 | ||
| Parent star | ||
|---|---|---|
| Star | Proxima Centauri | |
| Constellation | Centaurus | |
| Right ascension | (α) | 14h 29m 42.94853s |
| Declination | (δ) | −62° 40′ 46.1631″ |
| Apparent magnitude | (mV) | 11.13 |
| Distance | 4.224 ly (1.295[1] pc) | |
| Spectral type | M6Ve[2] | |
| Mass | (m) | 0.123 (± 0.006)[3] M☉ |
| Radius | (r) | 0.141 (± 0.007)[4] R☉ |
| Temperature | (T) | 3042 (± 117)[3] K |
| Metallicity | [Fe/H] | 0.21[5] |
| Age | 4.85[6] Gyr | |
| Physical characteristics | ||
| Minimum mass | (m sin i) | 1.27+0.19 −0.17[1] M⊕ |
| Radius | (r) | ≥1.1 (± 0.3)[7] R⊕ |
| Stellar flux | (F⊙) | 0.65[1] ⊕ |
| Temperature | (T) | 234 K (−39 °C; −38 °F) |
| Orbital elements | ||
| Semi-major axis | (a) | 0.0485+0.0041 −0.0051[1] AU |
| Eccentricity | (e) | <0.35[1] |
| Orbital period | (P) | 11.186+0.001 −0.002[1] d |
| Argument of periastron | (ω) | 310 (± 50)[1]° |
| Semi-amplitude | (K) | 1.38 (± 0.21)[1] m/s |
| Discovery information | ||
| Discovery date | 24 August 2016 | |
| Discoverer(s) | Anglada-Escudé (ca) et al. | |
| Discovery method | Doppler spectroscopy | |
| Discovery site | European Southern Observatory | |
| Discovery status | Confirmed | |
| Other designations | ||
Alpha Centauri Cb, Proxima b, GL 551 b, HIP 70890 b
| ||
| Database references | ||
| Extrasolar Planets Encyclopaedia | data | |
| SIMBAD | data | |
| Exoplanet Archive | data | |
| Open Exoplanet Catalogue | data | |
Proxima Centauri b (also
called Proxima b[8][9]) is an exoplanet orbiting within the habitable zone of
the red dwarf star Proxima Centauri, the closest star to the Sun.[10][11] It is
located about 4.2 light-years (1.3 parsecs, 40 trillion km, or 25 trillion
miles) from Earth in the constellation of Centaurus, making it the closest
known exoplanet to the Solar System. It is unlikely to be habitable, as the
planet is subject to stellar wind pressures of more than 2,000 times those
experienced by Earth from the solar wind. More information about the planet's
physical characteristics is needed for a proper evaluation.[12][13][14]
In August 2016, the European
Southern Observatory announced the discovery of the planet.[1][10][15][16][17]
The planet was found using the radial velocity method, where periodic Doppler
shifts of spectral lines of the host star suggest an orbiting object. From
these readings, the radial velocity of the parent star relative to the Earth is
varying with an amplitude of about 2 metres (7 feet) per second.[1]
Researchers think that its
proximity to Earth offers an opportunity for robotic exploration of the planet
with the Starshot project[10][11] or, at least, "in the coming
centuries".[11]
Characteristics
Mass, radius and temperature
The apparent inclination of
Proxima Centauri b's orbit has not yet been measured. The minimum mass of
Proxima b is 1.27 M⊕, which would be the actual
mass if its orbit were seen edge on from the Earth, producing the maximum
Doppler shift.[1] Once its orbital inclination is known, the mass will be
calculable. More tilted orientations imply a higher mass, with 90% of possible
orientations implying a mass below 3 M⊕.[18] The planet's
exact radius is unknown. If it has a rocky composition and a density equal to
that of the Earth, then its radius is at least 1.1 R⊕. It could be larger if it has a lower density than the Earth, or a mass
higher than the minimum mass.[7] The planet has an equilibrium temperature of
234 K (−39 °C; −38 °F).[1]
Host star
The planet orbits a (M-type)
red dwarf star named Proxima Centauri. The star has a mass of 0.12 M☉ and a radius of
0.14 R☉.[1] It has a surface temperature of 3042 K [3] and is 4.85 billion years
old.[19] In comparison, the Sun is 4.6 billion years old [20] and has a surface
temperature of 5778 K.[21] Proxima Centauri rotates once roughly every 83
days,[22] and has a luminosity about 0.0015 L☉.[1] Like the two larger
stars in the triple star system, Proxima Centauri is rich in metals, relative
to the Sun, something not normally found in low-mass stars like Proxima. Its
metallicity ([Fe/H]) is 0.21, or 1.62 times the amount found in the Sun's
atmosphere.[5][note 1]Even though Proxima Centauri is the closest star to the
Sun, it is not visible to the unaided eye from Earth because of its low
luminosity (apparent magnitude of 11.13[23]).Proxima Centauri is a flare star
that undergoes occasional dramatic increases in brightness and high-energy
emissions because of magnetic activity[24] that would create large solar
storms, possibly irradiating the surface of the exoplanet if it does not
possess a strong magnetic field or a protective atmosphere.
Orbit
Proxima Centauri b orbits
its host star every 11.186 days at a semi-major axis distance of approximately
0.05 astronomical units (7,000,000 km; 5,000,000 mi), which means the distance
from the exoplanet to its host star is one-twentieth of the distance from the
Earth to its own host star, the Sun.[1] Comparatively, Mercury, the closest
planet to the Sun, has a semi-major axis distance of 0.39 AU. Proxima Centauri
b receives about 65% of the amount of radiative flux from its host star that
the Earth receives from the Sun. Most of the radiative flux from Proxima
Centauri is in the infrared spectrum. In the visible spectrum, the exoplanet
only receives 2% of the light Earth does, so it would never get brighter than
twilight anywhere on Proxima Centauri b's surface.[note 2] However, because of
its tight orbit, Proxima Centauri b receives about 400 times more X-ray
radiation than the Earth does.[1]
Habitability
Artist's conception of
Proxima Centauri b, with Proxima Centauri and the Alpha Centauri binary system
in the background
It is unlikely that Proxima
Centauri b is habitable, as the planet is subject to stellar wind pressures of
more than 2,000 times those experienced by Earth from the solar wind.[12][25]
This radiation and the stellar winds would likely blow any atmosphere away,
leaving the undersurface as the only vaguely habitable location on that
planet.[26] But the habitability of Proxima Centauri b has not been
established.[12][13] Depending on the volatile reservoirs and the rotation rate
of the planet, 3D global climate models and theoretical arguments can be
contemplated.
The exoplanet is orbiting
within the habitable zone of Proxima Centauri, the region where, with the
correct planetary conditions and atmospheric properties, liquid water may exist
on the surface of the planet. The red dwarf host star, with about an eighth of
the mass of the Sun, has a habitable zone between ∼0.0423–0.0816 AU.[1]
Even though Proxima Centauri
b is in the habitable zone, the planet's habitability has been questioned because
of several potentially hazardous physical conditions. The exoplanet is close
enough to its host star that it might be tidally locked.[27][28] If the
planet's orbital eccentricity is 0, this could result in synchronous rotation,
with one blazing hot side permanently facing towards the star, while the
opposite side is in permanent darkness and freezing cold.[29][30] In this case,
scientists think that any habitable areas on the planet, if they exist, would
be confined to the border region between the two extreme sides, generally
referred to as the terminator line. Only here, temperatures might be suitable
for liquid water to exist.[28] However, Proxima Centauri b's orbital
eccentricity is not known with certainty, only that it is below 0.35[31] – potentially
high enough for it to have a significant chance of being captured into a 3:2
spin-orbit resonance similar to that of Mercury, where a Proxima b day would be
roughly equivalent to that of 7.5 Earth days.[14][32][33] Resonances as high as
2:1 are possible and an initial inclination of the planetary orbit to the plane
of the ecliptic could contribute to this.[14][33]
The European Southern
Observatory estimates that if water and an atmosphere are present, a far more
clement environment would result from such a configuration. In a world
including oceans, with average temperatures similar to those on Earth, assuming
an atmospheric N2 pressure of 1 bar and ∼0.01 bar of CO2, a
wide equatorial belt (non-synchronous rotation), or the majority of the sunlit
side (synchronous rotation), would be permanently ice-free.[31][33] A large
portion of the planet may be habitable if it has an atmosphere thick enough to
transfer heat to the side facing away from the star.[28] If it has an
atmosphere, simulations suggest that the planet could have lost about as much
as the amount of water that Earth has due to the early irradiation in the first
100–200 million years after the planet's formation. Liquid water may be present
only in the sunniest regions of the planet's surface in pools either in an area
in the hemisphere of the planet facing the star or diurnally in the equatorial
belt (3:2 resonance rotation).[14][33] All in all, astrophysicists consider the
ability of Proxima Centauri b to retain water from its formation as the most
crucial point in evaluating the planet's present habitability.[34] The planet
may be within reach of telescopes and techniques that could reveal more about
its composition and atmosphere, if it has any.[12]
In October 2016, researchers
at France's CNRS research institute stated that there is a considerable chance
of the planet harboring surface oceans and having a thin atmosphere.[35]
However, unless the planet transits, it is difficult to confirm these
hypotheses.
Formation
It seems implausible that
Proxima Centauri b originally formed in its current orbit since disk models for
small stars like Proxima Centauri would contain less than one M⊕ within the central one AU. This implies that Proxima Centauri b was either
formed elsewhere in a way still to be determined or that the current disk
models for stellar formation have to be revised.[1]
Discovery
The first indications of the
exoplanet were found in 2013 by Mikko Tuomi of the University of Hertfordshire
from archival observation data.[22][36] To confirm the possible discovery, the
European Southern Observatory launched the Pale Red Dot[note 3] project in
January 2016.[37] On 24 August 2016 the team of 31 scientists from all around
the world,[38] led by Guillem Anglada-Escudé of Queen Mary University of
London, confirmed the existence of Proxima Centauri b[19] through a
peer-reviewed article published by Nature.[1][27] The measurements were done
using two spectrographs, HARPS on the ESO 3.6 m Telescope at La Silla
Observatory and UVES on the 8-metre Very Large Telescope.[1] The peak radial
velocity of the host star combined with the orbital period allowed for the
minimum mass of the exoplanet to be calculated. The odds of a false positive
detection is less than one in ten million.[22]
Observational complications
of the system still leave theoretical room for additional large planets to
orbit Proxima Centauri. Calculations suggest that another super-Earth planet
around the star cannot be ruled out and that its presence would not destabilize
the orbit of Proxima Centauri b.[1] A second signal in the range of 60 to 500
days was also detected, but its nature is still unclear due to stellar
activity.[1]
Observations
A team of scientists think
they can image Proxima Centauri b and probe the planet's atmosphere for signs
of oxygen, water vapor and methane, combining ESPRESSO and SPHERE on the
VLT.[39] The JWST may be able to characterize the atmosphere of Proxima
Centauri b[40] and there is no conclusive evidence for transits combining MOST
and HATSouth photometry giving it less than a 1 percent chance of being a
transiting planet.[41] The planet might be in the reach of telescopes and other
techniques.[12] Although no current technology allows a detailed observation of
Proxima b, the potential habitability of the planet prompts the development of
mechanisms and techniques to achieve new discoveries. Future telescopes (the
European Extremely Large Telescope, the Giant Magellan Telescope, and the
Thirty Meter Telescope) could have the capability to characterize Proxima
Centauri b.
Exploration
The discovery of Proxima b
was significant to Breakthrough Starshot, a recent project aiming to send a
fleet of miniature probes to the Alpha Centauri system. The project, led by
Breakthrough Initiatives, a research company funded by Russian entrepreneur
Yuri Milner, plans to develop and launch a fleet of miniature unmanned
spacecraft called StarChips,[42] which could travel at up to 20% of the speed
of light,[43][44][45][46] arriving at the system in 20 years with notification reaching
Earth 4 years later.[10]
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