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Orbital Determination for Proxima Centauri

Let’s talk this morning about the relationship of Proxima Centauri to nearby Centauri A and B, because it’s an important issue in our investigations of Proxima b, not to mention the evolution of the entire system. Have a look at the image below, which shows Proxima Centauri’s orbit as determined by Pierre Kervella (CNRS/Universidad de Chile), Frédéric Thévenin (Observatoire de la Côte d’Azur) and Christophe Lovis (Observatoire astronomique de l’Université de Genève). The three astronomers have demonstrated that all three stars — Proxima Centauri as well as Centauri A and B — form a single, gravitationally bound system.

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Image: Proxima Centauri’s orbit (shown in yellow) around the Centauri A and B binary. Credit: Kervella, Thévenin and Lovis.

A couple of things to point out here, the first being the overall image. You’ll see Alpha Centauri clearly labeled within the yellow ellipse of Proxima’s orbit. Off to the right of the ellipse, you’ll see Beta Centauri. I often see the image of these two stars identified as Centauri A and B, but Kervella et al have it right. The single bright ‘star’ within the ellipse is the combined light of Centauri A and B. Beta Centauri, at the right, is an entirely different star, itself a triple system in the constellation Centaurus, at a distance of about 400 light years.

Now as to that orbit — 550,000 years for a single revolution — things get interesting. One reason it has been important to firm up Proxima’s orbit is that while a bound star would have affected the development of the entire system, the question has until now been unresolved. Was Proxima Centauri actually bound to Centauri A and B, or could it simply be passing by, associated with A and B only by happenstance? Back in 1993 Robert Matthews and Gerard Gilmore found this to be a borderline case, calling for further kinematic data to clarify the issue.

When Jeremy Wertheimer and Gregory Laughlin (UC-Santa Cruz) attacked the problem in 2006, they found it ‘quite likely’ that Proxima Centauri was bound to the A/B pair. If this were the case, it would mean that the trio probably formed together out of the same nearby material, with the result that we could expect them to have the same age and metallicity. Laughlin and Wertheimer assumed that future, yet more accurate kinematic measurements would make it clear ‘that Proxima Cen is currently near the apastron of an eccentric orbit…’

And now we have Kervella and team, who have used the HARPS instrument (High Accuracy Radial Velocity Planet Searcher) on ESO’s 3.6m instrument at La Silla to make the call. Using radial velocity and astrometry, the researchers have surmounted the main problem with determining Proxima’s bound state. The lack of high-precision radial velocity measurements has been the result of Proxima’s relative faintness, but drilling down into HARPS data has produced a new radial velocity of −21.700 ± 0.027 km s−1, which tracks nicely with the prediction of Wertheimer and Laughlin, and is low enough to indicate a bound state.

As we consider that interesting planet around Proxima Centauri, we now can ponder that its star is the same age as Centauri A and B, and that its age is a comparable 6 billion years, making the planet about a billion years older than our Earth. Exactly how the planet formed becomes an interesting issue as well, because we have interactions between three stars to think about. From the paper:

The orbital motion of Proxima could have played a significant role in the formation and evolution of its planet. Barnes et al. (2016) proposed that a passage of Proxima close to α Cen may have destabilized the original orbit(s) of Proxima’s planet(s), resulting in the current position of Proxima b. Conversely, it may also have influenced circumbinary planet formation around α Cen (Worth & Sigurdsson 2016). Alternatively, Proxima b may also have formed as a distant circumbinary planet of α Cen, and was subsequently captured by Proxima. In these scenarios, it could be an ocean planet resulting from the meltdown of an icy body (Brugger et al. 2016). Proxima b may therefore not have been located in the habitable zone (Ribas et al. 2016) for as long as the age of the α Cen system (5 to 7 Ga; Miglio & Montalbán 2005; Eggenberger et al. 2004; Kervella et al. 2003; Thévenin et al. 2002).

So there we are. Plenty of alternatives to ponder as we look into the origins of the nearest known planet to our Solar System. Just how the researchers tuned up the radial velocity data to avoid the problem of convective blueshift — where the star’s unstable surface can shift the observed wavelength of spectral lines – and gravitational redshift, which can likewise be misleading, is covered in the paper’s appendix. The selection of four strong very high signal-to-noise emission lines made the difference in this exquisitely tight measurement.

The paper is Kervella, Thévenin & Lovis, “Proxima’s orbit around α Centauri,” accepted at Astronomy & Astrophysics (preprint).

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Comments on this entry are closed.

  • Bruce Mayfield December 27, 2016, 12:13

    Nice work. Thought we might have needed Gaia’s data to make this call.

  • Gideon Marcus December 27, 2016, 13:51

    Very exciting news. I’ve been eagerly anticipating this news. We can finally drop the asterisk (Proxima Centauri may not actuallu be bound to Alpha Centauri…)

  • Harry R Ray December 27, 2016, 13:58

    Rounding off some figures: Age of system 5 Billion years. Number of orbits of Proxima Centauri around the Regil Kentaurus binary 10,000. Periastron 3,000 AU. Apastron 10,000 AU. NOW: CAN THE KOZAI-LIDOV PUMPING EFFECT BE FINALLY QUANTICIZED with respect to Proxima b? A SEPARATE analysis(using EXISTING data, but; with a DIFFERENT analysis CODE)reveals an eccentricity of 17% for Proxima b. THE GOOD NEWS: The NEW orbit STILL fits ENTIRELY in the CONSERVATIVE habitable zone. THE BAD NEWS: Unless the Kozai-Lidov pumping effect is either VERY WEAK or NON-EXISTANT, that eccentricity may have been in the past, or may be in the future, FAR GREATER than its EXISTING VALUE!

  • Coacervate December 27, 2016, 18:38

    Good science creates more questions. S0 exciting to learn more about our Neighbours. Great to see you back Paul. I look forward to reading up on the breakthroughs 2017 may hold!

    I have a question that I only feel safe to ask here: Since my childhood I have observed the constellation Orion from both hemispheres. Is it possible that the 3 stars forming his belt have moved enough that a casual observer could discern the change over a 50+ year period.

    It seems to me that when I was in scouts the 3 points formed a very-nearly straight line. Now, to my ancient eyes it seems distinctly non-linear.

    Given the distances and my limited knowledge I think it must be my mind that has become bent yet I must ask.

    • David B. Benson December 28, 2016, 0:45

      Surely this is due to your aging eyesight and that alone.

    • Bruce Mayfield December 28, 2016, 16:59

      The greatest total proper motion of the stars in this asterism is that of Alnitak; RA 3.19 mas/yr, DEC 2.03 mas/yr. So in 50 years it would have only moved about 365 milliarcseconds. Way too small to notice in such a tiny span of time.

      • Coacervate December 28, 2016, 18:40

        Thank you Bruce. 1/3 of an arcsecond is probably like the diameter of a human hair at arms length. That does put things into perspective. Now we know what has really drifted out of alignment :)

  • Bruno Raymundo December 28, 2016, 8:17

    Probably there are more people thinking the same: this makes Liu Cixin’s book “The Three-Body Problem” all the more interesting, doesn’t it? :)

    • Matt McIrvin January 1, 2017, 10:06

      Unfortunately (or fortunately, given how it plays out in the book) Liu Cixin got the dynamics of the Alpha Centauri system completely wrong. It certainly wouldn’t be chaotically unpredictable to the degree he describes from year to year; Alpha Cen is just a relatively tightly bound binary with a third partner in a wide orbit much further out.

  • djlactin December 28, 2016, 9:44

    My next question: is Sol bound to the Centauri system?

    • Bruce Mayfield December 28, 2016, 17:06

      No. We’re just (star)ships passing in the night.

  • Joe December 28, 2016, 20:44

    If Proxima b is a billion years older than Earth, then this is an excellent example of the synchronicity problem described in the Citizen SETI article. Given that it is impossible to predict what life on Earth will be like in a billion years, all bets are off when it comes to Proxima b.

  • Michael December 29, 2016, 13:17

    I am surprised if Proxima is so lightly bound that it has survived in orbit for so long. There should have been many close encounters over such a long time scale to disrupt the system, perhaps in the past it was more tightly bound or it could have drifted in.

    • Bruce Mayfield December 29, 2016, 18:00

      Maybe so Michael, but AC’s two main stars combine for twice the pull of Sol. Our outer Oort cloud is assumed to extend at least out to 50 kau. (1ly=63.239kau) Proxima’s newly found orbit ranges between 4.3 & 13 kau out from AC. If our system’s been able to retain its long period comets for 4.5By why couldn’t Proxima have been held on to for 6?

    • Mark December 29, 2016, 22:26

      Michael, I had the same thought and wondered if Proxima was captured more recently. I also wonder how this new information squares with the previously determined age for Proxima is 4.85 Gyr, probably making it too young to have formed with Alpha Centuri A and B?

      • Bruce Mayfield December 30, 2016, 13:29

        The new paper (based on improved messurements) does not square with the old 4.85 Gyr age for Proxima, which was just an estimate inside an error range of possible ages derived from older, less precise data. The authors conclude that Proxima is indeed as old as the AC pair; 6 Gyr.

        The wikipedia article on Proxima needs a rewrite per this new much improved info.

        • Michael December 31, 2016, 6:05

          In stellar nurseries the stars are much closer together, being closer to globular cluster densities than we see today. We would need AC&PC to have been in a low density part of the star formation region to not have the orbit altered greatly.

          • Bruce Mayfield December 31, 2016, 11:54

            Early encounters is a reasonable suggestion. That could help to explain the orbital excentricities in this system.

          • Bruce Mayfield December 31, 2016, 12:09

            Your idea about the density of the area of the cloud out of which these stars formed is interesting too Michael.

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