# Alpha Centauri Bb

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Not to be confused with [Alpha Centauri Ab](/source/Alpha_Centauri_Ab) or [Alpha Centauri Cb](/source/Alpha_Centauri_Cb).

**Alpha Centauri Bb** (**α Cen Bb**) was a proposed [exoplanet](/source/Exoplanet) orbiting the [K-type main-sequence star](/source/K-type_main-sequence_star) [Alpha Centauri B](/source/Alpha_Centauri_B) (Toliman), located 4.37 [light-years](/source/Light-year) from Earth in the southern [constellation](/source/Constellation) of [Centaurus](/source/Centaurus), but there has not been enough evidence to support the claim.[1][2]

The claimed discovery of the planet was announced in October 2012 by a team of European observers, and the finding received widespread media attention.[3][4][5][6] However, the announcement was met with scepticism by some astronomers, who thought that the European team was over-interpreting its data.[7]

In October 2015, astronomers from the [University of Oxford](/source/University_of_Oxford) published a scientific paper disproving the existence of the planet. They observed that an identical statistical analysis of randomly-generated synthetic data gave the same results as the actual astronomical data.[8] This led Xavier Dumusque, the lead author of the original paper, to concede "We are not 100 percent sure, but probably the planet is not there."[9]

## Presumed detection

Starting in February 2008, and continuing through July 2011,[1] a team of European astronomers,[2][4] mainly from the [Observatory of Geneva](/source/Observatory_of_Geneva) and from the [Centre for Astrophysics of the University of Porto](/source/Centre_for_Astrophysics_of_the_University_of_Porto), recorded measurements of Alpha Centauri B's [radial velocity](/source/Radial_velocity) with [European Southern Observatory](/source/European_Southern_Observatory)'s [HARPS](/source/HARPS) [echelle](/source/Echelle_grating) [spectrograph](/source/Spectrograph) at the [La Silla Observatory](/source/La_Silla_Observatory) in [Chile](/source/Chile).[10] The team made 459 observations of Alpha Centauri B's color spectrum over a four-year period, then used statistical filters to remove known sources of variance.[11]

On 16 October 2012, the team announced they had detected an Earth-mass planet in orbit around Alpha Centauri B.[2] The discovery was presented in the scientific journal *[Nature](/source/Nature_(journal))*, with lead author Xavier Dumusque, a graduate student at the University of Porto.[12] He called their findings "a major step towards the detection of a [twin Earth](/source/Earth_analog) in the immediate vicinity of the Sun".[2]

The team used the [Doppler spectroscopy](/source/Doppler_spectroscopy) method to search for a planet.[13] As the body orbits the host star, its gravity would cause extremely small (semi-amplitude of about 0.5 m/s) periodic shifts in the host star's velocity. Variations of the line-of-sight velocity component cause tiny shifts in the star's spectrum. Using an extremely sensitive [spectrometer](/source/Spectrometer), the team was able to infer variations of 0.51 m/s in Alpha Centauri B's radial velocity. [European Southern Observatory](/source/European_Southern_Observatory) called the findings the most precise measurements ever recorded using the technique.[2] The team estimated the probability of a spurious detection at 0.02%.[1]

## Scepticism and further analysis

Dumusque used a variety of mathematical transformations to isolate the planet's gravitational signal and remove the surrounding stellar noise. The transformations filtered out other sources of radial-velocity variance, including effects of [starspots](/source/Starspot), photospheric [granulation](/source/Solar_granule) and rotation, as well as interference from the neighboring star Alpha Centauri A.[11][14] The final, derived radial-velocity measurements showed the periodic influence of the planet on the star, but were near the threshold of the HARPS instrument's sensory capabilities.[15] (The long-term precision of HARPS is about 0.8 m/s.[1])

In a commentary piece that ran in *Nature* alongside the original paper, American exoplanet hunter [Artie Hatzes](/source/Artie_P._Hatzes) praised the team's technical achievements and admitted he observed a planetlike signal amongst the data, but called it "a weak signal in the presence of a larger, more complicated signal". He stressed the need for further empirical confirmation, and opined that the existence of the planet was "still open to debate".[7] (Hatzes had previously expressed scepticism about unproven methods of data analysis during the dispute over the unconfirmed planet [Gliese 581g](/source/Gliese_581g).)[16]

In October 2015, three years after the announcement of the initial finding, researchers from the University of Oxford published a paper entitled "Ghost in the Time Series", demonstrating flaws in the original analysis.[8] Using the same data as Dumusque, the Oxford team found the signal was caused not by a planet, but instead was a mathematical artifact of the [window function](/source/Window_function) used to create the time series for the original observations. To prove this assertion, they replaced the actual astronomical data with randomly-generated noise, and obtained identical results. The team concluded that their analysis "underscores the difficulty of detecting weak planetary signals in [RV](/source/Radial_velocity) data". Dumusque agreed with this analysis, and conceded that the planet he had identified probably did not exist.[9]

## See also

- [Proxima Centauri b](/source/Proxima_Centauri_b) (Alpha Centauri Cb)
- [Proxima Centauri c](/source/Proxima_Centauri_c)
- [Alpha Centauri Bc](/source/Alpha_Centauri_Bc)
- [Alpha Centauri in fiction](/source/Alpha_Centauri_in_fiction)
- [Project Longshot](/source/Project_Longshot)

## References

1. Dumusque, X.; Pepe, F.; Lovis, C.; Ségransan, D.; Sahlmann, J.; Benz, W.; Bouchy, F.; Mayor, M. et al. (17 October 2012). ["An Earth mass planet orbiting Alpha Centauri B"](http://www.eso.org/public/archives/releases/sciencepapers/eso1241/eso1241a.pdf). *[Nature](/source/Nature_(journal))*. **490** (7423): 207–11. [Bibcode:2012Natur.491..207D](https://ui.adsabs.harvard.edu/abs/2012Natur.491..207D). [doi:10.1038/nature11572](https://doi.org/10.1038/nature11572). [PMID 23075844](https://pubmed.ncbi.nlm.nih.gov/23075844). [S2CID 1110271](https://api.semanticscholar.org/CorpusID:1110271). Retrieved 17 October 2012.

1. ["Planet Found in Nearest Star System to Earth"](http://www.eso.org/public/news/eso1241/). [European Southern Observatory](/source/European_Southern_Observatory). 16 October 2012. Retrieved 17 October 2012.

1. Wall, Mike (16 October 2012). ["Discovery! Earth-Size Alien Planet at Alpha Centauri Is Closest Ever Seen"](http://www.space.com/18089-earth-size-alien-planet-alpha-centauri.html). *Space.Com web site*. TechMediaNetwork. Retrieved 17 October 2012.

1. Overbye, Dennis (17 October 2012). ["New Planet in Neighborhood, Astronomically Speaking"](https://www.nytimes.com/2012/10/17/science/space/new-planet-found-in-alpha-centauri.html). *The New York Times*. Retrieved 17 October 2012.

1. Palmer, Jason (17 October 2012). ["Exoplanet around Alpha Centauri is nearest-ever"](https://www.bbc.co.uk/news/science-environment-19959531). BBC. Retrieved 17 October 2012.

1. Kaufman, Marc (17 October 2012). ["New Planet Is Closest Yet: Earth-Size Lava World a Space "Landmark""](https://web.archive.org/web/20121018213559/http://news.nationalgeographic.com/news/2012/10/121017-alpha-centauri-new-planet-science-space-udry/). National Geographic. Archived from [the original](http://news.nationalgeographic.com/news/2012/10/121017-alpha-centauri-new-planet-science-space-udry/) on 18 October 2012. Retrieved 18 October 2012.

1. Hatzes, A. P. (17 October 2012). "Meet our closest neighbour". *[Nature](/source/Nature_(journal))*. **491** (7423): 200–201. [Bibcode:2012Natur.491..200H](https://ui.adsabs.harvard.edu/abs/2012Natur.491..200H). [doi:10.1038/nature11636](https://doi.org/10.1038/nature11636). [PMID 23075846](https://pubmed.ncbi.nlm.nih.gov/23075846). [S2CID 37768785](https://api.semanticscholar.org/CorpusID:37768785)

1. Rajpaul, Vinesh (19 October 2015). "Ghost in the time series: no planet for Alpha Cen B". *Monthly Notices of the Royal Astronomical Society: Letters*. **456** (1): L6–L10. [arXiv:1510.05598](https://arxiv.org/abs/1510.05598). [Bibcode:2016MNRAS.456L...6R](https://ui.adsabs.harvard.edu/abs/2016MNRAS.456L...6R). [doi:10.1093/mnrasl/slv164](https://doi.org/10.1093/mnrasl/slv164)

1. Powell, Devin (29 October 2015). ["Poof! The Planet Closest To Our Solar System Just Vanished"](https://web.archive.org/web/20151030010115/http://news.nationalgeographic.com/2015/10/151028-planet-disappears-alpha-centauri-astronomy-science/). National Geographic. Archived from [the original](http://news.nationalgeographic.com/2015/10/151028-planet-disappears-alpha-centauri-astronomy-science/) on 30 October 2015. Retrieved 15 January 2015.

1. Borenstein, Seth (17 October 2012). ["Earth-Sized Planet Found Just Outside Solar System"](https://abcnews.go.com/Technology/wireStory/earth-sized-planet-found-solar-system-17492821#.UH3VzFHwF8E). *[ABC News](/source/ABC_News_(United_States))*. Retrieved 17 October 2012.

1. Dumusque, X.; Pepe, F.; Lovis, C.; Ségransan, D.; Sahlmann, J.; Benz, W.; Bouchy, F.; Mayor, M. et al. (17 October 2012). ["An Earth mass planet orbiting Alpha Centauri B (Supplementary Information)"](http://www.nature.com/nature/journal/vaop/ncurrent/extref/nature11572-s1.pdf). *[Nature](/source/Nature_(journal))*. **490** (7423): 207–11. [Bibcode:2012Natur.491..207D](https://ui.adsabs.harvard.edu/abs/2012Natur.491..207D). [doi:10.1038/nature11572](https://doi.org/10.1038/nature11572). [PMID 23075844](https://pubmed.ncbi.nlm.nih.gov/23075844). [S2CID 1110271](https://api.semanticscholar.org/CorpusID:1110271). Retrieved 17 October 2012.

1. ["Xavier Dumusque C.V."](http://www.astro.up.pt/caup/index.php?WID=114&CID=1&ID=82&Lang=uk). Centro de Astrofisica da Universidade do Porto. [Archived](https://web.archive.org/web/20110616091129/http://www.astro.up.pt/caup/index.php?WID=114&CID=1&ID=82&Lang=uk) 16 June 2011 at the Wayback Machine. Retrieved 18 October 2012.

1. Plait, Phil (16 October 2012). ["Alpha Centauri Has a Planet!"](http://blogs.discovermagazine.com/badastronomy/2012/10/16/alpha-centauri-has-a-planet/). *[Discover](/source/Discover_(magazine))*. Retrieved 18 October 2012.

1. Drake, Nadia (3 November 2012). ["The alien next door: Newly discovered planet is just 4.4 light-years distant"](http://www.sciencenews.org/view/generic/id/345756/title/The_alien_next_door). Science News. Retrieved 21 October 2012.

1. ["ESPRESSO – Searching for other Worlds"](https://web.archive.org/web/20101017053751/http://espresso.astro.up.pt/). University of Porto. Archived from [the original](http://espresso.astro.up.pt) on 17 October 2010. Retrieved 18 October 2012.

1. Overbye, Dennis (20 August 2012). ["A Planet 'Just Right' for Life? Perhaps, if It Exists"](https://www.nytimes.com/2012/08/21/science/space/just-right-or-nonexistent-dispute-over-goldilocks-planet-gliese-581g.html). *The New York Times*. Retrieved 18 October 2012.

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Adapted from the Wikipedia article [Alpha Centauri Bb](https://en.wikipedia.org/wiki/Alpha_Centauri_Bb) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Alpha_Centauri_Bb?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.