Alpha Centauri Bb (α Cen B b) was a proposed
exoplanet orbiting the
K-type main-sequence star
A K-type main-sequence star, also referred to as a K-type dwarf or an orange dwarf, is a main-sequence (hydrogen-burning) star of spectral type K and luminosity class V. These stars are intermediate in size between red M-type main-sequence star ...
Alpha Centauri B
Alpha Centauri ( Latinized from α Centauri and often abbreviated Alpha Cen or α Cen) is a triple star system in the constellation of Centaurus. It consists of 3 stars: Alpha Centauri A (officially Rigil Kentaurus), Alpha Centaur ...
, located 4.37
light-years from Earth in the southern
constellation of
Centaurus
Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. ...
, but there has not been enough evidence to support the claim.
The claimed discovery of the planet was announced in October 2012 by a team of European observers, and the finding received widespread media attention.
However, the announcement was met with scepticism by some astronomers, who thought that the European team was over-interpreting its data.
In October 2015, astronomers from the
University of Oxford
, mottoeng = The Lord is my light
, established =
, endowment = £6.1 billion (including colleges) (2019)
, budget = £2.145 billion (2019–20)
, chancellor ...
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.
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."
Presumed detection
![ESO - The Radial Velocity Method (by)](https://upload.wikimedia.org/wikipedia/commons/f/f2/The_radial_velocity_method_%28artist%E2%80%99s_impression%29.jpg)
Starting in February 2008, and continuing through July 2011,
a team of European astronomers,
mainly from the
Observatory of Geneva and from the
Centre for Astrophysics of the University of Porto, recorded measurements of Alpha Centauri B's
radial velocity with
European Southern Observatory's HARPS
The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision echelle planet-finding spectrograph installed in 2002 on the ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003. ...
echelle
An echelle grating (from French ''échelle'', meaning "ladder") is a type of diffraction grating characterised by a relatively low groove density, but a groove shape which is optimized for use at high incidence angles and therefore in high diffract ...
spectrograph at the
La Silla Observatory
La Silla Observatory is an astronomical observatory in Chile with three telescopes built and operated by the European Southern Observatory (ESO). Several other telescopes are located at the site and are partly maintained by ESO. The observatory is ...
in
Chile
Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...
.
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.
On 16 October 2012, the team announced they had detected an Earth-mass planet in orbit around Alpha Centauri B.
The discovery was presented in the scientific journal ''
Nature
Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...
'', with lead author Xavier Dumusque, a graduate student at the University of Porto.
He called their findings "a major step towards the detection of a
twin Earth in the immediate vicinity of the Sun".
The team used the
Doppler spectroscopy
Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in t ...
method to search for a planet.
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
A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...
, the team was able to infer variations of 0.51 m/s in Alpha Centauri B's radial velocity.
European Southern Observatory called the findings the most precise measurements ever recorded using the technique.
The team estimated the probability of a spurious detection at 0.02%.
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
starspot
Starspots are stellar phenomena, so-named by analogy with sunspots.
Spots as small as sunspots have not been detected on other stars, as they would cause undetectably small fluctuations in brightness. The commonly observed starspots are in gene ...
s, photospheric
granulation
Granulation is the process of forming grains or granules from a powdery or solid substance, producing a granular material. It is applied in several technological processes in the chemical and pharmaceutical industries. Typically, granulation invo ...
and rotation, as well as interference from the neighboring star Alpha Centauri A.
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.
(The long-term precision of HARPS is about 0.8 m/s.
)
In a commentary piece that ran in ''Nature'' alongside the original paper, American exoplanet hunter
Artie 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".
(Hatzes had previously expressed scepticism about unproven methods of data analysis during the dispute over the unconfirmed planet
Gliese 581g
Gliese 581g , unofficially known as Zarmina (or Zarmina's World), was a candidate exoplanet postulated to orbit within the Gliese 581 system, twenty light-years from Earth. It was discovered by the Lick–Carnegie Exoplanet Survey, and was the si ...
.)
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.
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
In signal processing and statistics, a window function (also known as an apodization function or tapering function) is a mathematical function that is zero-valued outside of some chosen interval, normally symmetric around the middle of the int ...
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 data".
Dumusque agreed with this analysis, and conceded that the planet he had identified probably did not exist.
See also
*
Proxima Centauri b
Proxima Centauri b (or Proxima b), sometimes referred to as Alpha Centauri Cb, is an exoplanet orbiting in the habitable zone of the red dwarf star Proxima Centauri, which is the closest star to the Sun and part of the triple star system ...
(Alpha Centauri Cb)
*
Proxima Centauri c
Proxima Centauri c (also called Proxima c or Alpha Centauri Cc) is a controversial exoplanet candidate claimed to be orbiting the red dwarf star Proxima Centauri, which is the closest star to the Sun and part of a triple star syste ...
*
Alpha Centauri Bc
*
Alpha Centauri in fiction
*
Project Longshot
Project Longshot was a conceptual interstellar spacecraft design. It would have been an uncrewed starship (about 400 tonnes), intended to fly to and enter orbit around Alpha Centauri B powered by nuclear pulse propulsion.
History
Developed ...
References
{{2012 in space
Alpha Centauri
Centaurus (constellation)
Exoplanets detected by radial velocity
Exoplanets discovered in 2012
Astronomical controversies
Articles containing video clips
Disproven exoplanets