# New Planet: CoRoT’s Interesting Find

by on July 25, 2008

Finding transiting planets is no longer a surprise, and we can expect a host of transits from the CoRoT mission, which has the advantage of observing from a space-based platform. Moreover, CoRoT will, in the course of its lifetime, survey as many as 120,000 stars for up to five months. Driving home the advantage is the announcement of a new CoRoT planet known as CoRoT-Exo-4b.

We’re dealing with another Jupiter-sized planet orbiting in close proximity to its star, but this one has a unique claim to distinction: Its host star is rotating at the same pace as the planet’s orbit.

Image: Fixated upon a star: An artists impression of the satellite CoRoT in orbit around the Earth. Credit: CNES.

Moreover, for a transiting world, CoRoT-Exo-4b is a relatively long-period planet, orbiting its F-class primary in 9.2 days. Thus far most transiting worlds have had orbits below about five days, two major exceptions being HD 147506b and HD 17156b, the latter with a period of 21.2 days — both of these planets are in highly eccentric orbits. Working out what’s going on in this system is going to prove interesting, and doubtless illuminating when it comes to other planetary systems. Says Suzanne Aigrain (University of Exeter), who discussed the find at the Cool Stars, Stellar Systems, and the Sun conference being held in St. Andrews:

“We don’t know if CoRoT-Exo-4b and its star have always been rotating in synch since their formation about 1 billion years ago, or if the star became synchronized later. CoRoT will no doubt find many more transiting planets, and by systematically measuring their host stars’ rotation periods we will gain valuable insight into how stars interact with their planets.”

CoRoT’s observations are backed by ground telescopes, but the lack of atmospheric distortion that the spacecraft experiences means that we may be finding interesting worlds not all that much larger than the Earth. Moreover, in the case of larger worlds like CoRoT-Exo-4b, the continuous coverage CoRoT can bring has allowed scientists to make precise studies of the host star between transits, thus providing a way to measure the star’s rotation by tracking dark spots on its surface. So we’ve moved beyond planetary detections into the science of planetary interactions with the primary, as noted in the paper on this work:

This system clearly warrants further observational and theoretical investigation to pin down its tidal and rotational evolution status. For example, more detailed analysis of the out-of-transit light curve should enable the active regions on the stellar surface to be mapped in a time-resolved fashion… to search for signs of star-planet magnetic interaction.

The paper is Aigrain et al., “Transiting exoplanets from the CoRoT space mission IV. CoRoT-Exo-4b: A transiting planet in a 9.2 day synchronous orbit,” now available on the arXiv site and accepted for publication in Astronomy & Astrophysics. A news release from St. Andrews is here.

John Hunt July 25, 2008 at 21:13

It seems to me that, for interstellar mission advocates, the relevance of exoplanet discoveries has to do with finding a target planet close enough worthy of aiming our craft at.

I think that the criteria of worthiness would be something sufficiently Earth-like. But I’m doubting that we’ll actually find a planet with liquid water and continents within 20 ly of Earth. So we should discuss what is our range of planets worthy of spending the millions to billions of dollars to launch a craft toward.

I for one think that the low end of the criteria should be any planet (or moon) capable of supporting human life using modern technology. I would add that it should have a benign atmosphere. So a sterile Mars planet able to support a paraterraformed environment would be sufficient.

If we can set such criteria then we can look at the pace and nature of discoveries and get some estimate of when we expect our criteria to be met. I would hope that when this happens then someone will be able to immediately come out with a plausible interstellar mission plan to generate and capitalize on the public interest that would result.

daniel July 26, 2008 at 14:58

this planet CoRoT-Exo-4b its already been announce before in 19 may 2008 http://exoplanet.eu/corot.html the question is: why they announced this planet again?
i was expect planet around cool stars beem announced in this meet, 15th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun. http://star-www.st-and.ac.uk/coolstars15/ for me wasn’t nothing of so extraordinary. i just hope that corot will discovery earth-size planets and planet in cimcurbinary orbit by eclipsing binary timing. that will go to be a extraodinary discovery by corot and this is what i hope corot will find.

MaDeR July 28, 2008 at 3:19

Almost 2 years and only 5 bagged planets? This is pitful.

ljk July 30, 2008 at 0:08

A Search for Exotrojans in Transiting Exoplanetary systems

Authors: N. Madhusudhan, Joshua N. Winn

(Submitted on 29 Jul 2008)

Abstract: We present a search for Trojan companions to 25 transiting exoplanets. We use the technique of Ford & Gaudi 2006, in which a difference is sought between the observed transit time and the transit time that is calculated by fitting a two-body Keplerian orbit to the radial-velocity data. This technique is sensitive to the imbalance of mass at the L4/L5 points of the planet-star orbit.

No companions were detected. The median 2$\sigma$ upper limit is 60 Earth masses, and the most constraining limit is 2.5 Earth masses for the case of GJ 436.

Comments: To appear in Proceedings of the IAU Symposium No. 253, “Transiting Planets”, May 19 – May 23, 2008, Cambridge, MA

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0807.4570v1 [astro-ph]

Submission history

[v1] Tue, 29 Jul 2008 19:12:36 GMT (26kb)

http://arxiv.org/abs/0807.4570

ljk July 31, 2008 at 19:48

Searching for the secondary eclipse of CoRoT-Exo-2b and its transit timing variations

Authors: R. Alonso, S. Aigrain, F. Pont, T. Mazeh, the CoRoT Exoplanet Science Team

(Submitted on 30 Jul 2008)

Abstract: With more than 80 transits observed in the CoRoT light curve with a cadence of 32 s, CoRoT-Exo-2b provides an excellent case to search for the secondary eclipse of the planet, with an expected signal of less than 10^-4 in relative flux. The activity of the star causes a modulation on the flux that makes the detection of this signal challenging.

We describe the technique used to seek for the secondary eclipse, that leads to a tentative 2.5 sigma detection of a 5.5×10^-5 eclipse. If the effect of the spots are not taken into account, the times of transit centers will also be affected. They could lead to an erroneous detection of periodic transit timing variations of ~20 s and with a 7.45 d period. By measuring the transit central times at different depths of the transit (transit bisectors), we show that there are no such periodic variations in the CoRoT-Exo-2b O-C residuals larger than ~10 s.

Comments: 6 pages, 5 figures, to appear in the Proceedings of IAU Symposium 253, “Transiting Planets”, May 2008, Cambridge, MA

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0807.4828v1 [astro-ph]

Submission history

From: Roi Alonso [view email]

[v1] Wed, 30 Jul 2008 10:26:07 GMT (224kb)

http://arxiv.org/abs/0807.4828

ljk October 27, 2008 at 23:49

The Transit Light Curve Project. X. A Christmas Transit of HD 17156b

Authors: Joshua N. Winn, Matthew J. Holman, Gregory W. Henry, Guillermo Torres, Debra Fischer, John Asher Johnson, Geoffrey W. Marcy, Avi Shporer, Tsevi Mazeh

(Submitted on 26 Oct 2008)

Abstract: Photometry is presented of the Dec. 25, 2007 transit of HD 17156b, which has the longest orbital period and highest orbital eccentricity of all the known transiting exoplanets.

New measurements of the stellar radial velocity are also presented. All the data are combined and integrated with stellar-evolutionary modeling to derive refined system parameters.

The planet’s mass and radius are found to be 3.212_{-0.082}^{+0.069} Jupiter masses and 1.023_{-0.055}^{+0.070} Jupiter radii. The corresponding stellar properties are 1.263_{-0.047}^{+0.035} solar masses and 1.446_{-0.067}^{+0.099} solar radii.

The planet is smaller by 1 sigma than a theoretical solar-composition gas giant with the same mass and equilibrium temperature, a possible indication of heavy-element enrichment.

The midtransit time is measured to within 1 min, and shows no deviation from a linear ephemeris (and therefore no evidence for orbital perturbations from other planets).

We provide ephemerides for future transits and superior conjunctions. There is an 18% chance that the orbital plane is oriented close enough to edge-on for secondary eclipses to occur at superior conjunction.

Observations of secondary eclipses would reveal the thermal emission spectrum of a planet that experiences unusually large tidal heating and insolation variations.

Comments: To appear in ApJ [26 pages]

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0810.4725v1 [astro-ph]

Submission history

From: Joshua N. Winn [view email]

[v1] Sun, 26 Oct 2008 22:09:01 GMT (131kb)

http://arxiv.org/abs/0810.4725

ljk October 27, 2008 at 23:56

The BAST algorithm for transit detection

Authors: S. Renner, H. Rauer, A. Erikson, P. Hedelt, P. Kabath, R. Titz, H. Voss

(Submitted on 27 Oct 2008)

Abstract: The pioneer space mission for photometric exoplanet searches, CoRoT, steadily monitors about 12000 stars in each of its fields of view. Transit detection algorithms are applied to derive promising planetary candidates, which are then followed-up with ground-based observations.

We present BAST (Berlin Automatic Search for Transits), a new algorithm for periodic transit detection, and test it on simulated CoRoT data. BAST searches for box-shaped signals in normalized, filtered, variability-fitted, and unfolded light curves. A low-pass filter is applied to remove high-frequency signals, and linear fits to subsections of data are subtracted to remove the star’s variability. A search for periodicity is then performed in transit events identified above a given detection threshold. Some criteria are defined to better separate planet candidates from binary stars.

From the analysis of simulated CoRoT light curves, we show that the BAST detection performance is similar to that of the Box-fitting Least-Square (BLS) method if the signal-to-noise ratio is high. However, the BAST box search for transits computes 10 times faster than the BLS method. By adding periodic transits to simulated CoRoT data, we show that the minimum periodic depth detectable with BAST is a linearly increasing function of the noise level. For low-noise light curves, the detection limit corresponds to a transit depth d~0.01%, i.e. a planet of 1 Earth radius around a solar-type star.

Comments: 4 pages, 3 figures, to be published in A&A

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0810.4760v1 [astro-ph]

Submission history

From: Stefan Renner [view email]

[v1] Mon, 27 Oct 2008 09:04:22 GMT (186kb)

http://arxiv.org/abs/0810.4760

ljk January 30, 2009 at 0:32

Photospheric activity and rotation of the planet-hosting star CoRoT-Exo-4a

Authors: A. F. Lanza, S. Aigrain, S. Messina, G. Leto, I. Pagano, M. Auvergne, A. Baglin, P. Barge, A. S. Bonomo, A. Collier Cameron, G. Cutispoto, M. Deleuil, J. R. De Medeiros, B. Foing, C. Moutou

(Submitted on 29 Jan 2009)

Abstract: The space experiment CoRoT has recently detected a transiting hot Jupiter in orbit around a moderately active F-type main-sequence star (CoRoT-Exo-4a). This planetary system is of particular interest because it has an orbital period of 9.202 days, the second longest one among the transiting planets known to date.

We study the surface rotation and the activity of the host star during an uninterrupted sequence of optical observations of 58 days. Our approach is based on a maximum entropy spot modelling technique extensively tested by modelling the variation of the total solar irradiance. It assumes that stellar active regions consist of cool spots and bright faculae, analogous to sunspots and solar photospheric faculae, whose visibility is modulated by stellar rotation.

The modelling of the light curve of CoRoT-Exo-4a reveals three main active longitudes with lifetimes between about 30 and 60 days that rotate quasi-synchronously with the orbital motion of the planet. The different rotation rates of the active longitudes are interpreted in terms of surface differential rotation and a lower limit of 0.057 \pm 0.015 is derived for its relative amplitude.

The enhancement of activity observed close to the subplanetary longitude suggests a magnetic star-planet interaction, although the short duration of the time series prevents us from drawing definite conclusions.

Comments: 8 pages, 6 figures, accepted by Astronomy & Astrophysics

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

Cite as: arXiv:0901.4618v1 [astro-ph.SR]

Submission history

From: Antonino Francesco Lanza [view email]

[v1] Thu, 29 Jan 2009 07:58:45 GMT (72kb)

http://arxiv.org/abs/0901.4618

ljk September 21, 2009 at 9:02

Transiting exoplanets from the CoRoT space mission: VII. The “hot-Jupiter”-type planet CoRoT-5b

Authors: H. Rauer, D. Queloz, Sz. Csizmadia, M. Deleuil, R. Alonso, S. Aigrain, J. M. Almenara, M. Auvergne, A. Baglin, P. Barge, P. Borde, F. Bouchy, H. Bruntt, J. Cabrera, L. Carone, S. Carpano, R. De la Reza, H. J. Deeg, R. Dvorak, A. Erikson, M. Fridlund, D. Gandolfi, M. Gillon, T. Guillot, E. Guenther, A. Hatzes, G. Hebrard, P. Kabath, L. Jorda, H. Lammer, A. Leger, A. Llebaria, P. Magain, T. Mazeh, C. Moutou, M. Ollivier, M. Paetzold, F. Pont, M. Rabus, S. Renner, D. Rouan, A. Shporer, B. Samuel, J. Schneider, A. H. M. J. Triaud, G. Wuchterl

(Submitted on 18 Sep 2009)

Abstract: Aims. The CoRoT space mission continues to photometrically monitor about 12 000 stars in its field-of-view for a series of target fields to search for transiting extrasolar planets ever since 2007. Deep transit signals can be detected quickly in the “alarm-mode” in parallel to the ongoing target field monitoring.
CoRoT’s first planets have been detected in this mode.

Methods. The CoRoT raw lightcurves are filtered for orbital residuals, outliers, and low-frequency stellar signals. The phase folded lightcurve is used to fit the transit signal and derive the main planetary parameters. Radial velocity follow-up observations were initiated to secure the detection and to derive the planet mass.

Results. We report the detection of CoRoT-5b, detected during observations of the LRa01 field, the first long-duration field in the galactic anticenter direction. CoRoT-5b is a “hot Jupiter-type” planet with a radius of 1.388(+0.046, -0.047) R_Jup, a mass of 0.467(+0.047, -0.024) M_Jup, and therefore, a mean density of 0.217(+0.031, -0.025) g cm-3. The planet orbits an F9V star of 14.0 mag in 4.0378962 +/- 0.0000019 days at an orbital distance of 0.04947(+0.00026,
-0.00029) AU.

Comments: 6 pages, 6 figures, 4 tables, accepted at A&A

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Cosmology and Extragalactic Astrophysics (astro-ph.CO)

Cite as: arXiv:0909.3397v1 [astro-ph.EP]

Submission history