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On Planets in Binary Systems

Alpha Centauri A and B, the two primary stars of the Alpha Centauri threesome, orbit a common center of gravity, with an average separation of 23.7 AU. But bear in mind that this average covers wider ground. The separation can close to about 11 AU or widen to as far as 36 AU. I bring these distances up because it’s an open question whether there are planets around either of these stars. The possibility exists that we might find planets around both, and of course we already know of that interesting planet circling nearby Proxima Centauri.

Do we have examples of close binaries in which we find a planet around each star? Until late August, the closest known binary system with planets orbiting both individual stars showed a separation of 1000 AU. But now we have the twin stars HD 133131A and HD 133131B. Around the former we have two planets, one whose minimum mass is about 1.5 times Jupiter’s mass, the other with a minimum of about half Jupiter’s mass. The second star hosts a planet of about 2.5 Jupiter’s mass. These stars aren’t nearly as close as the Alpha Centauri stars, but their separation is still small, about 360 AU.

This is an interesting find, pulled off by a team of Carnegie scientists and scheduled to appear in The Astronomical Journal. The work draws on data from the Planet Finder Spectrograph mounted on instruments at Las Campanas Observatory in Chile. The Carnegie work has focused on large planets in elliptical or long-duration orbits, in an attempt to learn whether Jupiter-sized planets are as comparatively rare as current statistics suggest.

“We are trying to figure out if giant planets like Jupiter often have long and, or eccentric orbits,” said Johanna Teske (Carnegie Observatories). “If this is the case, it would be an important clue to figuring out the process by which our Solar System formed, and might help us understand where habitable planets are likely to be found.”


Image: An illustration of this highly unusual system, which features the smallest-separation binary stars that both host planets ever discovered. Only six other metal-poor binary star systems with exoplanets have ever been found. Credit: Timothy Rodigas.

We’re pushing radial velocity methods hard in this work. In fact, the authors note that we have only a dozen or so planets detected with this technique that have periods of 3600 days or more. The outer planets around HD 133131A and B thus become interesting data points as we try to analyze how frequently giant planets in eccentric orbits and long orbital periods appear.

As to the stars themselves, both are believed to be metal-poor and probably in the range of 9.5 billion years old. And that raises questions in itself, as the paper notes, because we’re used to thinking of giant planets forming in a more metal-rich environment. In the segment below, Tc stands for condensation temperatures:

…we find a small but significant depletion of high Tc (> 1000 K) elements in HD 133131A versus B, and explore how this could be related to differences in planet formation, interactions between the planets and the two stars, and/or the composition of the planets orbiting the two stars. This system is the smallest separation “twin” binary system for which such a detailed abundance analysis has been conducted. Overall, HD 133131A and B are especially noteworthy because they are metal-poor but are orbited by multiple giant planets, contradictory to the predicted giant planet-metallicity correlation. The planets detected here will be important benchmarks in studies of host star metallicity, binarity of host stars, and long period giant planet formation.

Understanding the behavior of gas giants will eventually help us piece together our own Solar System’s formation, because we already know that Jupiter and Saturn helped to shape the asteroid belt in the system’s early days, and also were factors in the delivery of volatile materials to the inner rocky worlds. Whether or not our Solar System’s configuration is rare depends largely on the occurrence of giant planets in long-period orbits. Yet recent studies suggest that true Jupiter analogs in similar orbits occur around only one to four percent of host stars.

What we are now learning is that even if Jupiter analogs are rare, there are numerous possibilities among giant planets on highly eccentric orbits, which could have major consequences on the formation of inner rocky worlds. We now try to learn whether long-period gas giants are as unusual as they seem, and whether they are more frequently eccentric, all factors that play into key questions of planetary habitability as infant systems emerge.

The paper is Teske et al., “The Magellan PFS Planet Search Program: Radial Velocity and Stellar Abundance Analyses of the 360 AU, Metal-Poor Binary “Twins” HD 133131A & B,” accepted at The Astronomical Journal (preprint).


Comments on this entry are closed.

  • spaceman September 12, 2016, 14:08

    Interesting find, Paul. I thought that only in the case of significantly unequal mass binaries the smaller star/disk is less likely to form planets than its bigger counterpart. What reason would we expect one rather than both members of a binary system consisting of equal or approximately equal mass stars to generate planets and not the other?

  • DJ Kaplan September 12, 2016, 16:59

    It would be helpful to know if the orbital planes of exoplanets in such systems generally mimic the orbital planes of their host stars. Obviously this would give us a clue as to whether we can catch the a-Cen planets transiting their host stars.

  • Michael September 13, 2016, 1:15

    We would also need to know if the discs of material where Co-rotating or not,if they did co-rotate they could collide impacting planet formation. It could be a quick way of determining if planets are likely to be there in binary systems by looking out for the spin direction of the stars via spectroscopy.

    • DJ Kaplan September 13, 2016, 12:23

      Interesting point. Yes, that co-rotation would produce some intense forces on neighboring sets of planets.

      Consider if the two stars were formed at some distance, with a more cooperative “anti-rotation”. As they drew closer, couldn’t planets be whipped into a figure 8 orbit? I’m sure we will find systems like that eventually.

  • Jim Franklin September 13, 2016, 7:36

    This is interesting, I recall a computer simulation study conducted a few years ago, the results were published in one of the noted journals, in which the team had used what we knew about planet formation and ran it for the A Cen A and A Cen B stars. They demonstrated that both stars could host planets up to about 1.5 Earth Mass as long as they were in orbits no more than 3 AU from their host star. What was interesting, and wholly unexpected, was that in some of the simulations ran, a few came up with planets orbiting each star, and one or two planets orbiting the common centre of gravity of the system..effectively orbiting nothing in space, held in a Lagrangian type pattern. Whether nature actually does this for real is another question, and we have also learnt a lot since then so new simulations may not produce similar results, but that would be a weird world to live on to say the least.

    I am disappointed to read people still falling into the trap of believing that metallicity is a major factor in giant planet formation. It is reasonable to conclude that some giant gas worlds form using the same process as stars do, but their formation to larger objects is cut short, either by a lack of material, instabilities in the formation process caused by the nearby formation of other planets or the fact the host star “fire up” and blows the remaining material away before accretion can take the proto-planet to the Brown Dwarf stage or larger.

    With regards to A Cen A and A Cen B, a lot rides on how the stars formed and whether they have undergone any form of orbital migration. If their current orbits are original and not migrated in or out, then the possibility of planets close to one or both stars increases, but if the stars orbit around their common centre of mass has evolved over time, then the chances of planets diminish rapidly.

    • Ashley Baldwin September 14, 2016, 14:26

      Yes. The dreaded stellar age estimation raises its ugly head again ! Asteroseismology if detailed enough can give estimations of age within 10 %. Gyro chronology or spin rates used to be considered this accurate but seems to have fallen from grace since detailed asteroseismology became available. Chromospheric activity ( as measured here via the Ca III H and K absorption lines which are an accurate proxy for star spot activity ) can also be used but when active but Middle to old aged stars like Proxima Centauri still are despite this one wonders how accurate this method is too )

      Asteroseismology though requires uninterrupted and prolonged photometry , the type only available via space through telescopes like Kepler (which revolutionised the field ) and PLATO to come better still ( 2 and 3 years fixed observation periods during its 6 year primary mission) . Yet as of observing specific stars, ground based viewing is always hamstrung by breaks in observation due to the day /night cycle ( and/or the absence of worldwide telescope network coverage -something Las Cumbres observatories are currently trying to rectify . Most recently shown in their simultaneous use as part of the Proxima b RV discovery through their near uninterrupted photometry to exclude spurious signals caused by stellar activity ) . So the years long observations in this article don’t have the anything like the asteroseismology potency of Kepler despite great observation work to discover such long period “atypical” gas giant planets

      The age averages are often cited in the literature as precise when they in fact cover large ranges so the sooner we have extended asteroseismology the better for a whole host of reasons but not least exoplanet characterisation .

      The point made in this article is the significant difference in metallicity between the two binary constituents. “Pollution” caused by absorbing large planets can play a small part in variation but not excessively so.

  • ljk September 14, 2016, 10:16
  • ljk September 20, 2016, 11:34

    Stellar activity can mimic misaligned exoplanets

    2016 September 20

    The occultation of stellar active regions during the planetary transit1 can lead to inaccurate estimates of the characteristics of these exoplanets, especially the spin-orbit tilt angle. This was the conclusion of simulations made by a team2 of astronomers from the Instituto de Astrofísica e Ciências do Espaço (IA2) in Portugal, and Institute of Astrophysics of Georg-August University of Göttingen, in Germany.

    The Rossiter-Mclaughlin4 (RM) effect has been used to measure the spin-orbit tilt angle in exoplanets, a parameter which can provide crucial information about the planetary formation and evolution processes, and even help to discriminate between different planetary migration models.

    Mahmoudreza Oshagh, currently working at the Institute of Astrophysics of Georg-August-University of Göttingen, but developed the work in this paper5 as a post-doc at IA, commented: “Our results showed that the aligned transiting exoplanets are the ones that can be easily misinterpreted as misaligned owing to the stellar activity. Moreover, our study could provide a viable explanation for the few cases in the literature that obtained conflicting spin-orbit angles, for instance the case of exoplanet WASP-19b”.

    Full article here:


  • ljk September 22, 2016, 11:35

    heic1619 — Science Release

    Hubble helps find light-bending world with two suns

    22 September 2016

    A distant planet orbiting two stars, found by its warping of spacetime, has been confirmed using observations from the NASA/ESA Hubble Space Telescope. The planet’s mass caused what is known as a microlensing event, where light is bent by an object’s gravitational field. The event was observed in 2007, making this the first circumbinary planet to be confirmed following detection of a microlensing event.

    The majority of exoplanets detected so far orbit single stars. Only a few circumbinary planets — planets orbiting two stars — have been discovered to date. Most of these circumbinaries have been detected by NASA’s Kepler mission, which uses the transit method for detection [1].

    This newly discovered planet, however, is very unusual. “The exoplanet was observed as a microlensing event in 2007. A detailed analysis revealed a third lensing body in addition to the star and planet that were quite obvious from the data,” says David Bennett from the NASA Goddard Space Flight Center, USA, lead author of the study [2].

    The event, OGLE-2007-BLG-349, was detected during the Optical Gravitational Lensing Experiment (OGLE) [3]. OGLE searches for and observes effects from small distortions of spacetime, caused by stars and exoplanets, which were predicted by Einstein in his theory of General Relativity. These small distortions are known as microlensing.

    Full article here: