The Orbit of Proxima Centauri

The small red dwarf star Proxima Centauri has been in the news quite a bit because of the discovery of an Earth-size planet, designated Proxima Centauri b, found orbiting inside this star’s habitable zone (see “Habitable Planet Reality Check: Proxima Centauri b”). But long before this, Proxima Centauri was well known for being the closest known star to our solar system at a distance of “only” 4.24 light years.

Ever since its discovery in 1915 by Scottish astronomer Robert Innes, it has been generally assumed that Proxima Centauri was bound to the pair of larger, Sun-like stars, α Centauri A and B, some 4.37 light years from us (whose official designation, according to a recent IAU announcement, will be changed from its Bayer designation α Centauri back to its ancient name, Rigil Kentaurus – see this IAU press release). This association has been assumed because the three stars are about the same distance from us in the same part of the sky and have approximately the same proper motion. It seemed highly improbable that these stars were not part of a bound, triple star system and were instead making their closest approach to each other at the same time they were passing close to us. But proving that Proxima Centauri is actually gravitationally bound to its larger neighbors has been problematic over the last century. The measurement uncertainties associated with the physical properties as well as the relative positions and motions of these stars have been large enough that the possibility that Proxima Centauri does not have a closed orbit around α Centauri AB an estimated 15,000 AU away could not be eliminated with any certainty.

Diagram showing the relative sizes of the Sun and the three stars of the α Centauri system. (PHL@UPR Arecibo)

Whether or not Proxima Centauri actually orbits α Centauri AB is more than just a simple academic question for astronomers. Studies have shown that the presence of Proxima Centauri in a closed orbit could affect the formation and stability of any circumbinary planets present around α Centauri AB, not to mention any planets the red dwarf might have. In addition, there is one formation scenario for the newly discovered Proxima Centauri b which theorizes that this world may have originally formed around α Centauri AB and was subsequently captured by the red dwarf at some point in the past. This could have an impact on the initial volatile inventory, evolution and, ultimately, the potential habitability of Proxima Centauri b.

The last published study about the orbit of Proxima Centauri b was a 2006 paper by Jeremy Wertheimer and Gregory Laughlin (University of California Observatories/Lick Observatory). Using the best data available on the masses, distances, positions, proper motion and radial velocities of the three stars involved, Wertheimer and Laughlin determined that Proxima Centauri was moving at a speed of 530±140 meters per second relative to the center of mass (or barycenter) of α Centauri AB. Their Monte Carlo simulation results of the motions of these stars and their associated uncertainties suggested that there were about even odds that Proxima Centauri was gravitationally bound to α Centauri AB. If Proxima Centauri were bound to its pair of Sun-like neighbors, it was likely near apastron (the farthest point in its eccentric orbit).

A scatter plot of Proxima Centauri radial velocity versus orbital binding energy for the Monte Carlo simulations run by Wertheimer and Laughlin. Negative values of binding energy indicate closed orbits with the asterisks denoting orbits with Proxima Centauri be close to apastron. Click on image to enlarge. (Wertheimer and Laughlin)

Wertheimer and Laughlin found that the biggest source of uncertainty in their calculations resulted from the fairly large measurement uncertainty in the radial velocity of Proxima Centauri. While there were highly accurate data on the relative changes in radial velocity with respect to an Earth-bound standard from surveys searching for extrasolar planets (see “The Search for Planets Around Proxima Centauri”), the uncertainties in the absolute radial velocity measurements for this dim V-magnitude 11 red dwarf were still on the order of 200 meters per second.

A view of ESO’s VLT which was used by the Red-Optical Planet Survey (ROPS) to obtain new and much more accurate measurements of Proxima Centauri. (J.L. Dauvergne & G. Hüdepohl/ESO)

French astronomers Pierre Kervella (Unidad Mixta Internacional Franco-Chilena de Astronomía/Observatoire de Paris), Frederic Thévenin (Observatoire de la Côte d’Azur) and Christophe Lovis (Observatoire Astronomique de l’Université de Genève) have recently submitted a paper for publication which addresses the question of the orbit of Proxima Centauri using new data. Kervella et al., along with other collaborators, had already recently published their work on the dynamics of α Centauri AB to provide the best determination of the masses of these two stars and the motion of their barycenter. For the radial velocity of Proxima Centauri, Kervella and Thévenin used a new high-precision measurement determined by the team conducting the Red-Optical Planet Survey (ROPS) of 15 nearby red dwarfs (including Proxima) which employed the UVES spectrometer on the 8.2-meter Very Large Telescope (VLT, UT2) at the European Southern Observatory’s facility at Cerro Paranal in the Chilean Andes. In addition to using the best available values for the parameters of this system, Kervella et al. also took into account the systematic effects of convective blueshift and gravitational redshift on the radial velocity measurements of the stars in this system to derive actual space velocities.

Kervella et al. determined that the relative velocity of Proxima Centauri with respect to the barycenter of α Centauri AB is 293±43 meter per second. Since escape velocity is calculated to be 545±11 meter per second at Proxima Centauri’s newly determined linear separation of 12,947±260 AU, this new study finds that the relative velocity of Proxima Centauri is consistent with it being bound to α Centauri AB to a 5.9σ level of certainty. Even with the adoption of alternate values of the gravitational redshift of Proxima Centauri, which was the largest source of error in the calculations, the conclusion is still robust.

A 3D diagram showing the orbit of Proxima Centauri around α Centauri AB as derived by Kervella et al.. Click on image to enlarge (Kervella et al.)

Using their new results of the position and motions of the stars in this system, Kervella et al. were able to calculate the orbit of Proxima Centauri around α Centauri AB. The semimajor axis of the orbit was found to be 9,100 (+700/-400) AU with a corresponding orbital period of 591,000 (+65,000/-43,000) years. With its eccentric orbit ranging from 5,300 (+1,200/-900) to 12,900 (+300/-100) AU, Proxima Centauri is close to apastron today, just as many earlier investigators had suspected. The next periastron (or closest point) in its orbit is expected 298,000 (+35,000/-29,000) years from the present.

This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star α Centauri AB also appears in the image to the upper-right of Proxima itself. (ESO/M. Kornmesser)

While this result requires independent verification and will be subject to updates based on newer, more precise data that is surely to come, it does seem that Proxima Centauri is in fact bound to α Centauri A and B and is not, by chance, just passing close by these stars in about the same epoch these stars are making their closest approach to the Sun. With this new information in hand, dynamicist can begin to study how the interactions of these three stars may have affected the formation and subsequent evolution of any exoplanets in this system including the newly found Proxima Centauri b.

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Related Reading

“Habitable Planet Reality Check: Proxima Centauri b”, Drew Ex Machina, August 29, 2016 [Post]

“The Search for Planets Around Proxima Centauri”, Drew Ex Machina, February 23, 2015 [Post]

General References

P. Kervella, F. Thévenin and C. Lovis, “Proxima’s orbit around Alpha Centauri”, arXiv 1611.03495 (submitted to Astronomy & Astrophysics), November 10, 2016 [Preprint]

Jeremy G. Wertheimer and Gregory Laughlin, “Are Proxima and α Centauri Gravitationally Bound?”, The Astronomical Journal, Vol. 132, No. 5, pp. 1995-1997, November 2006