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Hyades Planet Challenges Formation Theories

At just over 150 light years from Earth in the constellation Taurus, the Hyades is the nearest open star cluster to Earth. We’ve been scouting the terrain in clusters recently, looking at globular clusters like 47 Tucanae and open clusters like M37, both of which are under intense scrutiny. But the first exoplanet to be identified definitively in either kind of cluster seems to be Epsilon Tauri b in the Hyades [but see below re a 2.5 Jupiter mass planet in the system comprising pulsar PSR B1620−26 and its white dwarf companion — that one is in the globular cluster M4].

It’s an interesting world, a gas giant that’s a little less than 2 AU out with an orbital period of 1.63 years. This is the first planet discovered around a red giant, its star the most massive of all planet hosts known. [My mistake: several planets evidently orbit red giants — see comments below, and check here for another example]. That leads to intriguing speculation: Should we expect planets around other red giants, and do our current planet formation theories work in this environment?

The latter point bears watching. Greg Laughlin (UC-Santa Cruz) argues that in the case of Epsilon Tauri b, the cluster’s harsh ultraviolet radiation should have disrupted the protplanetary nebula.

The UV radiation environment in the original Hyades cluster was fierce. The protostellar disks of the individual Hyads were likely photoevaporated before the growing planetary cores were able to reach the runaway gas accretion phase that gives rise to Jupiter-mass planets (see our paper on this topic). When we get the full inventory of planets in the Hyades, I think we’ll find plenty of Neptunes and terrestrial planets, but almost nothing in the Jovian range. Indeed, work by Bill Cochran and the Texas RV group has demonstrated that the Hyades are generally deficient in massive planets.

So how do we explain Epsilon Tauri b? Laughlin thinks this may be an example of a planet forming via the process known as gravitational instability, which can produce massive planets and is little affected by nearby ultraviolet radiation. Gravitational instability is a model in which instabilities within the protostellar disk can cause gas giants to coalesce. The rival core accretion model sees such planets growing from small cores of rock and ice that acquire new mass through collisions, eventually growing large enough for their gravity to draw in nearby gas.

Laughlin is saying that the likely dispersal of the protostellar disks in the Hyades stars (thanks to UV) makes core accretion less likely in the case of Epsilon Tauri b (there simply wasn’t time). Whereas if gravitational instability produces a planet for every few hundred stars formed, as Laughlin believes, then there is no reason not to expect such a world in an open cluster like the Hyades. That’s a win for gravitational instability, though Laughlin still sees core accretion as the dominant model, writing elsewhere that “…the weight of observational and theoretical evidence seems to be shifting against the gravitational instability hypothesis.”

The paper is Sato et al., “A Planetary Companion to the Hyades Giant Epsilon Tauri,” accepted by The Astrophysical Journal but not yet available at the arXiv preprint site.

Comments on this entry are closed.

  • Dunkleosteus February 21, 2007, 19:05

    A few corrections —

    * Epsilon Tauri b is not the first planet in a star cluster: that fame belongs to the planet of pulsar PSR B1620−26 and its white dwarf companion, which are located in the globular cluster M4. Interestingly, it is a relatively distant companion, one that is currently detectable only by the pulsar timing method.

    * Epsilon Tauri is certainly not the first red giant with a planet. There are several of them, Pollux being the most famous. In fact, the planet of Pollux was first suspected as early as in 1993.

    * Epsilon Tauri may not even be the most massive known star with a planet; that fame may belong to the bright giant HD 13189. However, the calculated mass of HD 13189 b depends on the mass of the star, which is poorly constrained (2–7 M_Sun) and may well be far above the planet/brown dwarf limit (8–20 M_J).

  • Administrator February 21, 2007, 19:53

    Thanks for the correction re red giants, which I just added to the original post. As to mass, I’ll stick with Laughlin’s assessment on Epsilon Tauri: “It weighs in at 2.7 solar masses, making it the most massive star known to harbor a planet,” though I take your point about the mass of the star.

    Re PSR B1620−26 b, you’re right — my impression had been that a brown dwarf was still a possibility, but evidently Hubble data rule that out.

  • tedfractus April 24, 2007, 12:17

    Cryptoplanet update

    Authors: Robert L. Kurucz

    (Submitted on 22 Apr 2007)

    Abstract: We have had several talks recently reviewing 11 years of exoplanet discoveries through radial velocity variations, or from transits, or from microlensing. More than 200 exoplanets have been found, including some around pulsars that we do not discuss here.

    My physical definition for a planet is a roughly spherical, self-gravitating body more massive than 10**26 g formed from the leftover material in a protostellar disk after the protostar forms. Radiation from the protostar pushes the inner wall of the disk outward. The material agglomerates and forms planets in radial sequence. The outer planets are formed slowly by classical dynamical mechanisms acting in the snow zone. Planets have dense cores because of agglomeration.

    Not one of the exoplanets discovered thus far is a planet. They are cryptoplanets formed from matter ejected by protostars. When protostars have excessive infall at high latitudes, they partially balance angular momentum through outflow at the equator as they spin up. The ejected matter is trapped in the magnetic torus formed between the star and the disk, like a tokamak. The tokamak eventully reconnects and magnetic compression forms self-gravitating remnants trapped and compressed by a closed spherical magnetic field, spheromaks. Cooled spheromaks are cryptoplanets.

    They orbit near the star. They can merge with each other or fall into the star or be ejected. They can grow by accreting gas. They have a low density core and abundances characteristic of the protostar. Their masses, radii, densities, and orbits are random, and are inconsistent with the parameters for planets. They tend to have lower density than planets.


    research presentation, 85 figures


    Astrophysics (astro-ph)

    Cite as:

    arXiv:0704.2860v1 [astro-ph]

    Submission history

    From: Robert Kurucz [view email]

    [v1] Sun, 22 Apr 2007 04:02:49 GMT (2942kb)


  • ljk January 21, 2008, 11:16

    The ages of L dwarfs

    Authors: R.F. Jameson, N. Lodieu, S.L. Casewell, N.P. Bannister, P.D. Dobbie

    (Submitted on 18 Jan 2008)

    Abstract: We present a new method to derive the age of young (<0.7 Gyr) L dwarfs based on their near-infrared photometry, colours, and distances. The method is based on samples of L dwarfs belonging to the Upper Sco association (5 Myr), the Alpha Per (85 Myr) and Pleiades (125 Myr) clusters, and the Ursa Major (400 Myr) and Hyades (625 Myr) moving groups. We apply our method to a number of interesting objects in the literature, including a known L dwarf binary, L dwarf companions, and spectroscopic members of the young sigma Orionis cluster.

    Comments: 8 pages, 5 figures. Accepted for publication in MNRAS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.2915v1 [astro-ph]

    Submission history

    From: Sarah Casewell [view email]

    [v1] Fri, 18 Jan 2008 15:43:29 GMT (39kb)


  • ljk January 30, 2008, 13:25

    Spitzer observations of the Hyades: Circumstellar debris disks at 625 Myr of age

    Authors: Lucas Cieza, William D. Cochran, Jean-Charles Augereau

    (Submitted on 29 Jan 2008 (v1), last revised 30 Jan 2008 (this version, v2))

    Abstract: We use the Spitzer Space Telescope to search for infrared excess at 24, 70, and 160 micron due to debris disks around a sample of 45 FGK-type members of the Hyades cluster. We supplement our observations with archival 24 and 70 micron Spitzer data of an additional 22 FGK-type and 11 A-type Hyades members in order to provide robust statistics on the incidence of debris disks at 625 Myr of age an era corresponding to the late heavy bombardment in the Solar System. We find that none of the 67 FGK-type stars in our sample show evidence for a debris disk, while 2 out of the 11 A-type stars do so.

    This difference in debris disk detection rate is likely to be due to a sensitivity bias in favor of early-type stars. The fractional disk luminosity, L_dust/L*, of the disks around the two A-type stars is ~4.0E-5, a level that is below the sensitivity of our observations toward the FGK-type stars. However, our sensitivity limits for FGK-type stars are able to exclude, at the 2-sigma level, frequencies higher than 12% and 5% of disks with L_dust/L* greater than 1.0E-4 and L_dust/L* greater than 5.0E-4, respectively. We also use our sensitivity limits and debris disk models to constrain the maximum mass of dust, as a function of distance from the stars, that could remain undetected around our targets.

    Comments: 33 pages, 11 figures, accepted by ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.4403v2 [astro-ph]

    Submission history

    From: Lucas Cieza [view email]

    [v1] Tue, 29 Jan 2008 00:08:31 GMT (605kb)

    [v2] Wed, 30 Jan 2008 04:13:58 GMT (605kb)


  • ljk March 14, 2008, 15:06

    The exoplanet-host star iota Horologii: an evaporated member of the primordial Hyades cluster

    Authors: S. Vauclair, M. Laymand, F. Bouchy, G. Vauclair, A. Hui Bon Hoa, S.
    Charpinet, M. Bazot

    (Submitted on 13 Mar 2008)

    Abstract: We show that the exoplanet-host star iota Horologii, alias HD17051, which belongs to the so-called Hyades stream, was formed within the primordial Hyades stellar cluster and has evaporated towards its present location, 40 pc away. This result has been obtained unambiguously by studying the acoustic oscillations of this star, using the HARPS spectrometer in La Silla Observatory (ESO, Chili). Besides the fact that $\iota$ Hor belongs to the Hyades stream, we give evidence that it has the same metallicity, helium abundance, and age as the other stars of the Hyades cluster. They were formed together, at the same time, in the same primordial cloud.

    This result has strong implications for theories of stellar formation. It also indicates that the observed overmetallicity of this exoplanet-host star, about twice that of the Sun, is original and not caused by planet accretion during the formation of the planetary system.

    Comments: to be published in A&A letters

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.2029v1 [astro-ph]

    Submission history

    From: Sylvie Vauclair [view email]

    [v1] Thu, 13 Mar 2008 18:45:20 GMT (216kb)


  • ljk September 24, 2008, 8:04

    Deep MMT Transit Survey of the Open Cluster M37 IV: Limit on the Fraction of Stars With Planets as Small as 0.3 R_J

    Authors: J.D. Hartman, B.S. Gaudi, M.J. Holman, B.A. McLeod, K.Z. Stanek, J.A. Barranco, M.H. Pinsonneault, S. Meibom, J.S. Kalirai

    (Submitted on 22 Sep 2008)

    Abstract: We present the results of a deep (15 ~< r ~< 23), 20 night survey for transiting planets in the intermediate age open cluster M37 (NGC 2099) using the Megacam wide-field mosaic CCD camera on the 6.5m MMT. We do not detect any transiting planets among the ~1450 observed cluster members.

    We do, however, identify a ~ 1 R_J candidate planet transiting a ~ 0.8 Msun Galactic field star with a period of 0.77 days. The source is faint (V = 19.85 mag) and has an expected velocity semi-amplitude of K ~ 220 m/s (M/M_J).

    We conduct Monte Carlo transit injection and recovery simulations to calculate the 95% confidence upper limit on the fraction of cluster members and field stars with planets as a function of planetary radius and orbital period. Assuming a uniform logarithmic distribution in orbital period, we find that < 1.1%, < 2.7% and < 8.3% of cluster members have 1.0 R_J planets within Extremely Hot Jupiter (EHJ, 0.4 < T < 1.0 day), Very Hot Jupiter (VHJ, 1.0 < T < 3.0 days) and Hot Jupiter (HJ, 3.0 < T < 5.0 days) period ranges respectively. For 0.5 R_J planets the limits are < 3.2%, and < 21% for EHJ and VHJ period ranges, while for 0.35 R_J planets we can only place an upper limit of < 25% on the EHJ period range. For a sample of 7814 Galactic field stars, consisting primarily of FGKM dwarfs, we place 95% upper limits of < 0.3%, < 0.8% and < 2.7% on the fraction of stars with 1.0 R_J EHJ, VHJ and HJ assuming the candidate planet is not genuine.

    If the candidate is genuine, the frequency of sim 1.0 R_J planets in the EHJ period range is 0.002% < f_EHJ < 0.5% with 95% confidence. We place limits of < 1.4%, < 8.8% and < 47% for 0.5 R_J planets, and a limit of < 16% on 0.3 R_J planets in the EHJ period range.

    This is the first transit survey to place limits on the fraction of stars with planets as small as Neptune.

    Comments: submitted to ApJ. 62 pages, 21 figures, 4 tables

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0809.3807v1 [astro-ph]

    Submission history

    From: Joel Hartman [view email]

    [v1] Mon, 22 Sep 2008 20:51:57 GMT (1167kb)