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A New ‘Hot Neptune’

Our second transiting Neptune-mass planet has been discovered via the HAT Network of small, automated telescopes maintained by the Harvard-Smithsonian Center for Astrophysics. HAT-P-11b is described by Greg Laughlin at systemic (thanks to many who sent this link):

HAT-P-11b is quite similar in mass and radius to Gliese 436b, and it’s actually somewhat larger than Neptune on both counts. When the mass and radius are compared to theoretical models, it’s clear that, like Gliese 436, it’s mostly made of heavy elements (that is, some combination of metal, rock and “ice”) with an envelope of roughly 3 Earth masses of hydrogen and helium). It’s completely dwarfed when placed next to an inflated hot Jupiter, HAT-P-9b, for instance…

The advantages of a detected transit are great. Couple the transit light curve with radial velocity measurements and you can work out the mass and radius of the transiting planet. Moreover, the opportunity to investigate planetary atmospheres comes into play through follow-up observations that can also tell us something about planetary temperatures. And because sixteen out of the twenty ‘hot Neptune’ planets we’ve found through various methods have been part of multiple planet systems, it’s no surprise that there is at least some evidence of a second planet to accompany HAT-P-11b.

It’s helpful indeed that HAT-P-11 will be in the Kepler mission’s observational field. With Kepler designed for a minimum mission of 3.5 years, the space-borne observatory should have this Neptune-class world available for more than 250 transits. That will firm up all our parameters for the planet based on the transit light curve, and will allow new evidence to be gathered for a second planet in the system. Knowing in advance that a particular Kepler star has a transiting world around it allows detailed observations to begin from the mission’s outset.

More in Bakos et al., “HAT-P-11b: A Super-Neptune Planet Transiting a Bright K Star in the Kepler Field,” submitted to the Astrophysical Journal and available online. Related: Stringellow et al., “Transit Timing Observations of the Extrasolar Hot-Neptune Planet GL 436b,” available here.

Comments on this entry are closed.

  • TheoA January 6, 2009, 15:49

    Isn’t TrES-2 also a transiting planet in the Kepler Field, though more Jupiter like in mass?

    The eccentricity still calls for an outside influence does it not? Any migration mechanism should have largely circularized the orbit during formation.

    This process of parking planets at the brink of oblivion is truly puzzling still.

  • andy January 8, 2009, 8:54

    Interesting that it has hints of a second companion: as yet, none of the transiting planets are in systems where a second planet has been characterised. This seems to be down to very small radial velocity datasets: since it’s harder to get RV observations, the measurements seem to be typically done only as far as is necessary to characterise known the transiting planet.

  • ljk January 16, 2009, 22:28

    [vsnet-alert 10954] Exoplanet Transit Database‏
    From: vsnet-alert-bounces@ooruri.kusastro.kyoto-u.ac.jp on behalf of Luboš Brát (brat@pod.snezkou.cz)
    Sent: Fri 1/16/09 5:46 PM
    To: vsnet-alert@ooruri.kusastro.kyoto-u.ac.jp

    Dear collegues, observers of transiting exoplanets. Let us introduce new tool for exoplanet transits observers. During year 2008, we have developed an Exoplanet Transit Database (ETD), where are collected all published transit observations. You can see the data both in table and in graphical output of relations: timings vs epoch, transit depth vs epoch, transit duration vs epoch.

    Besides this, there are two other main functions, especially for observers. Transit predictions and Data fitting procedure. Transit predictions are available for all locations around the world, just type your longitude and latitude. For each object, transit timings for next year are available (with highlighted observable transits vs bellow horizont or during daylight). Data fitting procedure is automatic on-line application form, where observer can upload his photometric observation of transit event.

    The application will fit your data with model light curve and find out time of midtransit, depth and lenght of the transit. There is also possibility to publish your transit observation in TRESCA database (TRansiting ExoplanetS and CAndidates) and after revision directly in ETD.

    Our database administrators are periodically checking for new transits both in literature and in on-line internet sources (TRESCA, AXA) and updates the ETD database. New transiting exoplanets published in literature are also included to ETD as soon possible after discovery.

    Feel free to use all features of Exoplanet Transit Database and in case of any suggestions, please contact us. And now the most important information: Exoplanet Transit Database can be found at


    Lubos Brat – president of Variable Star and Exoplanet Section of Czech Astronomical Society


  • ljk August 6, 2009, 19:36

    NASA Science News for August 6, 2009

    NASA’s new planet-hunting Kepler space telescope has detected the changing phases and atmosphere of a planet a thousand light years away.



    Check out our RSS feed at http://science.nasa.gov/rss.xml!

  • ljk September 14, 2009, 12:54

    Models of Neptune-Mass Exoplanets: Emergent Fluxes and Albedos

    Authors: David S. Spiegel (1), Adam Burrows (1), Laurent Ibgui (1), Ivan Hubeny (2), John A. Milsom (3) ((1) Princeton University, (2) Steward Observatory, (3) The University of Arizona)

    (Submitted on 11 Sep 2009)

    Abstract: There are now many known exoplanets with Msin(i) within a factor of two of Neptune’s, including the transiting planets GJ436b and HAT-P-11b. Planets in this mass-range are different from their more massive cousins in several ways that are relevant to their radiative properties and thermal structures.

    By analogy with Neptune and Uranus, they are likely to have metal abundances that are an order of magnitude or more greater than those of larger, more massive planets. This increases their opacity, decreases Rayleigh scattering, and changes their equation of state.

    Furthermore, their smaller radii mean that fluxes from these planets are roughly an order of magnitude lower than those of otherwise identical gas giant planets. Here, we compute a range of plausible radiative equilibrium models of GJ436b and HAT-P-11b.

    In addition, we explore the dependence of generic Neptune-mass planets on a range of physical properties, including their distance from their host stars, their metallicity, the spectral type of their stars, the redistribution of heat in their atmospheres, and the possible presence of additional optical opacity in their upper atmospheres.

    Comments: 16 pages, 6 figures, submitted to ApJ

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0909.2043v1 [astro-ph.EP]

    Submission history

    From: David Spiegel [view email]

    [v1] Fri, 11 Sep 2009 18:46:12 GMT (380kb)