Tuesday, February 03, 2009

Corot-Exo-7b: A Venus in another World

German science blogs today are abuzz with reports about the discovery of an Earth-like planet around a Sun-like star in the constellation of Monoceros, at a distance of about 450 light years.

The newly discovered planet Corot-Exo-7b transiting in front of its star (left, illustration by Klaudia Einhorn), and Venus in front of the disk of the Sun on June 8, 2004 (right, photo by Martin Sloboda). As the sizes of both the stars and the planets are similar, a transit of Corot-Exo-7b would look very similar to the Venus transit.

The planet has a radius which is 1.75 times larger than that of the Earth, and has six to thirteen times the mass of Earth. The star is a main sequence star with roughly the same composition as the Sun, with slightly less mass and a slightly lower temperature. However, distance of the planet to the star is only 1.7 percent of the distance of the Earth from the Sun - hence the revolution period, or "year", of the planet is only 20 hours, and its surface temperature is estimated to be between 1,000 and 1,500 degrees Celsius.

The planet was discovered by the European satellite mission Corot - hence its name, Corot-Exo-7b, meaning the first planet in the 7th planetary system discovered by Corot. Corot uses the transit method to search for new planets: When a planet passes in front of the disk of a star, the light of the star is slightly dimmed.

Here is the light curve of Corot-Exo-7, the star around which the planet is in orbit, showing a drop in brightness of the order of 10-4:

Mass, radius, and orbital parameters of the planet could be extracted from this measurement, and further observations and data analysis using the radial velocity method, the method which had led to the first detection of an exoplanet back in 1995.

So, it's indeed the first Earth-like planet at a Sun-like star - unfortunately, at a temperature close to the melting point of iron.

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  1. I think there is a bias towards discovery of such a planet. E.g., if a revolution of the planet took an year, we'd have to be really lucky to spot it; almost all of the time it would not be occluding its star.

    Deep in my heart, I do believe,
    We shall find another one some day

  2. Hi Stefan,

    That was certainly a nice find they’ve come across. However, I’m a tad confused for the orbit they seem to have it in from the time lapse they show has it around the bottom third of the star (it’s sun). Is there something I’m missing as I never imagined something could have such a orbit where it doesn’t divide the sun in half with the tracing of its orbit?



  3. Bad Astronomer says we do not knows its mass. How did you get that it is 6 times the mass of Earth?

  4. Hi Kurt,

    How did you get that it is 6 times the mass of Earth?

    good point - I've taken this mass from this press release by the German Tautenburg Observatory, which was involved in the measurements.

    Actually, the Corot-Exo-7b factsheet gives a value of

    M sini = 0.035(−0.017+0)M_Jupiter,

    where i is the inclination of the orbit. Now, given an inclination of about 70+/-10 degrees, the sine factor is larger than 0.93, and the mass less than about 11 Earth masses, which is an upper bound. The uncertainty in the M sini value translates roughly into a range of 5-11 Earth masses for the planet.

    I do not know much details about the technique to determine these data, but it is from a combination of both the transit and the radial Doppler method.

    Unfortunately, there seems no preprint available on the arxiv.

    Best, Stefan

  5. a radius which is 1.75 times larger than that of the Earth, and has six [5-11] times the mass of Earth.

    (1.75)^3 = 5.36 and Earth has an iron core the size of Mars. It sounds like the core of a gas giant whose volatiles have been blasted away.

  6. Sorry, in my comment above, the link to the Corot-Exo-7b fact sheet was corrupted, so here is the correct one:


    Moreover, it seems I was arithmetically challenged when trying to convert the published data about the mass of the planet into Earth masses, so here is another try:

    The mass range of 0.018 to 0.035 Jupiter masses corresponds to 5.7 to 11.1 Earth masses. The sin i factor from the projection of the orbit in our plane of view ranges, for i between 60 and 80 degrees, between 0.87 and 0.98. Taken together, this yields a mass range of 5.8 to 12.8 Earth masses...

    This means that the average density is indeed between the same and double the density of Earth.

    Cheers, Stefan

  7. Hi Arun,

    ... there is a bias towards discovery of such a planet.

    I agree! An Earth-like planet in an Earth-like orbit is probably like finding the needle in the haystack!

    Hi Phil,

    concerning this animation in the press release of the Observatoire de Paris, I also found it confusing - mainly so because of this silly cut where the planet just jumps back from the end of the transit to the start.

    Actually, the animation is only part of a much better one here, from the German Aerospace Center (DLR). The complete animation features one full orbit. I guess it also makes clear why the transit doe not exactly cut in half the disk of the star.

    Cheers, Stefan

  8. thanks for that explanation. it makes good sense.

  9. Hi Stefan,

    Thanks, yes the German Aerospace Centre certainly did a better job as you can see the orbital path arcing upward as it travels off the star’s face and around, so we are looking upward at the star/planet from this perspective. It is all very interesting leaving one to wonder if there are any planets just a bit further out. I hope the keep there eye on this one.

    The other thing it leaves me to wonder about is as the orbit is so close, will the period of the graph alter over time in accordance to GR since its orbit is much closer then Mercury’s (to our sun) such that the deviation (perihelion) to be even more pronounced? Imagine only 450 light years away, oh so close and yet so far.



  10. Another planet is suspected around the star with four times the orbital period of this one (according to a comment at oklo.org).

  11. Hi there,
    I was asked by Stefan to give some insights of what exactly is going on with CoRoT-Exo-7b from the perspective of somebody within the CoRoT team.

    Well the mass, that depends of course on the R_V measurements and some teams i.e. Geneva seem to have large error bars which is due to the fact that this star is relatively active.

    However, I talked with Artie Hatzes yesterday, he is responsible for the CoRoT followup and he says: "I say its 6 Earth masses." My dear colleague Eike Guenther is right now spending time in Chile making more oberservation on this interesting object.

    Yes, and we do suspect at least another albeit bigger planet around CoRoT-Exo-7.

    So stay tuned for more news coming from that system.

    There is a CoRoT special issue planned. The deadline for submission is 20th February - unless it is shifted ;-) So I suspect that a preprint will be available by then. However, since this is still very much work in progress I'm not sure if they can make it by that time.

  12. Hi Ludmila,

    It is so nice that Stefan has the contacts who are willing and allow us to learn of this from the horse mouth so to speak. I for one find this all truly fascinating. I also find it strange that many are disappointed that you haven’t found and earthlike planet at the orbital distance to perhaps foster life.

    The way I’ve looked at it is if even a large (Jupiter like) planet was found in such a location that one of the moons it might have could serve as such a place. For me it only serves to indicate that the old earth centered perspective didn’t vanish completely with Copernicus’ revelation or Darwin’s for that matter. It will be interesting as to what all of this yields as time proceeds.



  13. Very nice! However, I think that, from the point of view of the general public, it is unfortunate that they decided to draw a starspot on the parent star...

    Waiting for more exciting exoplanetary news from this star!

  14. Hi Ludmila,

    thank you very much for sharing your first-hand insights into what is going on! I guess this has been a very exciting time for you - and hard not to spread rumours beforehand ;-)

    About the mass, let me just add as an explantion for our readers (I hope I get it right...) - the discovery of the planet by Corot was via the slight drop of brightness - the light curve is by Corot, is that correct? Observing the transits allows to determine the orbital period, and probably also to estimate the diameter of the planet, via the slope of the dropping?

    But about the mass, the orbital period alone doesn't say anything, as the period doesn't depend (to lowest order) on the mass of the planet. However, while the planet orbits its star, the centre of mass of the system planet-star has to stay at rest, and hence, there must be a small motion of the star "mirroring" the orbit of the planet. This motion manifests itself for us as a small periodic change in the velocity of the star along the line of sight, the radial velocity R_V - and this change of radial velocity can be observed via the Doppler effect as a periodic shift of spectral lines. This then allows to calculate the mass of the planet. However, as one can only monitor radial motions, one can only measure the projection of the orbit into the plane containing the line of sight Earth-Star, hence the unavoidable factor sin i in the mass estimate.

    There is, however, a difficulty with the measurement of the changing radial velocity of the star over one period of the planet: The amplitude of the change is very small - on the order of 1 metre per second. This is tiny and easily blurred by convection in the atmosphere of the star, especially for active stars. So, one has to look for a small periodically changing offset of a very noisy signal, and this requires to observe the star over an extended period of time.

    But at the end of the day, all this results in an estimate for the mass of the planet, with quite a clear upper limit, but a large range of possible values which can be further constrained by long enough an observation of the radial velocity...

    So, I guess the mass estimate will be sharper soon, and maybe the Tautenburg team has better data already now, so that they can nail down the six Earth masses with some good confidence.

    We will see!



  15. Hi Phil,

    the German Aerospace Centre certainly did a better job as you can see the orbital path arcing upward as it travels off the star’s face and around

    exactly... the animation by Klaudia Einhorn at the DLR website is really much better. Actually, the snippet in the Observatoire de Paris press release is taken out of this full animation, but I think it is really not very helpful to understand what is going on...

    Hi Changcho,

    it is unfortunate that they decided to draw a starspot on the parent star...

    Hm... maybe it is supposed to show that it is a star similar to our Sun, and that it is quite active? And, though I am not sure about this, but I think Corot in principle could detect large "starspots".

    Cheers, Stefan

  16. Hi Stefan, what I meant by it being (slightly) unfortunate that they drew a starspot is this. When the figure is small, the starspot looks like another planet, so people are going to wonder whether it is 1 or 2 planets that they've discovered.

    However, when I click on the figure, the link takes me to the original source where it is an *animated* figure so it is very clear which is the starspot and which is the planet. Anyways, tiny nitpick, excellent and exciting science!

  17. Hi Changcho,

    When the figure is small, the starspot looks like another planet, ...

    Ah, I see what you mean! That's true!

    Curiously, I just thought, a starspot should move a bit over the period of on revolution of the planet - and having a closer look at the animation, I see: Indeed, it does so!

    It even moves much slower and changes a bit its shape when "appearing" from the limb of the star. Amazing!

    Cheers, Stefan

  18. Hi Stefan,

    yes that was a very good decription about the whole detection process and waht it takes to determine the parameter of the planet.

    but I think Corot in principle could detect large "starspots".

    CoRoT already did and these spots don't have to be that large. We already used observations of starspots during transits to nail down the stellar rotation period of some stars. These spots weren't larger than the ones we see on our own sun.

    I have to try and find one of the presentations where this was shown.

    Regarding the PR. Yes this was a mess, but it went actually better than I anticipated.

  19. Hi Ludmila,

    thanks for the feedback!

    We already used observations of starspots during transits to nail down the stellar rotation period of some stars. These spots weren't larger than the ones we see on our own sun.

    Ah, that's interesting! I was searching around NASA/ADS a tiny bit about starspots, but wasn't sure if the papers I've seen were about actual observations or "proofs of principle" that current space telescopes could see starspots...

    I have to try and find one of the presentations where this was shown.

    Cool, yes, that would be great if you can give some more details about this some time. Are there preprints about starspots? I have seen stuff about stellar oscillations, but not starspots...

    Cheers, Stefan

  20. So 450 light years is a loong ways, dark matter an eduring guess / hypothesis, and data degradation is a fact whether it be a fax or shared digital copy.

    Did I miss a theorem explaining why light cannot "forget?" Huge planets have been captured in super fast orbits which I assume must make them donut shaped (similar to a rain drop falling), and I can't perceive accretion explaining this as excess material, in my mind, should have been sucked in to make the star. Across the vast, dense?, vacuum of space, are photons maybe a little lazy and want their buddies catching up for long journeys? Envision a chunk of tofu where a hole could be drilled shaped as a sine wave. Each photon smacks in and digs a little deeper, with the one behind it following the same path. Momentum is conserved(?-?) until they get into "open" space at regular frequencies, 20 hour intervals per se, but it has taken years to dig through the tofu.

    I have not taken data as to the size / mass variations vs. distance from earth or orbiting star of the growing number of exoplanets being discovered, but would like to know the proof that light's characteristics are finite. I recently was informed that c is slow at near 0 K.


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