55 Cancri e
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55 Cancri e is an extrasolar planet with half the mass of Neptune orbiting the Sun-like star 55 Cancri A. Its mass is about 8.3 Earth masses. It takes less than 18 hours to complete an orbit and is the innermost known planet in its planetary system. 55 Cancri e was discovered on August 30, 2004. However, until the 2010 observations and recalculations, this planet had been thought to take about 2.8 days to orbit the star.[1]
Discovery
Like the majority of known extrasolar planets, 55 Cancri e was discovered by detecting variations in its star's radial velocity. This was achieved by making sensitive measurements of the Doppler shift of the spectrum of 55 Cancri A. At the time of its discovery, three other planets were known orbiting the star. After accounting for these planets, a signal at around 2.8 days remained, which could be explained by a planet of at least 14.2 Earth masses in a very close orbit.[2] The same measurements were used to confirm the existence of the uncertain planet 55 Cancri c.
55 Cancri e was one of the first extrasolar planets with a mass comparable to that of Neptune to be discovered. It was announced at the same time as another "hot Neptune" orbiting the red dwarf star Gliese 436 named Gliese 436 b.
Planet challenged
In 2005 the existence of planet e was questioned by Jack Wisdom in a reanalysis of the data.[3] According to him, instead of the 2.8-day planet there is a planet with a mass similar to that of Neptune in a 261-day orbit around 55 Cancri A. In 2007, Debra Fisher and colleagues at San Francisco State University published a new analysis[4] indicating that both planets existed; the planet in the 260-day orbit was accordingly designated 55 Cancri f.
Orbit and mass
55 Cancri e is located in a very close orbit around the star which takes less than 18 hours to complete and falls into the category of "hot Super-Earths". A limitation of the radial velocity method used to detect 55 Cancri e is that only a minimum mass can be obtained, in this case around 8.3 times that of Earth, or 48% of the mass of Neptune. Astrometric observations made with the Hubble Space Telescope suggest that the outer planet 55 Cancri d is inclined at 53° with respect to the plane of the sky.[2] If these measurements are confirmed and the system is assumed to be coplanar, the true mass of the planet would therefore be about 25% greater than this lower limit, at around 0.6 Neptune mass.
Characteristics
Since the planet has only been detected indirectly through its gravitational influence on 55 Cancri A, properties such as its radius and albedo are not well constrained, although the planet is smaller than 1.2 Jupiter radius. If it is 0.8 Jupiter radius, then its albedo is low; allowable albedo increases as the model for the planet radius shrinks. It receives more radiation than Gliese 436 b and is expected to be of comparable size.[5]
Without a fixed size and appearance, composition and temperature are also unknown. It is also unknown whether it is a small gas giant like Neptune or a large terrestrial planet. The formation models proposed for Mu Arae c do not apply to a planet this close to its star.[6] Large terrestrial planets may be formed from material compacted by the inward migration of the system's gas giants.[7]
Alternatively 55 Cancri e may be the core of a gas giant pushed inwards before it had time to accumulate a significant gaseous envelope.[2] This was dismissed in 2004, as gas giants can survive for long timescales in the inner regions of a planetary system.[8] However the star could have blown off a volatile layer via coronal mass ejection.[9] In this model (also proposed for Gliese 876 d), the original composition was richer in volatile substances, such as water. As it arrived in range, the planet boiled away to a pressurised ocean of water (in the form of a supercritical fluid) separated from the silicate core by a layer of ice kept frozen by the high pressures in the planetary interior. Such a planet would have an atmosphere containing water vapor and free oxygen produced by the breakdown of water by ultraviolet radiation.[10]
Distinguishing between these two models would require more information about the planet's radius or composition. The planet probably does not transit its star, which makes obtaining this information beyond current observational capabilities.
See also
References
- ^ Fischer, D. A.; et al. (2007-12-23). "Five Planets Orbiting 55 Cancri". Astrophysics. arXiv:0712.3917. Bibcode:2008ApJ...675..790F. doi:10.1086/525512.
{{cite journal}}: Explicit use of et al. in:|author=(help) - ^ a b c McArthur, B.; et al. (2004). "Detection of a NEPTUNE-mass planet in the ρ1 Cnc system using the Hobby-Eberly Telescope". The Astrophysical Journal. 614 (1): L81–L84. arXiv:astro-ph/0408585. Bibcode:2004ApJ...614L..81M. doi:10.1086/425561.
{{cite journal}}: Explicit use of et al. in:|author=(help) - ^ Wisdom, J. (2005). "Evidence of a Neptune-Sized Planet in the ρ1 Cancri System" (PostScript). The Astrophysical Journal Letters (submitted).
- ^ Minkel, JR (November 6, 2007). "Record Fifth Planet Discovered Around Distant Star". Scientific American.
- ^ Lucas, P. W.; Hough, J. H.; Bailey, J. A.; Tamura, M.; Hirst, E.; Harrison, D. (2007). "Planetpol polarimetry of the exoplanet systems 55 Cnc and tau Boo". arXiv:0807.2568 [astro-ph].
- ^ I. Baraffe, Y. Alibert, G. Chabrier, W. Benz (2008). "Birth and fate of hot-Neptune planets". Astronomy & Astrophysics. arXiv:0512091.
{{cite journal}}: Check|arxiv=value (help)CS1 maint: multiple names: authors list (link) - ^ Fogg, M., Nelson, R. (2005). "Oligarchic and giant impact growth of terrestrial planets in the presence of gas giant planet migration". Astronomy and Astrophysics. 441 (2): 791–806. arXiv:astro-ph/0507180. Bibcode:2005A&A...441..791F. doi:10.1051/0004-6361:20053453.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Bouchy, F.; et al. (2004). "Two new "very hot Jupiters" among the OGLE transiting candidates". Astronomy and Astrophysics. 421 (1): L13–L16. arXiv:astro-ph/0404264. Bibcode:2004A&A...421L..13B. doi:10.1051/0004-6361:20040170.
{{cite journal}}: Explicit use of et al. in:|author=(help) - ^ H. Lammer; et al. (2007). "The impact of nonthermal loss processes on planet masses from Neptunes to Jupiters" (PDF). Geophysical Research Abstracts. 9 (07850).
{{cite journal}}: Explicit use of et al. in:|author=(help) - ^ Zhou, J.-L.; et al. (2005). "Origin and Ubiquity of Short-Period Earth-like Planets: Evidence for the Sequential Accretion Theory of Planet Formation". The Astrophysical Journal. 631 (1): L85–L88. arXiv:astro-ph/0508305. Bibcode:2005ApJ...631L..85Z. doi:10.1086/497094.
{{cite journal}}: Explicit use of et al. in:|author=(help)
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