55 Cancri is the system'sFlamsteed designation. It also bears theBayer designation ρ1 Cancri (Latinised to Rho1 Cancri) and theBright Star Catalogue designation HR 3522. The two components are designated A and B,[15] though component A is sometimes referred to simply as 55 Cancri.[16] The first planet discovered orbiting 55 Cancri A was designated HR 3522b by its discoverers,[17] though it is more commonly referred to as 55 Cancri b.[18] Under the rules for naming objects in binary star systems it should be named 55 Cancri Ab[19] and this more formal form is occasionally used to avoid confusion with the secondary star 55 Cancri B. The other planets discovered were designated 55 Cancri c, d, e and f, in order of their discovery.
In July 2014 theInternational Astronomical Union launchedNameExoWorlds, a process for giving proper names to certain exoplanets and their host stars.[20] The process involved public nomination and voting for the new names.[21] In December 2015, the IAU announced the winning names were Copernicus for 55 Cancri A and Galileo, Brahe, Lipperhey, Janssen and Harriot for its planets (b, c, d, e and f, respectively).[22]
The winning names were those submitted by the Royal Netherlands Association for Meteorology and Astronomy of theNetherlands. They honor the astronomersNicolaus Copernicus,Galileo Galilei,Tycho Brahe andThomas Harriot and the spectacle makers and telescope pioneersHans Lipperhey andJacharias Janssen.[23] (The IAU originally announced the winning name was Lippershey for 55 Cancri d. In January 2016, in recognition that his actual name was Lipperhey (with Lippershey an error introduced in the 19th century), the exoplanet name was corrected to Lipperhey by the IAU and that name was submitted to the official sites that keep track of astronomical information).[22][23]
In 2016, the IAU organized aWorking Group on Star Names (WGSN)[24] to catalog and standardize proper names for stars. In its first bulletin of July 2016,[25] the WGSN explicitly recognized the names of exoplanets and their host stars approved by the Executive Committee Working Group Public Naming of Planets and Planetary Satellites, including the names of stars adopted during the 2015 NameExoWorlds campaign. This star is now so entered in the IAU Catalog of Star Names.[14]
The 55 Cancri system is located fairly close to theSolar System: theGaiaastrometrysatellite measured theparallax of 55 Cancri A as 79.45milliarcseconds, corresponding to a distance of 12.6parsecs (41light-years).[1] 55 Cancri A has anapparent magnitude of 5.95, making it just visible to thenaked eye under very dark skies. The red dwarf 55 Cancri B is of the 13th magnitude and only visible through atelescope. The two components are separated by85″, an estimated separation of1,065 AU[26] (6.15light-days). Despite their wide separation, the two stars appear to be gravitationally bound, as they share a commonproper motion.[16]
The primary star, 55 Cancri A, has aspectral type of K0IV-V, indicating amain-sequence orsubgiant star. It is smaller in radius and slightly less massive than theSun, and so is cooler and lessluminous. The star has only low emission from its chromosphere, and is not variable in the visible spectrum;[16] but it is variable in X-rays.[27] It is more enriched than the Sun inelements heavier thanhelium, with 186% the solar abundance ofiron; it is therefore classified as a rare "supermetal-rich" (SMR) star.[16] 55 Cancri A also has more carbon than the Sun, with a C/O ratio of 0.78,[28] compared to solar value of 0.55. This abundance of metal makes estimating the star's age and mass difficult, asevolutionary models are less well defined for such stars.[citation needed] 55 Cancri A is much older than the solar system, and its age has been estimated to values of 7.4–8.7 billion years[29] or 10.2 ± 2.5 billion years.[10]
A hypothesis for the high metal content in SMR dwarf stars is that material enriched in heavy elements fell into the atmosphere from aprotoplanetary disk. This would pollute the star's external layers, resulting in a higher than normal metallicity. The lack of a deepconvection zone would mean that the outer layers would retain higher abundance ratios of these heavy elements.[30]
Observations of 55 Cancri A in thesubmillimeter region of the spectrum have thus far failed to detect any associated dust. The upper limit on emissions within100 AU of this star is about 850 mJy, at a wavelength of 850 μm. This limits the total mass of fine dust around the star to less than 0.01% of the Earth's mass. However, this does not exclude the presence of an asteroid belt or aKuiper belt equivalent.[31]
The secondary, 55 Cancri B, is ared dwarf star much less massive and luminous than the Sun. There are indications that component B may itself be a double star, though this is uncertain.[15]
Comparison of the orbits of the inner planets of 55 Cancri A (black) with the planets of the Solar System
The 55 Cancri system was the first known to have four, and later five, planets, and may possibly have more. The innermost planet, e,transits 55 Cancri A as viewed from Earth.[32] The next planet, b, is non-transiting but there is tentative evidence that it is surrounded by an extended atmosphere that does transit the star.[27]
In 1997, the discovery of a51 Pegasi-like planetorbiting 55 Cancri A was announced, together with the planet ofTau Boötis and the inner planet ofUpsilon Andromedae.[17] The planet was discovered by measuring the star'sradial velocity, which showed a periodicity of around 14.7 days corresponding to a planet at least 78% of themass of Jupiter. These radial velocity measurements still showed a drift unaccounted for by this planet, which could be explained by thegravitational influence of a more distant object.
In 1998 the discovery of a possible dust disk around 55 Cancri A was announced.[35] Calculations gave the disk radius at least 40 AU, similar to theKuiper belt in the Solar System, with an inclination of 25° with respect to the plane of the sky. However, the discovery could not be verified and was later deemed to be spurious, caused instead by background galaxies.[36]
TheSolar System with onlyEarth andJupiter compared with the planetary system of 55 Cancri (Note: this depiction was made before planets e and f were discovered.)
After making further radial velocity measurements, a planet orbiting at a distance of around 5 AU was announced in 2002.[16] This planet received the designation55 Cancri d. At the time of discovery, the planet was thought to be in an orbit of mildeccentricity (close to 0.1), but this value was increased by later measurements. Even after accounting for these two planets, a periodicity at 43 days remained, possibly due to a third planet. Measurements of the star suggested that this was close to the star's rotation period, which raised the possibility that the 43-day signal was caused by stellar activity. This possible planet received the designation55 Cancri c.
Artist's rendition of 55 Cnc's planets
55 Cancri e was announced in 2004.[37] With 8.3 Earth masses, it is a largesuper-Earth which was originally thought to have an orbital period of 2.8 days, though it was later found that this was an alias of its true period of 0.74 of a day by observations of etransiting in 2011.[32] This planet was the first known instance of a fourth extrasolar planet in one system, and was the shortest-period planet until the discovery ofPSR J1719−1438 b. The measurements that led to the discovery of this planet also confirmed the existence of 55 Cancri c.
In 2005,Jack Wisdom combined three data sets and drew two distinct conclusions: that the 2.8-day planet was an alias and that there was aNeptune-scale planet with a period near 261 days. Fischer et al. (2008)[38] reported new observations that they said confirmed the existence of the 2.8-day planet, as first reported by McArthur et al. (2004), and a 260-day Neptune-sized planet, as first reported by Wisdom (2005).[39] However, Dawson and Fabrycky (2010)[33] concluded that the 2.8-day planet was indeed an alias, as suggested by Wisdom (2005), and that the correct period was 0.7365 of a day.
In 2007, Fisher et al. confirmed the existence of the 260-day planet proposed in 2005 by Wisdom. This planet, 55 Cancri f, was the first occurrence of a fifth extrasolar planet in one system. With a similar mass toc, it has a 260-day orbit, towards the inner edge of 55 Cancri A'shabitable zone.[40][41] The planet itself is not thought to be conducive to life, but hypotheticalmoons in principle could maintain at least water and life.
The planet e's eccentricity is poorly defined; varying values between 0 and 0.4 does not significantly improve the fit, so an eccentricity of 0.2 was assumed. Taking interactions between the planets into account results in a near-zero orbital eccentricity.
Astrometric observations with theHubble Space Telescope measured an inclination of 53° of the outer planet d,[37] though this result relies on the precise orbital parameters which have been substantially revised since this was published.[42] The observed transits of e suggest an orbit normal inclined within 9° to the line-of-sight, and a possible detection of the transit of an extended atmosphere around 55 Cancri b would, if confirmed, imply that it too is in an orbit that is close to edge-on.[27] Between them, no measurement of c's nor f's inclinations have been made. It had been thought that with five planets, the system cannot deviate far fromcoplanar in order to maintain stability.[41] An attempt to measure the spin-orbit misalignment of the innermost planet reported that it was in a nearly polar orbit,[43] but this interpretation of the data has since been challenged by a subsequent study, with noted inconsistencies between the implied and measured stellar rotation.[44]
The approximate ratios of periods of adjacent orbits are (proceeding outward): 1:20, 1:3, 1:6, 1:20. The nearly 1:3 ratio between 55 Cancri b and c is apparently anear resonance, rather than a genuinemean motion resonance.[41]
Between planets f and d, there appears to be a huge gap of distance where no planets are known to orbit. A 2008 paper found that as many as 3 additional planets of up to 50 times the mass of Earth could orbit at a distance of 0.9 to 3.8 AU from the star, and stable resonances of a hypothetical planet g with the known planets were found to be 3f:2g, 2g:1d, and 3g:2d.[45] A study released in 2019 showed that undiscovered terrestrial planets may be able to orbit safely in this region at 1 to 2 AU; this space includes the outer limits of 55 Cancri'shabitable Zone.[46]In 2021, it was found that terrestrial planets with comparable water content to Earth may have indeed been able to form and survive between the planets f and d.[47]As for the space outside d's orbit, its stability zone begins beyond 10 AU, though there is a stability zone between 8.6 and 9 AU due to a 2:1 resonance.[45]
Since 55 Cancri e orbits less than 0.1 AU from its host star, some scientists hypothesized that it may cause stellar flaring synchronized to the orbital period of the exoplanet. A 2011 search for these magnetic star-planet interactions that would result incoronal radio emissions resulted in no detected signal. Furthermore, nomagnetospheric radio emissions were detected from any exoplanet within the system.[48]
AMETI message was sent to 55 Cancri. It was transmitted fromEurasia's largestradar—the 70 m (230 ft)Evpatoria Planetary Radar. The message was namedCosmic Call 2; it was sent on July 6, 2003, and it will arrive at 55 Cancri in May 2044.[49]
^abZacharias, N.; Finch, C. T.; Girard, T. M.; Henden, A.;Bartlett, J. L.; Monet, D. G.; Zacharias, M. I. (2012). "VizieR Online Data Catalog: UCAC4 Catalogue (Zacharias+, 2012)".VizieR On-line Data Catalog.Bibcode:2012yCat.1322....0Z.
^Gray, R. O.; Corbally, C. J.; Garrison, R. F.; McFadden, M. T.; Robinson, P. E. (2003). "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I".The Astronomical Journal.126 (4): 2048.arXiv:astro-ph/0308182.Bibcode:2003AJ....126.2048G.doi:10.1086/378365.S2CID119417105.
^abHoffleit, D.; Warren, W. H. (1995). "VizieR Online Data Catalog: Bright Star Catalogue, 5th Revised Ed. (Hoffleit+, 1991)".VizieR On-line Data Catalog: V/50. Originally Published in: 1964BS....C......0H.5050.Bibcode:1995yCat.5050....0H.
^abvon Braun, Kaspar; Tabetha, S. Boyajian; ten Brummelaar, Theo; Kane, Stephen R.; van Belle, Gerard T.; Ciardi, David R.; Raymond, Sean N.; López-Morales, Mercedes; McAlister, Harold A.; Schaefer, Gail (2011). "55 Cancri: Stellar Astrophysical Parameters, a Planet in the Habitable Zone, and Implications for the Radius of a Transiting Super-Earth".The Astrophysical Journal.740 (1):49–54.arXiv:1106.1152.Bibcode:2011ApJ...740...49V.doi:10.1088/0004-637X/740/1/49.S2CID2856228.
^Fischer, Debra A.; Marcy, Geoffrey W.; Butler, R. Paul; Vogt, Steven S.; Laughlin, Greg; Henry, Gregory W.; Abouav, David; Peek, Kathryn M. G.; Wright, Jason T.; Johnson, John A.; McCarthy, Chris; Isaacson, Howard (2008). "Five Planets Orbiting 55 Cancri".The Astrophysical Journal.675 (1):790–801.arXiv:0712.3917.Bibcode:2008ApJ...675..790F.doi:10.1086/525512.S2CID55779685.
^Wisdom, J. (2005). "A Neptune-sized Planet in the ρ1 Cancri System".AAS/Division of Dynamical Astronomy Meeting #36.36: 05.08.Bibcode:2005DDA....36.0508W.
^Mercedes Lopez-Morales; Amaury H. M. J. Triaud; Florian Rodler; Xavier Dumusque; Lars A. Buchhave; A. Harutyunyan; Sergio Hoyer; Roi Alonso; Michael Gillon; Nathan A. Kaib; David W. Latham; Christophe Lovis; Francesco Pepe; Didier Queloz; Sean N.Raymond; Damien Segransan; Ingo P. Waldmann; Stephane Udry (2014). "Rossiter-McLaughlin Observations of 55 Cnc e".The Astrophysical Journal.792 (2): L31.arXiv:1408.2007.Bibcode:2014ApJ...792L..31L.doi:10.1088/2041-8205/792/2/L31.S2CID14634162.
