Proxima Centauri

Proxima Centauri is a small, low-mass

Proxima Centauri has two known exoplanets and one candidate exoplanet: Proxima Centauri b, Proxima Centauri d and the disputed Proxima Centauri c.^{[nb 3]} Proxima Centauri b orbits the star at a distance of roughly 0.05 AU (7.5 million km) with an orbital period of approximately 11.2 Earth days. Its estimated mass is at least 1.07 times that of Earth.^[16] Proxima b orbits within Proxima Centauri's habitable zone—the range where temperatures are right for liquid water to exist on its surface—but, because Proxima Centauri is a red dwarf and a flare star, the planet's habitability is highly uncertain. A candidate super-Earth, Proxima Centauri c, roughly 1.5 AU (220 million km) away from Proxima Centauri, orbits it every 1,900 d (5.2 yr).^[17][18] A sub-Earth, Proxima Centauri d, roughly 0.029 AU (4.3 million km) away, orbits it every 5.1 days.^[16]

General characteristics

A 1998 study of photometric variations indicates that Proxima Centauri completes a full rotation once every 83.5 days.^[28] A subsequent time series analysis of chromospheric indicators in 2002 suggests a longer rotation period of 116.6±0.7 days.^[29] Later observations of the star's magnetic field subsequently revealed that the star rotates with a period of 89.8±4 days, consistent with a measurement of 92.1+4.2
−3.5 days from radial velocity observations.^[12][30]

Structure and fusion

Because of its low mass, the interior of the star is completely

Proxima Centauri has a relatively weak stellar wind, no more than 20% of the mass loss rate of the solar wind. Because the star is much smaller than the Sun, the mass loss per unit surface area from Proxima Centauri may be eight times that from the solar surface.^[43]

Life phases

A red dwarf with the mass of Proxima Centauri will remain on the main sequence for about four trillion years. As the proportion of helium increases because of hydrogen fusion, the star will become smaller and hotter, gradually transforming into a so-called "blue dwarf". Near the end of this period it will become significantly more luminous, reaching 2.5% of the Sun's luminosity (L_☉) and warming up any orbiting bodies for a period of several billion years. When the hydrogen fuel is exhausted, Proxima Centauri will then evolve into a helium white dwarf (without passing through the red giant phase) and steadily lose any remaining heat energy.^[32][44]

The Alpha Centauri system may form naturally through a low-mass star being dynamically captured by a more massive binary of 1.5–2 M_☉ within their embedded star cluster before the cluster disperses.^[45] However, more accurate measurements of the radial velocity are needed to confirm this hypothesis.^[46] If Proxima Centauri was bound to the Alpha Centauri system during its formation, the stars are likely to share the same elemental composition. The gravitational influence of Proxima might have stirred up the Alpha Centauri protoplanetary disks. This would have increased the delivery of volatiles such as water to the dry inner regions, so possibly enriching any terrestrial planets in the system with this material.^[46]

Alternatively, Proxima Centauri may have been captured at a later date during an encounter, resulting in a highly eccentric orbit that was then stabilized by the galactic tide and additional stellar encounters. Such a scenario may mean that Proxima Centauri's planetary companions have had a much lower chance for orbital disruption by Alpha Centauri.^[11] As the members of the Alpha Centauri pair continue to evolve and lose mass, Proxima Centauri is predicted to become unbound from the system in around 3.5 billion years from the present. Thereafter, the star will steadily diverge from the pair.^[47]

Motion and location

Based on a parallax of 768.0665±0.0499 mas, published in 2020 in

Among the known stars, Proxima Centauri has been the closest star to the Sun for about 32,000 years and will be so for about another 25,000 years, after which Alpha Centauri A and Alpha Centauri B will alternate approximately every 79.91 years as the closest star to the Sun. In 2001, J. García-Sánchez et al. predicted that Proxima Centauri will make its closest approach to the Sun in approximately 26,700 years, coming within 3.11 ly (0.95 pc).[54] A 2010 study by V. V. Bobylev predicted a closest approach distance of 2.90 ly (0.89 pc) in about 27,400 years,^[55] followed by a 2014 study by C. A. L. Bailer-Jones predicting a perihelion approach of 3.07 ly (0.94 pc) in roughly 26,710 years.^[56] Proxima Centauri is orbiting through the Milky Way at a distance from the Galactic Centre that varies from 27 to 31 kly (8.3 to 9.5 kpc), with an orbital eccentricity of 0.07.^[57]

Alpha Centauri

Proxima Centauri has been suspected to be a companion of the Alpha Centauri

As of 2022, three planets (two confirmed and one candidate) have been detected in orbit around Proxima Centauri, with one being among the lightest ever detected by radial velocity ("d"), one close to Earth's size within the

Searches for exoplanets around Proxima Centauri date back to the late 1970s. In the 1990s, multiple measurements of Proxima Centauri's radial velocity constrained the maximum mass that a detectable companion could possess.

Proxima Centauri b, or Alpha Centauri Cb, orbits the star at a distance of roughly 0.05 AU (7.5 million km) with an orbital period of approximately 11.2 Earth days. Its estimated mass is at least 1.17 times that of the Earth.^[72] Moreover, the equilibrium temperature of Proxima Centauri b is estimated to be within the range where water could exist as liquid on its surface; thus, placing it within the habitable zone of Proxima Centauri.^[59][73][74]

The first indications of the

In 2016, in a paper that helped to confirm Proxima Centauri b's existence, a second signal in the range of 60 to 500 days was detected. However, stellar activity and inadequate sampling causes its nature to remain unclear.[59]

Planet c

Proxima Centauri c is a candidate

In 2019, a team of astronomers revisited the data from ESPRESSO about Proxima Centauri b to refine its mass. While doing so, the team found another radial velocity spike with a periodicity of 5.15 days. They estimated that if it were a planetary companion, it would be no less than 0.29 Earth masses.^[62] Further analysis confirmed the signal's existence leading up the discovery's announcement in February 2022.^[16]

Habitability

Prior to the discovery of Proxima Centauri b, the TV documentary

Proxima Centauri's flare outbursts could erode the atmosphere of any planet in its habitable zone, but the documentary's scientists thought that this obstacle could be overcome. Gibor Basri of the University of California, Berkeley argued: "No one [has] found any showstoppers to habitability." For example, one concern was that the torrents of charged particles from the star's flares could strip the atmosphere off any nearby planet. If the planet had a strong magnetic field, the field would deflect the particles from the atmosphere; even the slow rotation of a tidally locked planet that spins once for every time it orbits its star would be enough to generate a magnetic field, as long as part of the planet's interior remained molten.^[88]

Other scientists, especially proponents of the

Harold L. Alden in 1928, who confirmed Innes's view that it is closer, with a parallax of 0.783″±0.005″.^[94][96]

A size estimate for Proxima Centauri was obtained by the Canadian astronomer John Stanley Plaskett in 1925 using interferometry. The result was 207,000 miles (333,000 km), or approximately 0.24 R_☉.^[99]

In 1951, American astronomer Harlow Shapley announced that Proxima Centauri is a flare star. Examination of past photographic records showed that the star displayed a measurable increase in magnitude on about 8% of the images, making it the most active flare star then known.^[100][101] The proximity of the star allows for detailed observation of its flare activity. In 1980, the Einstein Observatory produced a detailed X-ray energy curve of a stellar flare on Proxima Centauri. Further observations of flare activity were made with the EXOSAT and ROSAT satellites, and the X-ray emissions of smaller, solar-like flares were observed by the Japanese ASCA satellite in 1995.^[102] Proxima Centauri has since been the subject of study by most X-ray observatories, including XMM-Newton and Chandra.^[35]

Because of Proxima Centauri's southern declination, it can only be viewed south of

apparent visual magnitude 11, so a telescope with an aperture of at least 8 cm (3.1 in) is needed to observe it, even under ideal viewing conditions—under clear, dark skies with Proxima Centauri well above the horizon.^[105] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars.^[106] The WGSN approved the name Proxima Centauri for this star on August 21, 2016, and it is now so included in the List of IAU approved Star Names.^[107]

In 2016, a superflare was observed from Proxima Centauri, the strongest flare ever seen. The optical brightness increased by a factor of 68× to approximately magnitude 6.8. It is estimated that similar flares occur around five times every year but are of such short duration, just a few minutes, that they have never been observed before.^[19] On 2020 April 22 and 23, the New Horizons spacecraft took images of two of the nearest stars, Proxima Centauri and Wolf 359. When compared with Earth-based images, a very large parallax effect was easily visible. However, this was only used for illustrative purposes and did not improve on previous distance measurements.^[108][109]

Future exploration

Main articles:

Proxima Centauri in fiction and Interstellar travel

Because of the star's proximity to Earth, Proxima Centauri has been proposed as a flyby destination for interstellar travel.[110] If non-nuclear, conventional propulsion technologies are used, the flight of a spacecraft to Proxima Centauri and its planets would probably require thousands of years.^[111] For example, Voyager 1, which is now travelling 17 km/s (38,000 mph)^[112] relative to the Sun, would reach Proxima Centauri in 73,775 years, were the spacecraft travelling in the direction of that star and Proxima was standing still. Proxima's actual galactic orbit means a slow-moving probe would have only several tens of thousands of years to catch the star at its closest approach, before it recedes out of reach.^[113]

swing-by's around Proxima Centauri or Alpha Centauri are to be employed.^[115] Then the probes would take photos and collect data of the planets of the stars, and their atmospheric compositions. It would take 4.25 years for the information collected to be sent back to Earth.^[116]

Explanatory notes

1. ^ From knowing the absolute visual magnitude of Proxima Centauri, ${\displaystyle \scriptstyle M_{V_{\ast }}=15.6}$, and the absolute visual magnitude of the Sun, ${\displaystyle \scriptstyle M_{V_{\odot }}=4.83}$, the visual luminosity of Proxima Centauri can therefore be calculated: ${\displaystyle \scriptstyle {\frac {L_{V_{\ast }}}{L_{V_{\odot }}}}=10^{0.4\left(M_{V_{\odot }}-M_{V_{\ast }}\right)}=4.92\times 10^{-5}}$
2. ^ If Proxima Centauri was a later capture into the Alpha Centauri star system then its metallicity and age could be quite different to that of Alpha Centauri A and B. Through comparing Proxima Centauri to other similar stars it was estimated that it had a lower metallicity, ranging from less than a third, to about the same, of our Sun's.^[10][11]
3. ^ Extrasolar planet names are designated following the International Astronomical Union's naming conventions in alphabetical order according to their respective dates of discovery, with 'Proxima Centauri a' being the star itself.
4. ^ The density (ρ) is given by the mass divided by the volume. Relative to the Sun, therefore, the density is:

   ${\displaystyle \rho }$	= ${\displaystyle {\begin{smallmatrix}{\frac {M}{M_{\odot }}}\cdot \left({\frac {R}{R_{\odot }}}\right)^{-3}\cdot \rho _{\odot }\end{smallmatrix}}}$
   	= 0.122 · 0.154−3 · (1.41 × 103 kg/m3)
   	= 33.4 · (1.41 × 103 kg/m3)
   	= 4.71 × 104 kg/m3

   where ${\displaystyle {\begin{smallmatrix}\rho _{\odot }\end{smallmatrix}}}$ is the average solar density. See:

   • Munsell, Kirk; Smith, Harman; Davis, Phil; Harvey, Samantha (11 June 2008). "Sun: facts & figures". Solar system exploration. NASA. Archived from the original on 2 January 2008. Retrieved 12 July 2008.
   • Bergman, Marcel W.; Clark, T. Alan; Wilson, William J. F. (2007). Observing projects using Starry Night Enthusiast (8th ed.). Macmillan. pp. 220–221.
     ISBN 978-1-4292-0074-5
     .
5. ^ The standard surface gravity on the Earth is 980.665 cm/s², for a 'log g' value of 2.992. The difference in logarithms is 5.20 − 2.99 = 2.21, yielding a multiplier of 10^2.21 = 162. For the Earth's gravity, see:
   • Taylor, Barry N., ed. (2001). The International System of Units (SI) (PDF). United States Department of Commerce: National Institute of Standards and Technology. p. 29. Retrieved 8 March 2012. {{cite book}}: |work= ignored (help)
6. ^ The coordinates of the Sun would be diametrically opposite Proxima Centauri, at α=02^h 29^m 42.9487^s, δ=+62° 40′ 46.141″. The absolute magnitude M_v of the Sun is 4.83, so at a parallax π of 0.77199 the apparent magnitude m is given by 4.83 − 5(log₁₀(0.77199) + 1) = 0.40. See: Tayler, Roger John (1994). The Stars: Their Structure and Evolution. Cambridge University Press. p. 16.
   ISBN 978-0-521-45885-6
   .
7. ^ Pale Red Dot is a reference to Pale Blue Dot, a distant photo of Earth taken by Voyager 1.
8. ^ For a star south of the zenith, the angle to the zenith is equal to the Latitude minus the Declination. The star is hidden from sight when the zenith angle is 90° or more, i.e., below the horizon. Thus, for Proxima Centauri:

   Highest latitude = 90° + (−62.68°) = 27.32°.

   See: Campbell, William Wallace (1899). The elements of practical astronomy. London: Macmillan. pp. 109–110. Retrieved 12 August 2008.

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Further reading

• Marcy, Geoffrey W.; et al. (January 2022). "Laser communication with Proxima and Alpha Centauri using the solar gravitational lens". Monthly Notices of the Royal Astronomical Society. 509 (3): 3798–3814.
  doi:10.1093/mnras/stab3074
  .
• Smith, Shane; et al. (October 2021). "A radio technosignature search towards Proxima Centauri resulting in a signal of interest". Nature Astronomy. 5 (11): 1148–1152.
  S2CID 239948037
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• Marcy, G. W. (August 2021). "A search for optical laser emission from Proxima Centauri". Monthly Notices of the Royal Astronomical Society. 505 (3): 3537–3548.
  doi:10.1093/mnras/stab1440
  .
• Kavanagh, Robert D.; et al. (June 2021). "Planet-induced radio emission from the coronae of M dwarfs: the case of Prox Cen and AU Mic". Monthly Notices of the Royal Astronomical Society. 504 (1): 1511–1518.
  doi:10.1093/mnras/stab929
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• Pérez-Torres, M.; et al. (January 2021). "Monitoring the radio emission of Proxima Centauri". Astronomy & Astrophysics. 645: A77.
  S2CID 227255606
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• Zic, Andrew; et al. (December 2020). "A Flare-type IV Burst Event from Proxima Centauri and Implications for Space Weather". The Astrophysical Journal. 905 (1): 23.
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• Lalitha, S.; et al. (November 2020). "Proxima Centauri - the nearest planet host observed simultaneously with AstroSat, Chandra, and HST". Monthly Notices of the Royal Astronomical Society. 498 (3): 3658–3663.
  doi:10.1093/mnras/staa2574
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• Vida, Krisztián; et al. (October 2019). "Flaring Activity of Proxima Centauri from TESS Observations: Quasiperiodic Oscillations during Flare Decay and Inferences on the Habitability of Proxima b". The Astrophysical Journal. 884 (2): 160.
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• Banik, Indranil; Kroupa, Pavel (August 2019). "Directly testing gravity with Proxima Centauri". Monthly Notices of the Royal Astronomical Society. 487 (2): 1653–1661.
  doi:10.1093/mnras/stz1379
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• Pavlenko, Ya. V.; et al. (June 2019). "Temporal changes of the flare activity of Proxima Centauri". Astronomy & Astrophysics. 626: A111.
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• Feliz, Dax L.; et al. (June 2019). "A Multi-year Search for Transits of Proxima Centauri. II. No Evidence for Transit Events with Periods between 1 and 30 days". The Astronomical Journal. 157 (6): 226.
  doi:10.3847/1538-3881/ab184f
  . 226.
• Kielkopf, John F.; et al. (June 2019). "Observation of a possible superflare on Proxima Centauri". Monthly Notices of the Royal Astronomical Society: Letters. 486 (1): L31–L35.
  doi:10.1093/mnrasl/slz054
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• Meng, Tong; et al. (January 2019). "Dynamical evolution and stability maps of the Proxima Centauri system". Monthly Notices of the Royal Astronomical Society. 482 (1): 372–383.
  doi:10.1093/mnras/sty2682
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• Schwarz, R.; et al. (November 2018). "Exocomets in the Proxima Centauri system and their importance for water transport". Monthly Notices of the Royal Astronomical Society. 480 (3): 3595–3608.
  doi:10.1093/mnras/sty2064
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• Howard, Ward S.; et al. (June 2018). "The First Naked-eye Superflare Detected from Proxima Centauri". The Astrophysical Journal Letters. 860 (2): L30.
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• MacGregor, Meredith A.; et al. (March 2018). "Detection of a Millimeter Flare from Proxima Centauri". The Astrophysical Journal Letters. 855 (1): L2.
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  . L2.
• Damasso, M.; Del Sordo, F. (March 2017). "Proxima Centauri reloaded: Unravelling the stellar noise in radial velocities". Astronomy & Astrophysics. 599: A126.
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External links

Wikimedia Commons has media related to Proxima Centauri.

• Nemiroff, R.; Bonnell, J., eds. (15 July 2002). "Proxima Centauri: the closest star". Astronomy Picture of the Day. NASA. Retrieved 25 June 2008.
• "Proxima Centauri: The Nearest Star to the Sun". Harvard University. Chandra X-ray Observatory. 1 July 2008. Retrieved 1 July 2008.
• James, Andrew (11 March 2008). "Voyage to Alpha Centauri". The Imperial Star – Alpha Centauri. Southern Astronomical Delights. Retrieved 5 August 2008.
• "Alpha Centauri 3". SolStation. Retrieved 5 August 2008.
• "O Sistema Alpha Centauri". Astronomia & Astrofísica (in Portuguese). Archived from the original on 3 March 2016. Retrieved 25 June 2008.
• "Image of Proxima Centauri". Wikisky. Retrieved 1 July 2017.
• "Proxima Centauri". Constellation-guide.com. Retrieved 25 August 2016.
• "Planet Found in Habitable Zone Around Nearest Star". European Southern Observatory. 24 August 2016. Retrieved 6 September 2016.
• Wall, Mike (24 April 2016). "Found! Potentially Earth-Like Planet at Proxima Centauri Is Closest Ever". Space.com.