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A Baffling New Planetary Discovery

A Jupiter-sized planet with the density of cork? The idea seems farcical, but it’s under discussion as I write at a news conference held by the Harvard-Smithsonian Center for Astrophysics (CfA). The planet, called HAT P-1, revolves around ADS 16402, a star much like our Sun that is part of a binary system some 450 light years away in the constellation Lacerta. The first planet found by the Hungarian Automated Telescope observing network, HAT P-1 may represent a new class of planet entirely.

For despite a radius of 1.38 times Jupiter’s, HAT P-1 has only half its mass. “This planet is about one-quarter the density of water,” said Gaspar Bakos (CfA). “In other words, it’s lighter than a giant ball of cork! Just like Saturn, it would float in a bathtub if you could find a tub big enough to hold it, but it would float almost three times higher.” Intriguingly, the new world isn’t the first planet with oddly low density. Another planet found by the transit method, HD 209458b, is also about 20 percent larger than predicted by theory.

New planet HAT P-1

So how do you puff up a planet? Additional heat in its interior would do the job, and that could be accomplished through tidal heating if the planet were titled in a Uranian-style orbit on its side. But that scenario doesn’t seem likely. As Smithsonian astronomer Matthew Holman puts it, “The circumstances required to tip over a planet are so unusual that this would seem unlikely to explain both known examples of inflated worlds.” Another possibility: tidal heating due to an eccentric orbit, but the current observations do not favor this possibility.

Image: The newly discovered world HAT-P-1 has baffled astronomers, since it is puffed up much larger than theory predicts. HAT-P-1 has a radius about 1.38 times Jupiter’s but contains only half Jupiter’s mass. Credit: David A. Aguilar (CfA).

HAT P-1 revolves around its primary once every 4.5 days at a distance twenty times closer to the Sun than the Earth. What we’ll hope to gain from detailed follow-up studies of this bizarre world is new information about how giant planets are formed and evolve. Right now there seem to be more questions than answers, and it will be fascinating to read this work in its entirety when it appears in The Astrophysical Journal. A preprint has just been posted at the arXiv site.

We’ll discuss the second announcement from this morning’s news conference — on biomarkers and the analysis of exoplanet atmospheres — tomorrow.

Comments on this entry are closed.

  • hdeasy September 18, 2006, 8:41

    Could the lowdensity be because it’s a disk? If this had been built by an alien civilisation they might have expanded lebensraum a la Diskworld a la Ringworld… that way incerase solar energy enormously etc.

  • Dylan Taylor September 26, 2006, 19:58

    I Don’t Believe It! That’s Probably Made in Photoshop!

  • Ian October 10, 2006, 7:51

    Dylan – That’s an artist’s impression. I think there are only two pics of planets, and they’re both virtually pixels against brighter pixels.

  • Jim Banklaan November 1, 2006, 9:21

    OK then, it’s made in Paintsho pro!

  • ljk May 3, 2007, 9:04

    CfA Release No: 2007-11

    For Release: Wednesday, May 2, 2007


    Cambridge, MA – Today, astronomers at the Harvard-Smithsonian Center for
    Astrophysics (CfA) announced that they have found the most massive known
    transiting extrasolar planet. The gas giant planet, called HAT-P-2b,
    contains more than eight times the mass of Jupiter, the biggest planet in
    our solar system. Its powerful gravity squashes it into a ball only slightly
    larger than Jupiter.

    HAT-P-2b shows other unusual characteristics. It has an extremely oval orbit
    that brings it as close as 3.1 million miles from its star before swinging
    three times farther out, to a distance of 9.6 million miles. If Earth’s
    orbit were as elliptical, we would loop from almost reaching Mercury out to
    almost reaching Mars. Because of its orbit, HAT-P-2b gets enormously heated
    up when it passes close to the star, then cools off as it loops out again.
    Although it has a very short orbital period of only 5.63 days, this is the
    longest period planet known that transits, or crosses in front of, its host

    “This planet is so unusual that at first we thought it was a false alarm –
    something that appeared to be a planet but wasn’t,” said CfA astronomer
    Gáspár Bakos. “But we eliminated every other possibility, so we knew we had
    a really weird planet.”

    Bakos is lead author of a paper submitted to the Astrophysical Journal
    describing the discovery. That paper is available online at


    HAT-P-2b orbits an F-type star, which is almost twice as big and somewhat
    hotter than the Sun, located about 440 light-years away in the constellation
    Hercules. Once every 5 days and 15 hours, it crosses directly in front of
    the star as viewed from Earth – a sort of mini-eclipse. Such a transit
    offers astronomers a unique opportunity to measure a planet’s physical size
    from the amount of dimming.

    Brightness measurements during the transit show that HAT-P-2b is about 1.18
    times the size of Jupiter. By measuring how the star wobbles as the planet’s
    gravity tugs it, astronomers deduced that the planet contains about 8.2
    times Jupiter’s mass. A person who weighs 150 pounds on Earth would tip the
    scale at 2100 pounds, and experience 14 times Earth’s gravity, by standing
    on the visible surface (cloud tops) of HAT-P-2b.

    CfA astronomer and co-author Robert Noyes said, “All the other known
    transiting planets are like ‘hot Jupiters.’ HAT-P-2b is hot, but it’s not a
    Jupiter. It’s much denser than a Jupiter-like planet; in fact, it is as
    dense as Earth even though it’s mostly made of hydrogen.”

    “This object is close to the boundary between a star and a planet,” said
    Harvard co-author Dimitar Sasselov. “With 50 percent more mass, it could
    have begun nuclear fusion for a short time.”

    An intriguing feature of HAT-P-2b is its highly eccentric (e=0.5) orbit.
    Gravitational forces between star and planet tend to circularize the orbit
    of a close-in planet. There is no other planet known with such an eccentric,
    close-in orbit. In addition, all other known transiting planets have
    circular orbits.

    The most likely explanation is the presence of a second, outer world whose
    gravity pulls on HAT-P-2b and perturbs its orbit. Although existing data
    cannot confirm a second planet, they cannot rule it out either.

    HAT-P-2b orbits the star HD 147506. With visual magnitude 8.7, HD 147506 is
    the fourth brightest star known to harbor a transiting planet, making the
    star (but not the planet) visible in a small, 3-inch telescope.

    HAT-P-2b was discovered using a network of small, automated telescopes known as HATNet, which was designed and built by Bakos. The HAT network consists of six telescopes, four at the Smithsonian Astrophysical Observatory’s
    Whipple Observatory in Arizona and two at its Submillimeter Array facility
    in Hawaii. As part of an international campaign, the Wise HAT telescope,
    located in the Negev desert (Israel) also took part in the discovery. The
    HAT telescopes conduct robotic observations every clear night, each covering
    an area of the sky 300 times the size of the full moon with every exposure.

    About 26,000 individual observations were made to detect the periodic dips
    of intensity due to the transit.

    Major funding for HATnet was provided by NASA. More information about HAT is available online at http://www.hatnet.hu

    Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
    Astrophysics (CfA) is a joint collaboration between the Smithsonian
    Astrophysical Observatory and the Harvard College Observatory. CfA
    scientists, organized into six research divisions, study the origin,
    evolution and ultimate fate of the universe.

  • ljk July 17, 2007, 11:03

    The Transit Light Curve Project. VII. The Not-So-Bloated Exoplanet HAT-P-1b

    Authors: Joshua N. Winn, Matthew J. Holman, Gaspar A. Bakos, Andras Pal, John Asher Johnson, Peter K.G. Williams, Avi Shporer, Tsevi Mazeh, Jose Fernandez, David W. Latham

    (Submitted on 13 Jul 2007)

    Abstract: We present photometry of the G0 star HAT-P-1 during six transits of its close-in giant planet, and we refine the estimates of the system parameters. Relative to Jupiter’s properties, HAT-P-1b is 1.20 +/- 0.05 times larger and its surface gravity is 2.7 +/- 0.2 times weaker. Although it remains the case that HAT-P-1b is among the least dense of the known sample of transiting exoplanets, its properties are in accord with previously published models of strongly irradiated, coreless, solar-composition giant planets. The times of the transits have a typical accuracy of 1 min and do not depart significantly from a constant period.

    Comments: To appear in AJ [19pg, 3 figures]

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0707.1908v1 [astro-ph]

    Submission history

    From: Joshua N. Winn [view email]

    [v1] Fri, 13 Jul 2007 00:55:26 GMT (176kb)


  • ljk July 31, 2007, 17:19

    HAT-P-3b: A heavy-element rich planet transiting a K dwarf star

    Authors: G. Torres (1), G. A. Bakos (1,2), G. Kovacs (3), D. W. Latham (1), J. M. Fernandez (1), R. W. Noyes (1), G. A. Esquerdo (1), A. Sozzetti (1,4), D. A. Fischer (5), R. P. Butler (6), G. W. Marcy (7), R. P. Stefanik (1), D. D. Sasselov (1), J. Lazar (8), I. Papp (8), P. Sari (8) ((1)Harvard-Smithsonian Center for Astrophysics; (2) Hubble Fellow; (3) Konkoly Observatory, Hungary; (4) INAF – Osservatorio Astronomico di Torino, Italy; (5) San Francisco State University; (6) Carnegie Institute of Washington; (7) University of California, Berkeley; (8) Hungarian Astronomical Association, Hungary)

    (Submitted on 28 Jul 2007)

    Abstract: We report the discovery of a Jupiter-size planet transiting a relatively bright (V = 11.56) and metal-rich early K dwarf star with a period of about 2.9 days. On the basis of follow-up photometry and spectroscopy we determine the mass and radius of the planet, HAT-P-3b, to be M_p = 0.599 +/- 0.026 M_Jup and R_p = 0.890 +/- 0.046 R_Jup. The relatively small size of the object for its mass implies the presence of about 75 Earth masses worth of heavy elements (1/3 of the total mass) based on current theories of irradiated extrasolar giant planets, similar to the mass of the core inferred for the transiting planet HD 149026b. The bulk density of HAT-P-3b is found to be rho_p = 1.06 +/- 0.17 g/cm^3, and the planet orbits the star at a distance of 0.03894 AU. Ephemerides for the transit centers are T_c = 2,454,218.7594 +/- 0.0029 + N (2.899703 +/- 0.000054) (HJD).

    Comments: To appear in ApJL

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0707.4268v1 [astro-ph]

    Submission history

    From: Guillermo Torres [view email]

    [v1] Sat, 28 Jul 2007 23:32:48 GMT (108kb)


  • ljk June 12, 2008, 0:04

    Measurement of the Spin-Orbit Angle of Exoplanet HAT-P-1b

    Authors: John A. Johnson, Joshua N. Winn, Norio Narita, Keigo Enya, Peter K. G. Williams, Geoffrey W. Marcy, Bun’ei Sato, Yasuhiro Ohta, Atsushi Taruya, Yasushi Suto, Edwin L. Turner, Gaspar Bakos, R. Paul Butler, Steven S. Vogt, Wako Aoki, Motohide Tamura, Toru Yamada, Yuzuru Yoshii, Marton Hidas

    (Submitted on 10 Jun 2008)

    Abstract: We present new spectroscopic and photometric observations of the HAT-P-1 planetary system. Spectra obtained during three transits exhibit the Rossiter-McLaughlin effect, allowing us to measure the angle between the sky projections of the stellar spin axis and orbit normal, \lambda = 3.7 +/- 2.1 degrees. The small value of \lambda for this and other systems suggests that the dominant planet migration mechanism preserves spin-orbit alignment.

    Using two new transit light curves, we refine the transit ephemeris and reduce the uncertainty in the orbital period by an order of magnitude. We find a upper limit on the orbital eccentricity of 0.067, with 99% confidence, by combining our new radial-velocity measurements with those obtained previously.

    Comments: 28 total pages, 4 figures, 4 tables, ApJ Accepted

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0806.1734v1 [astro-ph]

    Submission history

    From: John Johnson [view email]

    [v1] Tue, 10 Jun 2008 19:56:29 GMT (90kb)