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CoRoT-7b

Coordinates:Sky map06h43m49.0s,−01° 03′ 46.0″
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CoRoT-7b
Size comparison of CoRoT-7b (center) with Earth (left) and Neptune (right)
Discovery
Discovered byRouan et al. (CoRoT)
Discovery sitePolar orbit
Discovery dateFebruary 3, 2009
Transit
Orbital characteristics
0.0172 ± 0.00029 AU (2.573 ± 0.043 million km; 1.599 ± 0.027 million mi)[1]
Eccentricity0
0.853585 ± 0.000024d(20.48604 ± 0.00058h)[1]
Inclination80.1 ± 0.3[1]
StarCoRoT-7
Physical characteristics
1.528±0.065R🜨[2]
Mass6.06±0.65ME[2]
Temperature1,300–1,800 K (1,030–1,530 °C; 1,880–2,780 °F)[3]

CoRoT-7b(previously namedCoRoT-Exo-7b)[3][4]is anexoplanetorbiting thestarCoRoT-7in theconstellationofMonoceros,489light-years(150parsecs) fromEarth.It was first detectedphotometricallyby the French-ledCoRoTmission and reported in February 2009.[5]Until the announcement ofKepler-10bin January 2011, it was the smallestexoplanetto have its diameter measured, at 1.58 times that of theEarth(which would give it a volume 3.95 times Earth's) and the first potential extrasolarterrestrial planetto be found. The exoplanet has a very shortorbital period,revolving around its host star in about 20 hours.[1]

Combination of the planet's diameter derived from transit data with the planet's mass derived from radial velocity measurements means that the density of CoRoT-7b is about the same as that of Earth; therefore, CoRoT-7b is aterrestrial planetlike Earth and not agas giantlikeJupiter.The radial velocity observations of CoRoT-7 also detected a secondsuper-Earth,CoRoT-7c,which has a mass 8.4 times that of Earth and orbits every 3.7 days at a distance of 6.9 million km (0.046 AU; 4.3 million mi).

Discovery

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Artist conception of CoRoT-7btransitingyellow dwarfCoRoT-7

CoRoT-7b was found by observing its parent star's periodic decrease inapparent magnitudecaused by the planet's transit in front of the star as seen from Earth. Measuring this dip in brightness, together with a size estimate for the star, allows calculating the planet's size. (SeeTransit method.) The space missionCoRoTobserved the starCoRoT-7,in the stellar field LRa01, from October 15, 2007, to March 3, 2008. During this period, 153 periodic transit signals of 1.3 h duration with a depth of 3.4 × 10−4were registered. After 40 days of data acquisition, the Alarm mode pipeline algorithm detected the shallow signal of CoRoT-7b, starting the follow-up observations from the ground to get a confirmation of the planetary nature of the transiting object.

The discovery of CoRoT-7b was announced a year later on February 3, 2009, during the CoRoT Symposium 2009 inParis.[5]It was published in a special issue of the journalAstronomy and Astrophysicsdedicated to results from CoRoT.[6]

Mass

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After the detection of CoRoT-7b in the lightcurve, follow-up observations carried out with a network of ground-based telescopes ruled out nearly completely the possibility of a false positive detection.[7]TheHARPSspectrographwas subsequently used to measure themassof CoRoT-7b with theradial velocitymethod. The strong activity of the host star, which perturbates radial velocity measurements, made the mass determination troublesome.

The discovery paper, by Quelozet al.,[8]weighed the planet at about 4.8Earth masses,giving it a density of 5.6 ± 1.3 g cm−3,similar to Earth's. The value was obtained using a pre-whitening procedure and harmonic decomposition. It was also inferred that there was a second non-transiting planet in the system,CoRoT-7c,with a 3.7-day orbital period.

A second paper, byHatzeset al.,[9]employing Fourier analysis, reported a likely mass of 6.9 Earth masses for CoRoT-7b, and found hints for the presence of a third planet in the system,CoRoT-7d,with mass similar toNeptune's and a 9-day orbital period.

Pontet al.[10]evidences larger-than-declared systematic errors in the HARPS measurements, estimating CoRoT-7b to be between one and four Earth masses. The radial velocity confirmation of the planet is in shaky ground too, with a tentative detection of only 1.2 sigma certainty.

Boisseet al.,[11]employing simultaneous fitting of stellar activity and planetary signals in the radial velocity data, calculate for CoRoT-7b a mass of 5.7 Earth masses, though with a very large uncertainty.

The CoRoT team then published a second paper on CoRoT-7b's mass,[12]removing stellar activity through analysis only of radial velocity data for which multiple measurements were taken in a given night. The planet is weighed at 7.42 Earth masses, yielding an average density of 10.4 ± 1.8 g cm−3,far higher than the Earth's and similar to that of the second rocky planet found,Kepler-10b.

A last study by Ferraz-Melloet al.[13]improved the approach used in the discovery paper, finding that it downsized the amplitude of the planets' induced radial velocities. It reports for CoRoT-7b a heavier mass of 8 Earth masses, in agreement with the second paper published by the CoRoT team. Thus, CoRoT-7b may be rocky with a large iron core, with an internal structure more like Mercury than Earth.

Spitzer observations

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An independent validation of CoRoT-7b as a planet is supplied by follow-up performed with the space basedSpitzertelescope. Its observations confirmed the transits of the planet, with the same depth, at different wavelengths than the ones observed by CoRoT.[14]The data then allows to validate CoRoT-7b as a bona-fide planet with a very high degree of confidence, independently from the noisy radial velocity data.

Characteristics

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Artist's impression of CoRoT-7b.
Credit:ESO/L. Calçada.

CoRoT-7b's mass is somewhat uncertain at 6.06±0.65ME,[2]while its radius and orbital period are well known fromCoRoTphotometry: it orbits very close to its star (1/23rd the distance from theSuntoMercury[15]) with anorbital periodof 20 hours, 29 minutes, and 9.7 seconds and has a radius of 1.58 Earth radii.[16]CoRoT-7b had the shortest orbit of any planet known at the time of its discovery.[17]

Due to the high temperature, it may be covered inlava.[3]The composition and density of the planet, though weakly constrained, make CoRoT-7b a probablyrockyplanet, like Earth. It could belong to a class of planets that are thought to contain up to 40% water (in the form of ice and/or vapor) in addition to rock.[18]However, the fact that it formed so close to its parent star may mean that it is depleted ofvolatiles.[19]A strong possibility exists that the planet's rotation istidally lockedto the orbital period, so that temperatures and geologic conditions on the sides of the planet facing towards and away from the star may be dramatically different. Theoretical work suggests that CoRoT-7b could be achthonian planet(the remains of a Neptune-like planet from which much of the initial mass has been removed due to close proximity to its parent star).[20][21]Other researchers dispute this, and conclude CoRoT-7b was always a rocky planet and not the eroded core of a gas or ice giant,[22]due to the young age of the star system.

Any departure from circularity of its orbit (due to the influence of host star and neighboring planets) could generate intense volcanic activity similar tothat of Io,viatidal heating.[23]

A detailed study of the extreme properties of CoRoT-7b has been published,[24]concluding that, despite the mass uncertainty, the planet is Earth-like in composition. The extreme proximity to the star should prevent the formation of a significantatmosphere,with the dayside hemisphere as hot as the tungsten filament of an incandescent bulb, resulting in the formation of alavaocean. The researchers propose to name this new class of planets, CoRoT-7b being the first of them, "lava-ocean planets".

Model of the interior

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CoRoT-7b artist view.

Assuming a 5-Earth-masses planet, the planet was modeled to haveconvectionin themantlewith a small core with no more than 15% the mass of the planet, or 0.75 M🜨.The lower mantle above the core-mantle boundary has more sluggish convection than the upper mantle because the greater pressure causes fluids to become more viscous. The temperature of the upper convecting mantle is different from one side of the planet to the other with lateral temperature differences for downwellings up to several hundred kelvins. However, the temperature of the upwelling is unaffected by downwelling and surface temperature variations. On the permanent dayside of the tidally locked planet where the surface temperature is hot from continuously facing its sun, the surface takes part in convection, which is the evidence that all the surface of this hemisphere being covered in oceans of lava. On the permanent nightside, the surface is cool enough for the formation of the crust with pools of lava above the convective mantle with intensevolcanism.The dayside of the planet has largerconvection cellsthan the nightside.[25]Researchers also investigated the physical state of the interior of CoRoT-7b,[26]indicating as likely a solid iron core, thus a self-generatedmagnetic fieldshould be absent on the planet.

Possible atmosphere

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Due to the high temperatures on the illuminated side of the planet, and the likelihood that all surface volatiles have been depleted, silicate rock vaporization may have produced a tenuous atmosphere (with a pressure approaching 1 Pa or 10−2mbar at 2,500 K [2,230 °C; 4,040 °F]) consisting predominantly ofsodium,O2,Oandsilicon monoxide,as well as smaller amounts ofpotassiumand other metals.[15][19][27]Magnesium(Mg),aluminium(Al),calcium(Ca),silicon(Si), andiron(Fe) may rain out from such an atmosphere on the planet's daylight side in the form of particles of minerals, such asenstatite,corundumandspinel,wollastonite,silica,andiron (II) oxide,that wouldcondenseat altitudes below 10 km (6.2 mi).Titanium(Ti) may be depleted (and possibly iron similarly) by being transported towards the night side before condensing asperovskiteandgeikielite.[19]Sodium (and to a lesser extent, potassium), being more volatile, would be less subject to condensation into clouds and would dominate the outer layers of the atmosphere.[15][19]Observations carried out with theUVES spectrographon CoRoT-7b in and out of transit, searching for emission and absorption lines originating in theexosphereof the planet, failed to detect any significant feature.[28]Spectral lines of calcium (Ca I, Ca II) and sodium (Na), expected for aMercury-like planet, are either absent or below detection limits, and even emission lines expected from volcanic activity, due totidal forcesexerted by the gravity of the nearby star, were not found. The lack of detections is in agreement with the previously cited theoretical work,[24]which points to a cloudless atmosphere made of rocky vapours with a very low pressure. From the data available, scientists can only infer that CoRoT-7b does not resemble any of the rocky planets of theSolar System.

See also

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References

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  1. ^abcdLéger, A; et al. (2009)."Transiting exoplanets from the CoRoT space mission VIII. CoRoT-7b: the first Super-Earth with measured radius".Astronomy and Astrophysics.506(1): 287–302.arXiv:0908.0241.Bibcode:2009A&A...506..287L.doi:10.1051/0004-6361/200911933.S2CID5682749.
  2. ^abcAnna John, Ancy; Collier Cameron, Andrew; Wilson, Thomas G. (2022), "The impact of two non-transiting planets and stellar activity on mass determinations for the super-Earth CoRoT-7b",Monthly Notices of the Royal Astronomical Society,515(3): 3975–3995,arXiv:2206.14216,doi:10.1093/mnras/stac1814
  3. ^abc European Space Agency (3 February 2009)."COROT discovers smallest exoplanet yet, with a surface to walk on".esa.int.European Space Agency.RetrievedMarch 17,2022.
  4. ^Schneider, J. (10 March 2009)."Change in CoRoT planets names".Exoplanets.(Mailing list). Archived fromthe originalon 18 January 2010.Retrieved19 March2009.
  5. ^ab Rouan, D.; et al. (3 February 2009)."Has CoRoT discovered the first transiting Super-Earth around a main sequence star?"(PDF).CoRoT-exo-7b.Corot Exoplanet Science Team. Archived fromthe original(PDF)on 20 July 2011.Retrieved13 May2009.
  6. ^ Bertout, C.; Forveille, T.; et al. (21 October 2009)."The CoRoT space mission: early results".Astronomy and Astrophysics.506(1). EDP Sciences: E1 - 1.doi:10.1051/0004-6361/200913351.Retrieved23 August2010.
  7. ^ Léger, A.; et al. (2009). "Transiting exoplanets from the CoRoT space mission VIII. CoRoT-7b: the first Super-Earth with measured radius".Astronomy and Astrophysics.506(1): 287–302.arXiv:0908.0241.Bibcode:2009A&A...506..287L.doi:10.1051/0004-6361/200911933.S2CID5682749.
  8. ^ Queloz, D.; et al. (2009)."The CoRoT-7 planetary system: two orbiting super-Earths"(PDF).Astronomy and Astrophysics.506(1): 303–319.Bibcode:2009A&A...506..303Q.doi:10.1051/0004-6361/200913096.
  9. ^ Hatzes, A. P.; et al. (2010). "An Investigation into the Radial Velocity Variations of CoRoT-7".Astronomy and Astrophysics.520:A93.arXiv:1006.5476.Bibcode:2010A&A...520A..93H.doi:10.1051/0004-6361/201014795.S2CID38803450.
  10. ^ Pont, F.; Aigrain, S.; Zucker, S. (2010)."Re-assessing the radial-velocity evidence for planets around CoRoT-7".Monthly Notices of the Royal Astronomical Society.411(3): 1953–1962.arXiv:1008.3859.Bibcode:2011MNRAS.411.1953P.doi:10.1111/j.1365-2966.2010.17823.x.S2CID118544414.
  11. ^ Boisse, I.; et al. (2011). "Disentangling stellar activity and planetary signals".Proceedings of the International Astronomical Union.273:281–285.arXiv:1012.1452.Bibcode:2011IAUS..273..281B.doi:10.1017/S1743921311015389.S2CID124197833.
  12. ^ Hatzes, A. P.; et al. (2011). "On the Mass of CoRoT-7b".Astrophysical Journal.743(1): 75.arXiv:1105.3372.Bibcode:2011ApJ...743...75H.doi:10.1088/0004-637X/743/1/75.S2CID118588616.
  13. ^ Ferraz-Mello, S.; et al. (2011). "On planetary mass determination in the case of super-Earths orbiting active stars. The case of the CoRoT-7 system".Astronomy & Astrophysics.531:A161.arXiv:1011.2144.Bibcode:2011A&A...531A.161F.doi:10.1051/0004-6361/201016059.S2CID55422304.
  14. ^ Fressin, F.; et al. (2011). "Spitzer Infrared Observations and Independent Validation of the Transiting Super-Earth CoRoT-7b".Astrophysical Journal.745(1): 81.arXiv:1110.5336.Bibcode:2012ApJ...745...81F.doi:10.1088/0004-637X/745/1/81.S2CID9496072.
  15. ^abc Lutz, D. (7 October 2009)."Forecast for discovered exoplanet: cloudy with a chance of pebbles".Washington University in St. Louis News & Information.Washington University in St. Louis.
  16. ^ Bruntt, J.; et al. (2010). "Improved stellar parameters of CoRoT-7".Astronomy and Astrophysics.519:A51.arXiv:1005.3208.Bibcode:2010A&A...519A..51B.doi:10.1051/0004-6361/201014143.S2CID54657440.
  17. ^ Brumfiel, G. (3 February 2009)."Tiniest exoplanet found".Nature.doi:10.1038/news.2009.78.Retrieved2009-02-07.
  18. ^ Queloz, D.; et al. (2009)."The CoRoT-7 planetary system: two orbiting Super-Earths"(PDF).Astronomy and Astrophysics.506(1): 303–319.Bibcode:2009A&A...506..303Q.doi:10.1051/0004-6361/200913096.
  19. ^abcd Schaefer, L.; Fegley, B. (2009). "Chemistry of Silicate Atmospheres of Evaporating Super-Earths".Astrophysical Journal Letters.703(2): L113–L117.arXiv:0906.1204.Bibcode:2009ApJ...703L.113S.doi:10.1088/0004-637X/703/2/L113.S2CID28361321.
  20. ^"Exoplanets Exposed to the Core".AstroBiology Magazine. 2009-04-25.Retrieved2018-01-07.
  21. ^ "Super-Earth 'began as gas giant'".BBC News.7 January 2010.Retrieved2010-01-10.
  22. ^ Odert, P. (2010)."Thermal mass-loss of exoplanets in close orbits"(PDF).EPSC Abstracts.5:582.Bibcode:2010epsc.conf..582O.
  23. ^Jaggard, V. (5 February 2010).""Super Earth" May Really Be New Planet Type: Super-Io ".National Geographic.Archived fromthe originalon February 9, 2010.Retrieved2010-02-12.
  24. ^abLéger, A.; et al. (2011). "The extreme physical properties of the CoRoT-7b super-Earth".Icarus.213(1): 1–11.arXiv:1102.1629.Bibcode:2011Icar..213....1L.doi:10.1016/j.icarus.2011.02.004.S2CID55348462.
  25. ^Noack, L.; et al. (2010)."CoRoT-7b: Convection in a Tidally Locked Planet"(PDF).Geophysical Research Abstracts.12:9759.Bibcode:2010EGUGA..12.9759N.
  26. ^Wagner, F. W.; Sohl, F.; Rückriemen, T.; Rauer, H. (2011). "Physical State of the Deep Interior of the CoRoT-7b Exoplanet".Proceedings of the International Astronomical Union.276:193–197.arXiv:1105.1271.Bibcode:2011IAUS..276..193W.doi:10.1017/S1743921311020175.S2CID118391441.
  27. ^Miguel, Y.; Kaltenegger, L.; et al. (1 December 2011). "Compositions of Hot Super-earth Atmospheres: Exploring Kepler Candidates".The Astrophysical Journal.742(2): L19.arXiv:1110.2426.Bibcode:2011ApJ...742L..19M.doi:10.1088/2041-8205/742/2/L19.S2CID119113934.
  28. ^Guenther, E. W.; et al. (2011). "Constraints on the exosphere of CoRoT-7b".Astronomy and Astrophysics.525:A24.arXiv:1009.5500.Bibcode:2011A&A...525A..24G.doi:10.1051/0004-6361/201014868.S2CID37218214.
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