55 Cancri

Starbox begin
name = 55 CancriStarbox observe 2s
epoch = J2000.0
constell = Cancer
component1 = 55 Cancri A
ra1 = RA|08|52|35.8
dec1 = DEC|+28|19|51
appmag_v1 = 5.95
component2 = 55 Cancri B
ra2 = RA|08|52|40.9
dec2 = DEC|+28|19|59
appmag_v2 = 13.15Starbox character
class = G8V / M3.5-4V
b-v = 0.86 / 1.21
u-b = 0.65 / 1.66Starbox astrometry
radial_v = 27.3
prop_mo_ra = -485.46
prop_mo_dec = -234.40
parallax = 79.80 ± 0.84
dist_ly = 40.9 ± 0.4
dist_pc = 12.5 ± 0.1
absmag_v = 5.46 / 12.66Starbox detail
mass = 0.95±0.1 / 0.13
radius = 0.96 / 0.30
luminosity_bolometric = 0.63 / 0.0076
temperature = 5243 ± 93
rotation = 42.2 d
age = 4.5×10⁹Starbox catalog
names = BD+28°1660, Gliese 324, HD 75732, Rho¹ CancriStarbox reference
Simbad = HD+75732

55 Cancri (pronEng|ˈkæŋkraɪ or IPA|/ˈkæŋkriː/), also catalogued Rho¹ Cancri or abbreviated 55 Cnc, is a binary star approximately 41 light-years away in the constellation of Cancer. The system consists of a yellow dwarf star and a smaller red dwarf star, separated by over 1,000 times the distance from the Earth to the Sun.

As of 2007, five extrasolar planets have been confirmed to be orbiting the primary, 55 Cancri A (the yellow dwarf). The innermost planet is thought to be a terrestrial "super-Earth" planet, with a mass similar to Neptune, while the outermost planets are thought to be Jovian planets with masses similar to Jupiter. The 55 Cancri system is as of September 2008 the only planetary system known to have five planets, [cite web|url=http://exoplanet.eu/catalog-all.php |title=The Extrasolar Planets Encyclopaedia |publisher=Exoplanet.eu |date= |accessdate=2008-09-14] and may possibly have more. 55 Cancri A is ranked 63rd in the list of top 100 target stars for the NASA Terrestrial Planet Finder mission. [cite web|url=http://sco.stsci.edu/tpf_top100/index.php|title=TPF-C Top 100|accessmonthday=4 July|accessyear=2006]

Name

The term "55 Cancri" (the star's Flamsteed designation) is seen more often used for the system, rather than its Bayer designation "Rho¹ Cancri". The latter is used by those who feel that it should be preferred to the Flamsteed number — as is usually done (e.g., Tau Bootis is almost never called "4 Bootis"). But in this case the superscript in "Rho¹" makes the Bayer designation rather cumbersome, so that "55 Cancri" is the more commonly used name for this star.

Distance and visibility

The 55 Cancri system is located fairly close to our solar system: the Hipparcos astrometry satellite measured the parallax of 55 Cancri A as 79.80 milliarcseconds, corresponding to a distance of 12.5 parsecs. [cite web|url=http://vizier.u-strasbg.fr/viz-bin/VizieR-S?HIP%2043587|title=HIP 43587|work=The Hipparcos and Tycho Catalogues|publisher=ESA|year=1997|accessmonthday=30 June|accessyear=2006] 55 Cancri A has an apparent magnitude of 5.95, making it visible through binoculars. It is just visible to the naked eye under very dark skies. The red dwarf 55 Cancri B is of the 13th magnitude and only visible through a telescope.

System components

The primary star 55 Cancri A is a yellow dwarf star of main sequence spectral type G8V. It is smaller in radius and slightly less massive than our Sun, and so is cooler and less luminous. The star has little or no variability and only low emission from its chromosphere.

55 Cancri A is more enriched than our sun in elements heavier than helium, with 186% the solar abundance of iron; it is therefore classified as a rare "super metal-rich" (SMR) star.cite journal | url=http://www.journals.uchicago.edu/doi/full/10.1086/344298 | author=Marcy, G. et al. | title=A planet at 5 AU Around 55 Cancri | journal=The Astrophysical Journal | volume=581 | issue=2 | pages=1375 – 1388 | year=2002 | doi=10.1086/344298 ] This abundance of metal makes estimating the star's age and mass difficult, as evolutionary models are less well defined for such stars. One estimate based on chromospheric activity suggests an age of around 5,500 million years. [cite journal|url=http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?2005A%26A...443..609S&db_key=AST&nosetcookie=1|author=Saffe, C. et al.|title=On the Ages of Exoplanet Host Stars|journal=Astronomy and Astrophysics|volume=443|issue=2|pages=609 – 626|year=2005|doi=10.1051/0004-6361:20053452]

A hypothesis for the high metal content in SMR dwarf stars is that material enriched in heavy elements fell into the atmosphere from a protoplanetary disk. This would pollute the star's external layers, resulting in a higher than normal metallicity. The lack of a deep convection zone would mean that the outer layers would retain higher abundance ratios of these heavy elements. [cite news
last=Pasquini | first=Luca | coauthors=Hatzes, Artie
date=2007-07-06 | publisher=ESO
title=Star Surface Polluted by Planetary Debris
url=http://www.eso.org/public/outreach/press-rel/pr-2007/pr-29-07.html
accessdate=2007-11-08 ]

Observation of 55 Cancri A in the submillimeter region of the spectrum have thus far failed to detect any associated dust. The upper limit on emissions within 100 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. Of course, this does not exclude the existence of an asteroid belt or a Kuiper belt equivalent. [cite journal
last=Jayawardhana | first=Ray
coauthors=Holland, W. S.; Kalas, P.; Greaves, J. S.; Dent, W. R. F.; Wyatt, M. C.; Marcy, G. W.
title=New Submillimeter Limits on Dust in the 55 Cancri Planetary System
journal=The Astrophysical Journal
year=2002 | volume=570 | issue=2 | pages=L93–L96
url=http://www.journals.uchicago.edu/doi/full/10.1086/341101
accessdate=2007-11-07 | doi=10.1086/341101 ]

55 Cancri B is a red dwarf star located at an estimated distance of 1065 AU from the primary star, [cite journal|url=http://www-lep.gsfc.nasa.gov/code693/esp_papers/web/ps2/eggenberger_43.ps|format=PostScript |author=Eggenberger, A. et al.|title=Planets in Binaries|journal=Scientific Frontiers in Research on Extrasolar Planets|volume=294|pages=43 – 46|year=2003] and is much less massive and luminous than our Sun. Despite their wide separation, the two stars appear to be gravitationally bound, as they share a common proper motion. There are indications that component B may itself be a double star, though this is by no means certain. [cite journal|url=http://fr.arxiv.org/abs/astro-ph/0603836|author=Raghavan, D. et al.|title=Two Suns in The Sky: Stellar Multiplicity in Exoplanet Systems|journal=The Astrophysical Journal (accepted)|year=2006|volume=646|pages=523–542|doi=10.1086/504823]

Planetary system

In 1997, the discovery of a 51 Pegasi-like planet orbiting 55 Cancri A was announced, together with the planet of Tau Boötis and the inner planet of Upsilon Andromedae. [cite journal|url=http://www.journals.uchicago.edu/doi/full/10.1086/310444|author=Butler, R. et al.|title=Three New 51 Pegasi-Type Planets|journal=The Astrophysical Journal|volume=474|year=1997|pages=L115 – L118|doi=10.1086/310444 ] The planet was discovered by measuring the star's radial velocity, which showed a periodicity of around 14.7 days corresponding to a planet at least 78% of the mass of Jupiter. This planet was designated 55 Cancri b, though to distinguish it from the star 55 Cancri B it is occasionally referred to as 55 Cancri Ab. The radial velocity measurements still showed a drift unaccounted-for by this planet, which could be explained by the gravitational influence of a more distant object.

In 1998 the discovery of a possible dust disk around 55 Cancri A was announced. [cite journal|url=http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v395/n6704/abs/395775a0_fs.html |author=Trilling, D., Brown. R|title=A circumstellar dust disk around a star with a known planetary companion|journal=Nature|volume=395|pages=775 – 777|year=1998|doi=10.1038/27389] Calculations gave the disk radius at least 40 AU, similar to the Kuiper belt in our 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 radiation. [cite journal |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001AJ....121..525S&db_key=AST&high=3cab10bb0320924 |author=Schneider, G. et al.|title=NICMOS Coronagraphic Observations of 55 Cancri|journal=The Astronomical Journal|volume=121|issue=1|pages=525 – 537|year=2001|doi=10.1086/318050]

After making further radial velocity measurements, a planet orbiting at a distance of around 5 AU was announced in 2002. This planet received the designation 55 Cancri d. At the time of discovery, the planet was thought to be in an orbit of mild eccentricity (close to 0.1), however 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 designation 55 Cancri c.

In 2004 a Neptune-mass planet designated 55 Cancri e was announced in a 2.8-day orbit. [cite journal|url=http://xxx.lanl.gov/abs/astro-ph/0408585|author=McArthur, B. et al.|title=Detection of a NEPTUNE-mass planet in the ρ¹ Cnc system using the Hobby-Eberly Telescope|journal=The Astrophysical Journal|volume=614|year=2004|pages=L81 – L84|doi=10.1086/425561] This planet may either be a small gas giant or a large terrestrial planet. The measurements that led to the discovery of this planet also confirmed the existence of 55 Cancri c. In addition, astrometric measurements made by the Hubble Space Telescope led to an estimate of the inclination of the orbit of the outer planet: around 53° with respect to the plane of the sky.

In 2005 the existence of planet e was questioned by Jack Wisdom in a reanalysis of the data. [cite journal|url=http://swiss.csail.mit.edu/users/wisdom/planet.ps|format=PostScript|journal=The Astrophysical Journal Letters (submitted)|author=Wisdom, J.|title=Evidence of a Neptune-Sized Planet in the ρ¹ Cancri System|year=2005] 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 (corresponding to 0.77 AU in distance). This analysis has been partially confirmed in November 2007 - a planet designated 55 Cancri f with half the mass of Saturn was announced in a 260-day orbit, right in 55 Cancri A's habitable zone. [cite web|url=http://www.sciencedaily.com/releases/2007/11/071106133058.htm |title=Astronomers Discover Record Fifth Planet Around Nearby Star 55 Cancri |publisher=Sciencedaily.com |date=ScienceDaily (November 6, 2007) |accessdate=2008-09-14] cite journal | author =Fischer, D. A. "et al." | title=Five Planets Orbiting 55 Cancri |journal=Astrophysical Journal |issue=675| page=790-801 |doi=10.1086/525512 |month=March |year=2008] The planet itself is not thought to be conducive to life, but hypothetical moons in principle could maintain at least microbial life.

With five planets, the system cannot deviate far from coplanar in order to maintain stability. Assuming the astrometric determination of the orbit of the outer planet is correct, this means the true masses of the planets are around 25% greater than the lower limits measured by the radial velocity method.

More planets are possible within the stable zone, between f and d at 0.9 to 3.8 AU with eccentricities below 0.4. Given hypothetical planet g of up to 50 Earth mass, stable mean motion resonance regions lie at 3f:2g, 2g:1d, and 3g:2d. As for the space outside d's orbit, its stability zone begins beyond 10 AU. [Cite arXiv | title=A dynamical perspective on additional planets in 55 Cancri |author=Sean N. Raymond, Rory Barnes, Noel Gorelick | class=astro-ph |year=2008 |eprint=0808.3295v1]

In 2008, astronomers investigated if the distances of the planets in this system fit a mathematical relationship, similar to the Titius-Bode law that was once proposed for our own solar system. A relationship was found and used to predict the existence of two additional planets: one between the orbits of planets f and d at a distance of ~2.0 AU (which would have an orbital period of ~1130 days), and one beyond the orbit of d at ~15 AU, which would have an orbital period of ~62 years. [cite journal|authors=Poveda, A.; Lara, P.|month=April | year=2008|title=The exo-planetary system of 55 Cancri and the Titius-Bode Law|journal=The Astrophysical Journal|volume=44|pages=243-246|url=http://adsabs.harvard.edu/abs/2008RMxAA..44..243P|accessdate=2008-07-23] As yet, there is no evidence for the existence of these postulated planets. Furthermore, in principle the relationship predicts an infinite number of planets at ever-increasing separations from the central star, so the use of this relationship to predict planets outside the orbits of the known system is not necessarily reasonable. The application of relationships such as the Titius-Bode law to real planetary systems is controversial. [cite journal|authors=Kotliarov, I.|month=June | year=2008|title=The Titius-Bode Revisited But Not Revived| url=http://arxiv.org/abs/0806.3532|accessdate=2008-07-25]

There was a METI message sent to 55 Cancri. It was transmitted from Eurasia's largest radar — 70-meter Eupatoria Planetary Radar. The message was named Cosmic Call 2, it was sent on July 6 2003, and it will arrive at 55 Cancri in May 2044. [cite web|url=http://www.cplire.ru/rus/ra&sr/VAK-2004.html |title=Передача и поиски разумных сигналов во Вселенной |publisher=Cplire.ru |date= |accessdate=2008-09-14]

OrbitboxPlanet begin
table_ref= (Keplerian fit) OrbitboxPlanet
exoplanet = e
mass = >0.034 ± 0.0036
period = 2.81705 ± 0.0001
semimajor = 0.038 ± 10⁻⁶
eccentricity = 0.07 ± 0.06
year = 2004OrbitboxPlanet
exoplanet = b
mass = >0.824 ± 0.007
period = 14.65162 ± 0.0007
semimajor = 0.115 ± 0.000011
eccentricity = 0.014 ± 0.008
year = 1996OrbitboxPlanet
exoplanet = c
mass = >0.169 ± 0.008
period = 43.93 ± 0.021
semimajor = 0.240 ± 0.00005
eccentricity = 0.086 ± 0.052
year = 2002OrbitboxPlanet
exoplanet = f
mass = >0.144 ± 0.04
period = 260.00 ± 1.1
semimajor = 0.781 ± 0.007
eccentricity = 0.2 ± 0.2
year = 2005OrbitboxPlanet
exoplanet = d
mass = >3.835 ± 0.08
period = 5218 ± 230
semimajor = 5.77 ± 0.11
eccentricity = 0.025 ± 0.03
year = 2002

See also

* 51 Pegasi
* Mu Arae
* PSR 1257+12
* Upsilon Andromedae

References

External links

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* [http://www.aphelion-webzine.com/shorts/2008/08/WhentheGodsFall.html When the Gods Fall] 55 Cancri in fiction.
* [http://www.lpl.arizona.edu/~rory/research/xsp/dynamics/ Extrasolar Planet Interactions] by Rory Barnes & Richard Greenberg, Lunar and Planetary Lab, University of Arizona