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The Search for Vulcan

40 Eridani is a triple-star system some 16 light years from Earth. If it rings a faint bell, that’s probably because of its association with Star Trek. In the universe of the show, 40 Eridani is home to Vulcan, birthplace of the inscrutable Mr. Spock (Gene Roddenberry himself signed off on the idea). Not so long ago, the existence of planets would have been doubted in such a system, but we’re learning that double and even triple star systems can and do support planets. So maybe there is a ‘Vulcan’ out there after all, though doubtless sans humanoids with pointy ears.

View of two habitable zones

In any case, the elements of this system are widely spaced, and 40 Eridani A is a K-class star not so different from Centauri B, a star that could well support Earth-mass planets. Recently Angelle Tanner (Caltech) embarked on simulations designed to show whether or not the Space Interferometery Mission (known as SIM PlanetQuest) might be able to detect such a world. Tanner’s work confirmed that a planet like this in the habitable zone (about 0.6 AU) would be detectible. But it will have to wait for later missions, perhaps armed with sunshade or advanced coronagraph technology, to make the key spectroscopic measurements that would reveal the presence of biomarkers in its atmosphere.

The image above (click to enlarge) offers a comparison of the larger habitable zone around our Sun and that of 40 Eridani A (Credit: JPL). The Jet Propulsion Laboratory also offers a nifty animation of ‘Spock’s home’ here. Tanner’s work is slated for Publications of the Astronomical Society of the Pacific (and thanks to Hans Bausewein for the heads-up). The future of SIM PlanetQuest itself is more problematic. As of May 1, the word is “Launch deferred indefinitely by NASA headquarters.”

Comments on this entry are closed.

  • andy May 12, 2007, 10:13

    As for habitable planets in the system, bear in mind that 40 Eridani B is a white dwarf, and the BC pair is in a relatively close orbit with a semimajor axis of 35 AU.

    During its red giant stage, 40 Eridani B could have had a severe effect on any planetary system around 40 Eridani A: aside from the increased luminosity, there is also the effect of having the highly-enhanced red giant winds. Around the star Mira, the companion star (Mira B, which has now been shown to be a K dwarf, not a white dwarf as once believed) is surrounded by a disc of material which has been accreted from the stellar wind of the red giant star. What would the effects of such a “born-again protoplanetary disc” be on an existing planetary system?

    Finding a planetary system around 40 Eridani A (especially one with a world in the habitable zone) would therefore be extremely interesting.

  • andy May 12, 2007, 11:05

    I’ll just add that planets have been found around stars with white dwarf companions (e.g. Gliese 86), so the possibility of gas giants in such an environment is not ruled out.

  • andy May 12, 2007, 11:10

    Ooops… got the orbital parameters wrong there! The white dwarf is separated by around 420 AU from 40 Eridani A, it is 40 Eridani B and C which are separated by 35 AU.

  • Administrator May 12, 2007, 14:40

    Andy, yes, the spacing seems OK. JPL’s PlanetQuest site quotes Sean Raymond (University of Colorado, Boulder) on this: “Since the three members of the triple star system are so far away from each other [hundreds of astronomical units – the Earth-Sun distance], I see no reason why an Earth-mass planet would not be able to form around the primary star, 40 Eridani A.”

  • andy May 12, 2007, 20:07

    Yes, definitely I’m not contesting the possibility that 40 Eridani A could have formed a planetary system – a few of the currently-known extrasolar planets are in triple systems. If we regard a triple system as a star (1) and a close binary (2,3), it is also interesting to note that the currently-known planets in triple systems all reside around star 1 – in the 40 Eridani system, this would correspond to 40 Eridani A.

    I’m more interested in the effects of the red giant on the evolution of the system. I’m going to guess that at 400 AU, the increased luminosity would not have a significant effect, but the stellar wind of the red giant would make the environment around the stars very dusty (could this trigger ice ages or something, I don’t know). For comparison, the separation between Mira A and Mira B is somewhere in excess of 60 AU, so maybe the 40 Eridani system is too wide a binary to form a born-again protoplanetary disc (again, this is just a guess).

    If there are terrestrial planets around 40 Eridani A, it would be extremely interesting to see if there are any signs in the geological record of the red giant winds… which would make this system would be an ideal target for an advanced interstellar probe.

  • taoist May 13, 2007, 5:09

    I seem to remember that 40 Erdani is also the home system of one of the alien species in Harry Turtledove’s World War series.

  • Administrator May 13, 2007, 7:42

    Andy, I just re-read and saw that I had missed your original point. But the effects of the stellar wind from a red giant do add to the picture here. A look at the kind of geological record you’re talking about would be fascinating indeed, and I think your thought on ice ages is certainly plausible as one possible effect. We must be talking about a truly dusty environment.

  • Ronald May 14, 2007, 6:14

    @andy;
    With regard to Gl 86 (well, you may know this already), from the Extrasolar Planet Encyclopedia: it has a brown dwarf companion at only about 20 AU and yet at least one large planet (in close orbit at 0.11 AU). Spectral type and metallicity, at only about 58% of solar, are comparable to 40 Eridani. This bodes well for 40 Eridani. However, what puzzles me, is that Gl 86 has a gas giant in close orbit, despite such low metallicity! Luckily, 40 Eridani, does not appear to have one like that, leaving room for a more ‘normal’ planetary system.

  • andy May 14, 2007, 7:59

    @Ronald:
    The companion of Gliese 86 was originally reported to be a brown dwarf. Based on astrometric and colour observations, Gliese 86 B may be either a massive brown dwarf (~70 Jupiter masses) or a white dwarf. If Gliese 86 B is responsible for the residual radial-velocity drift of Gliese 86 A, then the mass is around 0.5 solar masses, and it is thus a white dwarf. See Gl86B: a white dwarf orbits an exoplanet host star and New constrains on Gliese 86 B. VLT near infrared coronographic imaging survey of planetary hosts.

  • Ronald May 14, 2007, 8:34

    @andy:
    thanks; in that case (Gl 86 B being a massive brown dwarf of white dwarf at only 18-20 AU from Gl 86 A), it is even more reassuring that Gl 86 A still manages to have at least one planetary companion (be it in very close orbit). This bodes well for Alpha Centauri A and B (OK closest approach is approx. 11 AU, but not all hope is lost, as theoratical models also indicated).

    This brings me to the next question: what is presently the closest binary star system (of course I mean closest to each other, not nearest to us) known to host at least one planet?

  • andy May 14, 2007, 11:41

    @Ronald:
    Problem with finding out which is the lowest-separation binary is that we don’t actually have orbital elements for a lot of them: the only information that is known is the projected separation (in terms of an angle on the sky), which gives a lower limit on the distance between the stars at the current time. The true distance could be larger (since the separation along the line of sight is unknown), and in any case the orbits may be eccentric, so the separation now is not necessarily the semimajor axis of the orbit.

    Gamma Cephei, Gliese 86 and HD 41004 are good candidates for the smallest separation of exoplanet host stars where the planet orbits one of the components: the orbital elements of Gamma Cephei B have been fairly well determined (semimajor axis = 19.02 AU), the other two have a projected separation of about 20 AU.

    The triple system HD 188753 has a semimajor axis of around 12.3 AU, but the planet around HD 188753 A has not been detected in follow-up observations.

    The pulsar/white dwarf binary PSR B1620-26 on the other hand has a separation of around 0.8 AU, with the planet in a circumbinary orbit. The history of the system is probably very complicated – the planet probably didn’t form around the stars it is currently orbiting!

  • ljk May 29, 2007, 10:34

    SIM PlanetQuest Key Project Precursor Observations to Detect Gas Giant Planets Around Young Stars

    Authors: Angelle Tanner, Charles Beichman, Rachel Akeson, Andrea Ghez, Konstantin N. Grankin, William Herbst, Lynne Hillenbrand, Marcos Huerta, Quinn Konopacky, Stanimir Metchev, Subhanjoy Mohanty, L. Prato, Michal Simon}

    (Submitted on 25 May 2007)

    Abstract: We present a review of precursor observing programs for the SIM PlanetQuest Key project devoted to detecting Jupiter mass planets around young stars. In order to ensure that the stars in the sample are free of various sources of astrometric noise that might impede the detection of planets, we have initiated programs to collect photometry, high contrast images, interferometric data and radial velocities for stars in both the Northern and Southern hemispheres. We have completed a high contrast imaging survey of target stars in Taurus and the Pleiades and found no definitive common proper motion companions within one arcsecond (140 AU) of the SIM targets. Our radial velocity surveys have shown that many of the target stars in Sco-Cen are fast rotators and a few stars in Taurus and the Pleiades may have sub-stellar companions. Interferometric data of a few stars in Taurus show no signs of stellar or sub-stellar companions with separations of 0.1 mag) that would degrade the astrometric accuracy achievable for that star. While the precursor programs are still a work in progress, we provide a comprehensive list of all targets ranked according to their viability as a result of the observations taken to date. By far, the observable that moves the most targets from the SIM-YSO program is photometric variability.

    Comments:

    Accepted for publication in Publications of the Astronomical Society of the Pacific, 25 pages, 9 figures

    Subjects:

    Astrophysics (astro-ph)

    Cite as:

    arXiv:0705.3687v1 [astro-ph]

    Submission history

    From: Angelle Tanner [view email]

    [v1] Fri, 25 May 2007 03:57:34 GMT (978kb)

    http://arxiv.org/abs/0705.3687

  • Shaun June 29, 2007, 5:38

    ljk the radial velocity surveys mentioned by the paper’s authors may indeed be suggestive of the presence of substellar mass objects in orbit around the the young stars in Taurus and the Pleiades. But one needs to understand that substellar mass objects refer not only to Jovian class bodies but also Brown Dwarfs (BDs).

  • ljk June 2, 2008, 11:35

    Crowded-Field Astrometry with SIM PlanetQuest. II. An Improved Instrument Model

    Authors: R. Sridharan, Ronald J. Allen

    (Submitted on 29 May 2008)

    Abstract: In a previous paper we described a method of estimating the single-measurement bias to be expected in astrometric observations of targets in crowded fields with the future Space Interferometry Mission (SIM). That study was based on a simplified model of the instrument and the measurement process involving a single-pixel focal plane detector, an idealized spectrometer, and continuous sampling of the fringes during the delay scanning.

    In this paper we elaborate on this “instrument model” to include the following additional complications: spectral dispersion of the light with a thin prism, which turns the instrument camera into an objective prism spectrograph; a multiple-pixel detector in the camera focal plane; and, binning of the fringe signal during scanning of the delay. The results obtained with this improved model differ in small but systematic ways from those obtained with the earlier simplified model.

    We conclude that it is the pixellation of the dispersed fringes on the focal plane detector which is responsible for the differences. The improved instrument model described here suggests additional ways of reducing certain kinds of confusion, and provides a better basis for the evaluation of instrumental effects in the future.

    Comments: 15 pages, 10 figures, accepted for publication in PASP July 2008

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.4622v1 [astro-ph]

    Submission history

    From: Ronald J. Allen [view email]

    [v1] Thu, 29 May 2008 20:37:34 GMT (243kb)

    http://arxiv.org/abs/0805.4622

  • ljk May 19, 2009, 12:36

    Star Trek’s main Web source says Vulcan orbits 40 Eridani A, so make it so:

    http://memory-alpha.org/en/wiki/40_Eridani_A

    Lots of details – and Gene Roddenberry and Harvard’s blessings – here:

    http://www.projectrho.com/vulsun.htm