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CoRoT-7b: A Small, Rocky World Examined

I love to run into genuine enthusiasm when someone is doing cutting-edge science, and Didier Queloz (Observatoire de Geneve) has not let me down. Here the astronomer is discussing CoRoT-7b, which new studies have determined is a rocky world:

“This is science at its thrilling and amazing best. We did everything we could to learn what the object discovered by the CoRoT satellite looks like and we found a unique system.”

Amazing indeed. We knew from CoRoT’s transit measurements that the radius of this planet was about twice that of Earth. Queloz and team went to work with the HARPS spectroscope (High Accuracy Radial velocity Planet Searcher) at ESO’s La Silla site in Chile, gathering fully seventy hours of observations on the system. What emerged was the best mass measurement for an exoplanet yet.

Combining that revealed mass — five times that of the Earth — with CoRoT’s radius readings, we can deduce that CoRoT-7b is about as dense as Earth, and thus almost certainly a rocky world. This was tricky work, because stellar activity on the star’s surface can confuse the precise measurements needed for these radial velocity studies. Hence the need for the large number of observing hours.

The result, though, is clear. Team member Claire Moutou (Laboratoire d’Astrophysique de Marseille) sums up the method:

“Since the planet’s orbit is aligned so that we see it crossing the face of its parent star — it is said to be transiting — we can actually measure, and not simply infer, the mass of the exoplanet, which is the smallest that has been precisely measured for an exoplanet. Moreover, as we have both the radius and the mass, we can determine the density and get a better idea of the internal structure of this planet.”


The find, announced at the European Planetary Sciences Congress in Potsdam this week, marks the first time the density of an exoplanet this small has been measured. Interestingly, the small world Gliese 581e has a minimum mass about twice that of Earth, but because the exact shape of its orbit is unknown, its real mass is conjectural. CoRoT’s transit data, on the other hand, make the geometry of CoRoT-7b’s orbit definable and allow precise mass determination.

Image: The star CoRoT-7 is located towards the constellation of Monoceros (the Unicorn) at a distance of about 500 light-years. Slightly smaller and cooler than our Sun, CoRoT-7 is also thought to be younger, with an age about 1.5 billion years. It is now known to have two planets, one of them being the first to be found with a density similar to that of Earth. CoRoT-7 is in the centre of the image. Credit: ESO.

CoRoT-7b transits its star every 20.4 hours, orbiting a mere 2.5 million kilometers out, some 23 times closer than Mercury is to our Sun. Given that extreme proximity, this world is not a benign place to be, at least not for carbon-based lifeforms like ourselves. Says Queloz:

“CoRoT-7b is so close that the place may well look like Dante’s Inferno, with a probable temperature on its ‘day-face’ above 2000 degrees and minus 200 degrees on its night face. Theoretical models suggest that the planet may have lava or boiling oceans on its surface. With such extreme conditions this planet is definitively not a place for life to develop.”

But there is yet more news about this system. The HARPS dataset used in this work also reveals the presence of a second ‘super Earth’ orbiting the same star in a slightly wider orbit. CoRoT-7c orbits in three days and seventeen hours, with a mass about eight times that of Earth. Unfortunately, no transits are visible for this one, so we can make no measurements of planetary radius and density.

The paper on this work is Queloz et al., “The CoRoT-7 planetary system: two orbiting Super-Earths,” to appear in Astronomy & Astrophysics Volume 506-1 (22 October 2009). Check here for early access to this paper. An ESO news release is also available.


Comments on this entry are closed.

  • Circle 9 September 16, 2009, 16:29

    It isn’t really hot throughout the Inferno though, and Judas even says it’s cold.

  • yeti101 September 16, 2009, 16:37

    so theyre now down to 5 earth mass from ground observations? . One wonders what the limit really is

  • eqmddlh September 16, 2009, 20:39

    In the third paragraph: “at ESO’s La Silla site” is “ESA, European Sapace Agency).

    Thanks for the article.

  • NS September 17, 2009, 2:45

    I think it’s detectable from the ground because it orbits so close to the star. From what I’ve read a similar planet in the star’s habitable zone would only be detectable from space e.g. by the Kepler mission.

  • Phil September 17, 2009, 6:41

    Very exciting news!

    Are those temps Fs or Cs?

    “…‘day-face’ above 2000 degrees and minus 200 degrees on its…”

  • Administrator September 17, 2009, 9:02

    eqmddlh writes:

    In the third paragraph: “at ESO’s La Silla site” is “ESA, European Sapace Agency).

    Nope. ESO stands for European Southern Observatory, which operates the La Silla site.

  • Administrator September 17, 2009, 9:05

    Phil says:

    Are those temps Fs or Cs?

    “…‘day-face’ above 2000 degrees and minus 200 degrees on its…”

    Dr. Queloz is using Celsius in his quote, to the best of my knowledge.

  • Allan R. Schmitt September 17, 2009, 9:13

    So would this temperature profile preclude life being able to exist at the terminator?

    “CoRoT-7b is so close that the place may well look like Dante’s Inferno, with a probable temperature on its ‘day-face’ above 2000 degrees and minus 200 degrees on its night face. Theoretical models suggest that the planet may have lava or boiling oceans on its surface. With such extreme conditions this planet is definitively not a place for life to develop.”

    – ARS –

  • Mike September 17, 2009, 12:01

    Why no transits detected for the second planet?
    Considering Corot-7c is also very close to it’s sun what kind of unusual
    orbital arrangment would allow transits to be viewed for 7b but not for
    I really find this puzzling.


  • andy September 17, 2009, 13:58

    Smallest mass precisely determined for an exoplanet? Gravitational interactions between the outer two planets of the always-forgotten PSR B1257+12 system have enabled determination of the true masses, which are 3.9 and 4.2 Earth masses.

    It is slightly annoying that the PSR B1257+12 system always gets overlooked in these news releases… two of them are super-Earths, and it will be a long time before an exoplanet as small as PSR B1257+12A is discovered around a sunlike star.

  • andy September 17, 2009, 14:14

    Mike: the relative size of the orbit and the star suggest that transits of CoRoT-7c do not occur if the inclination is less than 85 degrees. The inclination of CoRoT-7b is determined to be 80 degrees with an error of 0.3 degrees. Therefore if the system is coplanar, we wouldn’t expect to see a transit of the second planet.

  • David September 17, 2009, 14:34

    Keep in mind that it is possible that planet c may transit in the future.
    Gravitational perturbations can cause the inclination of the orbit to slowly precess. This is thought to be happening in the GJ 876 system. Interesting that the ratio of the orbital periods of planets b and c is about 4:1, wonder how close they are to being resonant.

  • ljk September 19, 2009, 21:26

    A planetary eclipse map of CoRoT-2a. Comprehensive lightcurve modeling combining rotational-modulation and transits

    Authors: K. F. Huber, S. Czesla, U. Wolter, J. H. M. M. Schmitt

    (Submitted on 17 Sep 2009)

    Abstract: We analyze the surface structure of the planet host star CoRoT-2a using a consistent model for both the `global’ (i.e., rotationally modulated) lightcurve and the transit lightcurves, using data provided by the CoRoT mission.

    Selecting a time interval covering two stellar rotations and six transits of the planetary companion CoRoT-2b, we adopt a `strip’ model of the surface to reproduce the photometric modulation inside and outside the transits simultaneously.

    Our reconstructions show that it is possible to achieve appropriate fits for the entire sub-interval using a low-resolution surface model with 36 strips. The surface reconstructions indicate that the brightness on the eclipsed section of the stellar surface is (6 +/- 1) % lower than the average brightness of the remaining surface.

    This result suggests a concentration of stellar activity in a band around the stellar equator similar to the behavior observed on the Sun.

    Comments: accepted by A&A on 12/09/2009

    Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:0909.3256v1 [astro-ph.SR]

    Submission history

    From: Klaus Huber [view email]

    [v1] Thu, 17 Sep 2009 15:55:43 GMT (565kb,D)


  • ljk September 23, 2009, 10:40


    CoRoT Satellite Discovers Rocky Planet

    Illustration Credit: ESO/L. Calcada

    Explanation: How similar is exoplanet CoRoT-7b to Earth? The newly discovered extra-solar planet is the closest physical match yet, with a mass about five Earths and a radius of about 1.7 Earths. Also, the home star to CoRoT-7b, although 500 light years distant, is very similar to our Sun.

    Unfortunately, the similarities likely end there, as CoRoT-7b orbits its home star well inside the orbit of Mercury, making its year last only 20 hours, and making its peak temperature much hotter than humans might find comfortable.

    CoRoT-7b was discovered in February by noting a predictable slight decrease in the brightness of its parent star.

    Pictured above, an artist’s depiction shows how CoRoT-7b might appear in front of its parent star. The composition of CoRoT-7b remains unknown, but given its size and mass, it cannot be a gas giant like Jupiter, and is very likely composed predominantly of rock. Future observations will likely narrow the composition of one of the first known rocky planets discovered outside of our Solar System.

  • from581 September 24, 2009, 15:12

    Actually, dynamical considerations give an upper mass to Gliese 581e. This is unusual for a planet dicovered with the radial-velocity method (that only measures the minimum mass of planets). If Gliese 581e were to be more than 3.2 earth-masses, the interactions with Gliese 581b would render the system unstable and the planet would be ejected.

  • ljk December 12, 2009, 21:49

    CoRoT-7 b: Super-Earth or Super-Io?

    Authors: Rory Barnes, Sean N. Raymond, Richard Greenberg, Brian Jackson, Nathan A. Kaib

    (Submitted on 7 Dec 2009)

    Abstract: CoRoT-7 b, a planet about 70% larger than the Earth orbiting a Sun-like star, is the first-discovered rocky exoplanet, and hence has been dubbed a “super-Earth”.

    Some initial studies suggested that since the planet is so close to its host star, it receives enough insolation to partially melt its surface. However, these past studies failed to take into consideration the role that tides may play in this system.

    Even if the planet’s eccentricity has always been zero, we show that tidal decay of semi-major axis could have been large enough that the planet formed on a wider orbit which received less insolation. Moreover, CoRoT-7 b could be tidally heated at a rate that dominates its geophysics and drives extreme volcanism.

    In this case, CoRoT-7 b is a “super-Io” that, like Jupiter’s volcanic moon, is dominated by volcanism and rapid resurfacing. Such heating could occur with an eccentricity of just 10^-5. This small value could be driven by CoRoT-7 c if its own eccentricity is larger than ~10^-4. CoRoT-7 b may be the first of a class of planetary super-Ios likely to be revealed by the CoRoT and Kepler spacecraft.

    Comments: 13 pages, 3 figures, accepted to ApJ Letters

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

    Cite as: arXiv:0912.1337v1 [astro-ph.EP]

    Submission history

    From: Rory Barnes [view email]

    [v1] Mon, 7 Dec 2009 21:53:46 GMT (23kb)


  • ljk January 3, 2010, 1:16

    News from the CoRoT Space Mission

    Authors: R. Dvorak, J. Schneider, H. Lammer, P. Barge, G. Wuchterl

    (Submitted on 23 Dec 2009)

    Abstract: The up to 150 day uninterrupted high-precision photometry of about 100000 stars – provided so far by the exoplanet channel of the CoRoT space telescope – gave a new perspective on the planet population of our galactic neighbourhood.

    The seven planets with very accurate parameters widen the range of known planet properties in almost any respect. Giant planets have been detected at low metallicity, rapidly rotating and active, spotted stars. CoRoT-3 populated the brown dwarf desert and closed the gap of measured physical properties between standard giant planets and very low mass stars.

    CoRoT extended the known range of planet masses down to 5 Earth masses and up to 21 Jupiter masses, the radii to less than 2 Earth radii and up to the most inflated hot Jupiter found so far, and the periods of planets discovered by transits to 9 days.

    Two CoRoT planets have host stars with the lowest content of heavy elements known to show a transit hinting towards a different planet-host-star-metallicity relation then the one found by radial-velocity search programs.

    Finally the properties of the CoRoT-7b prove that terrestrial planets with a density close to Earth exist outside the Solar System. The detection of the secondary transit of CoRoT-1 at the $10^{-5}$-level and the very clear detection of the 1.7 Earth radii of CoRoT-7b at $3.5 10^{-4}$ relative flux are promising evidence of CoRoT being able to detect even smaller, Earth sized planets.

    Comments: 8 pages, 19 figures and 3 tables

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:0912.4655v1 [astro-ph.EP]

    Submission history

    From: Ákos Bazsó [view email]

    [v1] Wed, 23 Dec 2009 14:57:58 GMT (1063kb)