies of Proxima have so far ruled out anything with m*sin(i) above 2-3 Earth masses in the habitable zone, and anything with m*sin(i) above the mass of Neptune within 1 AU.

Obviously the sin(i) dependency means that more massive planets could exist if they are in face-on orbits, but the geometric probability of such a configuration is low.

Endl and Kürster (2008) “Toward detection of terrestrial planets in the habitable zone of our closest neighbor: proxima Centauri“.

Mike April 23, 2012 at 15:10

Considering the age of the system I find it amazing that such a distant stellar member as Alpha Centauri C could remain gravitionally bound after billions of years of orbiting galactic central and being occasionaly perturbed by other passing stars.
If the age and metallicity of Proxima could really be nailed down (not an easy thing to do with active red dwarfs) then we would have better evidence that Proxima is really a birth sister of A and B and not an adopted sibling or just a next door neighbor.
Especially if something was needed to trigger the delivery of water to any possible planets of A and/or B in the early days of the Alpha Centauri system.
It would be nice to know if Proxima was in fact present in bygone eras.

Michael April 23, 2012 at 15:33

I wonder if it would be possible to first visit Promixa first then use a very close to the star gravity assist to deflect it to Alpha Centauri, Promixa has a very high density and lower temperature enabling a very close approach -this however would be affected by dust/gas in the system.

Rob Henry April 23, 2012 at 17:12

Asimov’s astronomers on the Centauri system are not very bright. They seem to measure parallax by their own planets orbital movement rather than their systems. By this better second measure, an object at 10,000 au would measure about 100 to 200 seconds of parallax ( depending on its angle to the semi major axis of the Centauri system), not a measly 20!

stephen April 23, 2012 at 19:35

Gregory Benford’s story, “Dance to Strange Musics” is on a planet of Alpha Centauri, where the local life takes energy from piezo-electric effects enhanced by solar tides. I don’t remember if there are lunar tides as well, but the effects are complex and bizarre.

Phil April 23, 2012 at 21:26

“Obviously the sin(i) dependency means that more massive planets could exist if they are in face-on orbits, but the geometric probability of such a configuration is low”

Why? Is there any reason to expect anything other than a random orbital inclination to our line of site with Proxima? I know that A & B are inclined almost 80 degrees, but do we expect Proxima to be orbiting in the same plane?

P

Eniac April 23, 2012 at 22:38

Michael: Gravity assist is ineffectual at the velocities we are talking about here, given the distance.

Interstellar Bill April 24, 2012 at 1:00

Michael

That’s a big swing angle from the Sol-Proxima vector
to the Proxima-Alpha vector.

Swingby delta-vee is proportional to escape velocity,
which is much less at Proxima’s low stellar mass.

Compared to any reasonable mission velocity,
Proxima’s low swingby speed would be the same as stopping,
which you might as well do.

ljk April 24, 2012 at 9:18

And remember, despite what Star Trek: First Contact said, we will find the inventor of warp drive, Zephram Cochrane, on Alpha Centauri, so getting there is imperative!

http://en.memory-alpha.org/wiki/Zefram_Cochrane

andy April 24, 2012 at 12:46

Why? Is there any reason to expect anything other than a random orbital inclination to our line of site with Proxima?

A random orientation implies a lower probability for face-on orbits.

The orientation can be represented by a unit vector perpendicular to the orbital plane. This vector will lie on the surface of a sphere, and for a face-on orbit this vector will be pointing either towards or away from us. This situation is equivalent to the vector pointing towards the poles of the sphere (inclination 0º or 180º). An edge-on orbit corresponds to the vector lying on the sphere’s equator (inclination 90º). Inclination is equivalent to 90º−latitude.

Now consider that for the same interval of latitude there is less area at high latitudes than near the equator (there is more area between 0ºN and 10ºN than there is between 80ºN and 90ºN north). This implies that the probability of orbits that are nearly face-on is lower than the probability of orbits that are nearly edge-on.

stephen April 25, 2012 at 8:34

I wondered if anybody else thinks Benford’s scenario in that story I mentioned, seemed far-fetched?

And regarding Zefram Cochrane–he emigrated to Alpha Centauri much later in life, then he went for a joyride and was abducted by that mysterious cloud creature.

The original series Star Trek Technical Manual posits that the Alpha Centaurians use the Greek alphabet.

So apparently the Preservers took some ancient Greeks and deposited them on a planet of Alpha Centauri. Okay, I’ll stop now.

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