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Mu Arae c, also known as HD 160691 c, is an extrasolar planet orbiting around Mu Arae. Its discovery was announced on August 25, 2004.[1] At the time, its minimum mass was reported at just 14 times that of Earth,[2] although later work established a value of 10.5 Earth masses.[3] It orbits very close to Mu Arae, completing one revolution every 9.6 days. The discovery was made with the aid of the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph, at the European Southern Observatory's La Silla Observatory in Chile. The data that revealed the presence of this planet was gathered on 8 nights of observations in June 2004.

Assuming its true mass is comparable to those of Neptune and Gliese 436 b, 14 Earth masses is theoretically the maximum size for a terrestrial planet. A rocky planet this size could certainly have formed, since Mu Arae has a higher metallicity than our Sun. Also, it is thought to have formed well inside the system's "snow line" at 3.2 AU.[2] However, various models of the system's formation have since converged that the planet attracted large amounts of volatiles before its star had cleared out the ice, and that it now has a core of only 6 Earth masses.[4] Its core is likely enveloped in so much hot-ice and gas that the planet would behave more like Neptune.

Mu Arae c is too far from its sun to be subject to coronal mass ejections. There is disagreement as to whether it is and has always been a hot Neptune in mass (Lammer),[5] or if it could have developed from a gas giant, losing most of its mass on the way (Baraffe).

If an eroded gas giant, the sun would have boiled the planet from a larger protoplanet, of 20 Earth masses up to half Jupiter's mass. If the latter, its current radius could be as high as 0.6 Jupiter.[4]

The planet must be hot because of its proximity to its star. The discoverers chose for it an albedo of 0.35, lighter than albedos chosen to calculate the temperatures of hot Jupiters such as tau Boötis b. This was perhaps due to the discoverers' assumption that the planet would be a silicate super-terrestrial with neither clouds nor a deep Rayleigh-scattering atmosphere. If so the surface temperature would be about 900 K.[2]

None of the four planets orbiting Mu Arae are directly visible from Earth using currently available tools. All four were found using the radial velocity method of extrasolar planet detection.

See also Edit

References Edit

  1. Cite error: Invalid <ref> tag; no text was provided for refs named eso0427
  2. 2.0 2.1 2.2 N.C. Santos, F. Bouchy, M. Mayor, F. Pepe, D. Queloz, S. Udry, C. Lovis, M. Bazot, W. Benz, J.-L. Bertaux, G. Lo Curto, X. Delfosse, C. Mordasini, D. Naef, J.-P. Sivan, and S. Vauclair (2004). "A 14 Earth-masses exoplanet around μ Arae". Astronomy and Astrophysics 426: L19–L23. DOI:10.1051/0004-6361:200400076.
  3. F. Pepe, A.C.M. Correia, M. Mayor, O. Tamuz, W. Benz, J.-L. Bertaux, F. Bouchy, J. Couetdic, J. Laskar, C. Lovis, D. Naef, D. Queloz, N. C. Santos, J.-P. Sivan, D. Sosnowska, and S. Udry (2007). "μ Ara, a system with four planets". Astronomy and Astrophysics 462 (2): 769–776. DOI:10.1051/0004-6361:20066194.
  4. 4.0 4.1 I. Baraffe, Y. Alibert, G. Chabrier, W. Benz (2005). "Birth and fate of hot-Neptune planets". Astronomy and Astrophysics 450 (3): 1221–1229. DOI:10.1051/0004-6361:20054040.
  5. H. Lammer et al. (2007). "The impact of nonthermal loss processes on planet masses from Neptunes to Jupiters". Geophysical Research Abstracts 9 (07850). The 0.09 AU sub-Jup is there called "HD160691d",

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