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Journal articles on the topic 'Proxima Centauri b'

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Published: 4 June 2021

Last updated: 1 February 2022

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1

Ritchie, Raymond J., Anthony W. D. Larkum, and Ignasi Ribas. "Could photosynthesis function on Proxima Centauri b?" International Journal of Astrobiology 17, no. 2 (July 18, 2017): 147–76. http://dx.doi.org/10.1017/s1473550417000167.

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AbstractCould oxygenic and/or anoxygenic photosynthesis exist on planet Proxima Centauri b? Proxima Centauri (spectral type – M5.5 V, 3050 K) is a red dwarf, whereas the Sun is type G2 V (5780 K). The light regimes on Earth and Proxima Centauri b are compared with estimates of the planet's suitability for Chlorophylla(Chla) and Chld-based oxygenic photosynthesis and for bacteriochlorophyll (BChl)-based anoxygenic photosynthesis. Proxima Centauri b has low irradiance in the oxygenic photosynthesis range (400–749 nm: 64–132 µmol quanta m−2s−1). Much larger amounts of light would be available for BChl-based anoxygenic photosynthesis (350–1100 nm: 724–1538 µmol quanta m−2s−1). We estimated primary production under these light regimes. We used the oxygenic algaeSynechocystisPCC6803,Prochlorothrix hollandica,Acaryochloris marina,Chlorella vulgaris,Rhodomonassp. andPhaeodactylum tricornutumand the anoxygenic photosynthetic bacteriaRhodopseudomonas palustris(BChla),Afifella marina(BChla),Thermochromatium tepidum(BChla),Chlorobaculum tepidum(BChla + c) andBlastochloris viridis(BChlb) as representative photosynthetic organisms. Proxima Centauri b has only ≈3% of the PAR (400–700 nm) of Earth irradiance, but we found that potential gross photosynthesis (Pg) on Proxima Centauri b could be surprisingly high (oxygenic photosynthesis: earth ≈0.8 gC m−2h−1; Proxima Centauri b ≈0.14 gC m−2h−1). The proportion of PAR irradiance useable by oxygenic photosynthetic organisms (the sum of Blue + Red irradiance) is similar for the Earth and Proxima Centauri b. The oxygenic photic zone would be only ≈10 m deep in water compared with ≈200 m on Earth. ThePgof an anoxic Earth (gC m−2h−1) is ≈0.34–0.59 (land) and could be as high as ≈0.29–0.44 on Proxima Centauri b. 1 m of water does not affect oxygenic or anoxygenic photosynthesis on Earth, but on Proxima Centauri b oxygenicPgis reduced by ≈50%. Effective elimination of near IR limitsPgby photosynthetic bacteria (<10% of the surface value). The spectrum of Proxima Centauri b is unfavourable for anoxygenic aquatic photosynthesis. Nevertheless, a substantial aerobic or anaerobic ecology is possible on Proxima Centauri b. Protocols to recognize the biogenic signature of anoxygenic photosynthesis are needed.

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2

Jenkins, James S., Joseph Harrington, Ryan C. Challener, Nicolás T. Kurtovic, Ricardo Ramirez, Jose Peña, Kathleen J. McIntyre, et al. "Proxima Centauri b is not a transiting exoplanet." Monthly Notices of the Royal Astronomical Society 487, no. 1 (May 11, 2019): 268–74. http://dx.doi.org/10.1093/mnras/stz1268.

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Abstract We report Spitzer Space Telescope observations during predicted transits of the exoplanet Proxima Centauri b. As the nearest terrestrial habitable-zone planet we will ever discover, any potential transit of Proxima b would place strong constraints on its radius, bulk density, and atmosphere. Subsequent transmission spectroscopy and secondary-eclipse measurements could then probe the atmospheric chemistry, physical processes, and orbit, including a search for biosignatures. However, our photometric results rule out planetary transits at the 200 ppm level at 4.5 $\mu$m, yielding a 3σ upper radius limit of 0.4 R⊕ (Earth radii). Previous claims of possible transits from optical ground- and space-based photometry were likely correlated noise in the data from Proxima Centauri’s frequent flaring. Our study indicates dramatically reduced stellar activity at near-to-mid infrared wavelengths, compared to the optical. Proxima b is an ideal target for space-based infrared telescopes, if their instruments can be configured to handle Proxima’s brightness.

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3

Sadovski, A. M., A. B. Struminsky, and A. Belov. "Cosmic Rays near Proxima Centauri b." Astronomy Letters 44, no. 5 (May 2018): 324–30. http://dx.doi.org/10.1134/s1063773718040072.

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4

Ribas, Ignasi, Emeline Bolmont, Franck Selsis, Ansgar Reiners, Jérémy Leconte, Sean N. Raymond, Scott G. Engle, et al. "The habitability of Proxima Centauri b." Astronomy & Astrophysics 596 (December 2016): A111. http://dx.doi.org/10.1051/0004-6361/201629576.

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5

Turbet, Martin, Jérémy Leconte, Franck Selsis, Emeline Bolmont, François Forget, Ignasi Ribas, Sean N. Raymond, and Guillem Anglada-Escudé. "The habitability of Proxima Centauri b." Astronomy & Astrophysics 596 (December 2016): A112. http://dx.doi.org/10.1051/0004-6361/201629577.

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6

Zuluaga, Jorge I., and Sebastian Bustamante. "Magnetic properties of Proxima Centauri b analogues." Planetary and Space Science 152 (March 2018): 55–67. http://dx.doi.org/10.1016/j.pss.2018.01.006.

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7

Garraffo, C., J. J. Drake, and O. Cohen. "THE SPACE WEATHER OF PROXIMA CENTAURI b." Astrophysical Journal 833, no. 1 (November 30, 2016): L4. http://dx.doi.org/10.3847/2041-8205/833/1/l4.

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8

Banik, Indranil, and Pavel Kroupa. "Directly testing gravity with Proxima Centauri." Monthly Notices of the Royal Astronomical Society 487, no. 2 (May 17, 2019): 1653–61. http://dx.doi.org/10.1093/mnras/stz1379.

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ABSTRACT The wide binary orbit of Proxima Centauri around α Centauri A and B differs significantly between Newtonian and Milgromian dynamics (MOND). By combining previous calculations of this effect with mock observations generated using a Monte Carlo procedure, we show that this prediction can be tested using high precision astrometry of Proxima Centauri. This requires ≈10 yr of observations at an individual epoch precision of $0.5 \, \mu\rm as$, within the design specifications of the proposed Theia mission. In general, the required duration should scale as the 2/5 power of the astrometric precision. A long-period planet could produce a MOND-like astrometric signal, but only if it has a particular ratio of mass to separation squared and a sky position close to the line segment connecting Proxima Centauri with α Centauri. Uncertainties in perspective effects should be small enough for this test if the absolute radial velocity of Proxima Centauri can be measured to within ≈10 m s−1, better than the present accuracy of 32 m s−1. We expect the required improvement to become feasible using radial velocity zero-points estimated from larger samples of close binaries, with the Sun providing an anchor. We demonstrate that possible astrometric microlensing of Proxima Centauri is unlikely to affect the results. We also discuss why it should be possible to find sufficiently astrometrically stable reference stars. Adequately, addressing these and other issues would enable a decisive test of gravity in the currently little explored low acceleration regime relevant to the dynamical discrepancies in galactic outskirts.

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9

Kane, Stephen R., Dawn M. Gelino, and Margaret C. Turnbull. "ON THE ORBITAL INCLINATION OF PROXIMA CENTAURI b." Astronomical Journal 153, no. 2 (January 9, 2017): 52. http://dx.doi.org/10.3847/1538-3881/153/2/52.

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10

Noack, L., K. G. Kislyakova, C. P. Johnstone, M. Güdel, and L. Fossati. "Interior heating and outgassing of Proxima Centauri b: Identifying critical parameters." Astronomy & Astrophysics 651 (July 2021): A103. http://dx.doi.org/10.1051/0004-6361/202040176.

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Context. Since the discovery of a potentially low-mass exoplanet around our nearest neighbour star Proxima Centauri, several works have investigated the likelihood of a shielding atmosphere and therefore the potential surface habitability of Proxima Cen b. However, outgassing processes are influenced by several different (unknown) factors such as the actual planet mass, mantle and core composition, and different heating mechanisms in the interior. Aims. We aim to identify the critical parameters that influence the mantle and surface evolution of the planet over time, as well as to potentially constrain the time-dependent input of volatiles from mantle into the atmosphere. Methods. To study the coupled star–planet evolution, we analysed the heating produced in the interior of Proxima Cen b due to induction heating, which strongly varies with both depth and latitude. We calculated different rotation evolutionary tracks for Proxima Centauri and investigated the change in its rotation period and magnetic field strength. Unlike the Sun, Proxima Centauri possesses a very strong magnetic field of at least a few hundred Gauss, which was likely even stronger in the past. We applied an interior structure model for varying planet masses (derived from the unknown inclination of observation of the Proxima Centauri system) and iron weight fractions, that is, different core sizes, in the range of observed Fe-Mg variations in the stellar spectrum. We used a mantle convection model to study the thermal evolution and outgassing efficiency of Proxima Cen b. For unknown planetary parameters such as initial conditions, we chose randomly selected values. We took heating in the interior due to variable radioactive heat sources and induction heating into account and compared the heating efficiency to tidal heating. Results. Our results show that induction heating may have been significant in the past, leading to local temperature increases of several hundreds of Kelvin. This early heating leads to an earlier depletion of the interior and volatile outgassing compared to if the planet had not been subject to induction heating. We show that induction heating has an impact comparable to tidal heating when assuming latest estimates on its eccentricity. Furthermore, we find that the planet mass (linked to the planetary orbital inclination) has a first-order influence on the efficiency of outgassing from the interior.

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11

Feng, F., and H. R. A. Jones. "Was Proxima captured by Alpha Centauri A and B?" Monthly Notices of the Royal Astronomical Society 473, no. 3 (October 5, 2017): 3185–89. http://dx.doi.org/10.1093/mnras/stx2576.

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12

Brugger, B., O. Mousis, M. Deleuil, and J. I. Lunine. "POSSIBLE INTERNAL STRUCTURES AND COMPOSITIONS OF PROXIMA CENTAURI b." Astrophysical Journal 831, no. 2 (November 3, 2016): L16. http://dx.doi.org/10.3847/2041-8205/831/2/l16.

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13

Kreidberg, Laura, and Abraham Loeb. "PROSPECTS FOR CHARACTERIZING THE ATMOSPHERE OF PROXIMA CENTAURI b." Astrophysical Journal 832, no. 1 (November 14, 2016): L12. http://dx.doi.org/10.3847/2041-8205/832/1/l12.

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14

Queloz, Didier, and Mejd Alsari. "Are we going to Alpha Centauri?" Scientific Video Protocols 1, no. 1 (April 12, 2020): 1–4. http://dx.doi.org/10.32386/scivpro.000019.

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Nobel Prize Laureate Didier Queloz talks about realistic ways to explore Proxima Centauri b and other potentially habitable planetary systems such as TRAPPIST-1 using technologies that are currently available. He also discusses his interdisciplinary research activities on abiogenesis and the search for life on other planets.

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15

Herath, Mahesh, Saraj Gunesekera, and Chandana Jayaratne. "Characterizing the possible interior structures of the nearby Exoplanets Proxima Centauri b and Ross-128 b." Monthly Notices of the Royal Astronomical Society 500, no. 1 (October 15, 2020): 333–54. http://dx.doi.org/10.1093/mnras/staa3110.

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ABSTRACT We developed a new numerical model to constrain the interior structure of rocky Exoplanets, and applied it to the nearby planets Proxima Centauri b and Ross-128 b. The recently measured elemental abundances of red dwarfs and Alpha Centauri were utilized to infer the bulk composition of each planet, and to measure their core mass fractions (CMFs). The results of our model predicted that the radius of Proxima b at its minimum mass may be 1.036 ± 0.040 R⊕, and if its mass is as high as 2 M⊕, 1.170 ± 0.040 R⊕. The radius of Ross-128 b at minimum mass may be 1.034 ± 0.040 R⊕, with its radius at an upper bound mass of 2 M⊕ being 1.150 ± 0.040 R⊕. Both planets may have thin mantles with similar conditions to Earth, but not convecting as vigorously. The CMFs might lie in the ranges of 20–59 per cent and 34–59 per cent for Proxima b and Ross-128 b, respectively, making it very likely they have massive iron cores. Their central temperatures may be high enough to partially melt the cores, and possibly generate magnetic fields. If they have magnetic fields at present, they are most likely to be multipolar in nature due to slow rotation speeds resulting from stellar tidal effects. The field strengths were predicted to have values of 0.06–0.23 G for Proxima b, and 0.07–0.14 G for Ross-128 b. If either planet contains more than 10 per cent of their mass in volatiles, magnetic fields would either be non-existent or very weak. The conditions of both planets may be hostile for habitability.

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16

Dong, Chuanfei, Manasvi Lingam, Yingjuan Ma, and Ofer Cohen. "Is Proxima Centauri b Habitable? A Study of Atmospheric Loss." Astrophysical Journal 837, no. 2 (March 10, 2017): L26. http://dx.doi.org/10.3847/2041-8213/aa6438.

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17

Galuzzo, D., F. Berrilli, and L. Giovannelli. "Proxima Centauri b: infrared detectability in presence of stellar activity." Journal of Physics: Conference Series 1548 (May 2020): 012012. http://dx.doi.org/10.1088/1742-6596/1548/1/012012.

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18

Garcia-Sage, K., A. Glocer, J. J. Drake, G. Gronoff, and O. Cohen. "On the Magnetic Protection of the Atmosphere of Proxima Centauri b." Astrophysical Journal 844, no. 1 (July 24, 2017): L13. http://dx.doi.org/10.3847/2041-8213/aa7eca.

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19

Meadows, Victoria S., Giada N. Arney, Edward W. Schwieterman, Jacob Lustig-Yaeger, Andrew P. Lincowski, Tyler Robinson, Shawn D. Domagal-Goldman, et al. "The Habitability of Proxima Centauri b: Environmental States and Observational Discriminants." Astrobiology 18, no. 2 (February 2018): 133–89. http://dx.doi.org/10.1089/ast.2016.1589.

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20

Del Genio, Anthony D., Michael J. Way, David S. Amundsen, Igor Aleinov, Maxwell Kelley, Nancy Y. Kiang, and Thomas L. Clune. "Habitable Climate Scenarios for Proxima Centauri b with a Dynamic Ocean." Astrobiology 19, no. 1 (January 2019): 99–125. http://dx.doi.org/10.1089/ast.2017.1760.

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21

Galuzzo, Daniele, Chiara Cagnazzo, Francesco Berrilli, Federico Fierli, and Luca Giovannelli. "Three-dimensional Climate Simulations for the Detectability of Proxima Centauri b." Astrophysical Journal 909, no. 2 (March 1, 2021): 191. http://dx.doi.org/10.3847/1538-4357/abdeb4.

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22

Klein, Baptiste, Jean-François Donati, Élodie M. Hébrard, Bonnie Zaire, Colin P. Folsom, Julien Morin, Xavier Delfosse, and Xavier Bonfils. "The large-scale magnetic field of Proxima Centauri near activity maximum." Monthly Notices of the Royal Astronomical Society 500, no. 2 (October 30, 2020): 1844–50. http://dx.doi.org/10.1093/mnras/staa3396.

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ABSTRACT We report the detection of a large-scale magnetic field at the surface of the slowly rotating fully convective (FC) M dwarf Proxima Centauri. 10 circular polarization spectra, collected from 2017 April to July with the HARPS-Pol spectropolarimeter, exhibit rotationally modulated Zeeman signatures suggesting a stellar rotation period of 89.8 ± 4.0 d. Using Zeeman–Doppler Imaging, we invert the circular polarization spectra into a surface distribution of the large-scale magnetic field. We find that Proxima Cen hosts a large-scale magnetic field of typical strength 200 G, whose topology is mainly poloidal, and moderately axisymmetric, featuring, in particular, a dipole component of 135 G tilted at 51° to the rotation axis. The large-scale magnetic flux is roughly 3× smaller than the flux measured from the Zeeman broadening of unpolarized lines, which suggests that the underlying dynamo is efficient at generating a magnetic field at the largest spatial scales. Our observations occur ∼1 yr after the maximum of the reported 7 yr-activity cycle of Proxima Cen, which opens the door for the first long-term study of how the large-scale field evolves with the magnetic cycle in an FC very low mass star. Finally, we find that Proxima Cen’s habitable zone planet, Proxima-b, is likely orbiting outside the Alfvèn surface, where no direct magnetic star–planet interactions occur.

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23

Salazar, Andrea M., Stephanie L. Olson, Thaddeus D. Komacek, Haynes Stephens, and Dorian S. Abbot. "The Effect of Substellar Continent Size on Ocean Dynamics of Proxima Centauri b." Astrophysical Journal 896, no. 1 (June 11, 2020): L16. http://dx.doi.org/10.3847/2041-8213/ab94c1.

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24

Morel, Thierry. "The chemical composition of α Centauri AB revisited." Astronomy & Astrophysics 615 (July 2018): A172. http://dx.doi.org/10.1051/0004-6361/201833125.

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The two solar-like stars α Cen A and B have long served as cornerstones for stellar physics in virtue of their immediate proximity, association in a visual binary, and masses that bracket that of the Sun. The recent detection of a terrestrial planet in the cool, suspected tertiary Proxima Cen now makes the system also of prime interest in the context of planetary studies. It is therefore of fundamental importance to tightly constrain the properties of the individual stellar components. We present a fully self-consistent, line-by-line differential abundance analysis of α Cen AB based on high-quality HARPS data. Various line lists are used and analysis strategies implemented to improve the reliability of the results. Abundances of 21 species with a typical precision of 0.02–0.03 dex are reported. We find that the chemical composition of the two stars is not scaled solar (e.g. Na and Ni excess, depletion of neutron-capture elements), but that their patterns are strikingly similar, with a mean abundance difference (A – B) with respect to hydrogen of –0.01 ± 0.04 dex. Much of the scatter may be ascribed to physical effects that are not fully removed through a differential analysis because of the mismatch in parameters between the two components. We derive an age for the system from abundance indicators (e.g. [Y/Mg] and [Y/Al]) that is slightly larger than solar and in agreement with most asteroseismic results. Assuming coeval formation for the three components belonging to the system, this implies an age of about ~6 Gyrs for the M dwarf hosting the terrestrial planet Proxima Cen b. After correction for Galactic chemical evolution effects, we find a trend between the abundance ratios and condensation temperature in α Cen A akin to that of the Sun. However, taking this finding as evidence for the sequestration of rocky material locked up in planets may be premature given that a clear link between the two phenomena remains to be established. The similarity between the abundance pattern of the binary components argues against the swallowing of a massive planet by one of the stars after the convective zones have shrunk to their present-day sizes.

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25

Luger, Rodrigo, Jacob Lustig-Yaeger, David P. Fleming, Matt A. Tilley, Eric Agol, Victoria S. Meadows, Russell Deitrick, and Rory Barnes. "The Pale Green Dot: A Method to Characterize Proxima Centauri b Using Exo-Aurorae." Astrophysical Journal 837, no. 1 (March 3, 2017): 63. http://dx.doi.org/10.3847/1538-4357/aa6040.

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26

Vida, Krisztián, Katalin Oláh, Zsolt Kővári, Lidia van Driel-Gesztelyi, Attila Moór, and András Pál. "Flaring Activity of Proxima Centauri from TESS Observations: Quasiperiodic Oscillations during Flare Decay and Inferences on the Habitability of Proxima b." Astrophysical Journal 884, no. 2 (October 22, 2019): 160. http://dx.doi.org/10.3847/1538-4357/ab41f5.

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27

Scheucher, M., K. Herbst, V. Schmidt, J. L. Grenfell, F. Schreier, S. Banjac, B. Heber, H. Rauer, and M. Sinnhuber. "Proxima Centauri b: A Strong Case for Including Cosmic-Ray-induced Chemistry in Atmospheric Biosignature Studies." Astrophysical Journal 893, no. 1 (April 8, 2020): 12. http://dx.doi.org/10.3847/1538-4357/ab7b74.

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28

Liu, Hui-Gen, Peng Jiang, Xingxing Huang, Zhou-Yi Yu, Ming Yang, Minghao Jia, Supachai Awiphan, et al. "Searching for the Transit of the Earth-mass Exoplanet Proxima Centauri b in Antarctica: Preliminary Result." Astronomical Journal 155, no. 1 (December 12, 2017): 12. http://dx.doi.org/10.3847/1538-3881/aa9b86.

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29

Yates, Jack S., Paul I. Palmer, James Manners, Ian Boutle, Krisztian Kohary, Nathan Mayne, and Luke Abraham. "Ozone chemistry on tidally locked M dwarf planets." Monthly Notices of the Royal Astronomical Society 492, no. 2 (January 8, 2020): 1691–705. http://dx.doi.org/10.1093/mnras/stz3520.

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ABSTRACT We use the Met Office Unified Model to explore the potential of a tidally locked M dwarf planet, nominally Proxima Centauri b irradiated by a quiescent version of its host star, to sustain an atmospheric ozone layer. We assume a slab ocean surface layer, and an Earth-like atmosphere of nitrogen and oxygen with trace amounts of ozone and water vapour. We describe ozone chemistry using the Chapman mechanism and the hydrogen oxide (HOx, describing the sum of OH and HO2) catalytic cycle. We find that Proxima Centauri radiates with sufficient UV energy to initialize the Chapman mechanism. The result is a thin but stable ozone layer that peaks at 0.75 parts per million at 25 km. The quasi-stationary distribution of atmospheric ozone is determined by photolysis driven by incoming stellar radiation and by atmospheric transport. Ozone mole fractions are smallest in the lowest 15 km of the atmosphere at the substellar point and largest in the nightside gyres. Above 15 km the ozone distribution is dominated by an equatorial jet stream that circumnavigates the planet. The nightside ozone distribution is dominated by two cyclonic Rossby gyres that result in localized ozone hotspots. On the dayside the atmospheric lifetime is determined by the HOx catalytic cycle and deposition to the surface, with nightside lifetimes due to chemistry much longer than time-scales associated with atmospheric transport. Surface UV values peak at the substellar point with values of 0.01 W m−2, shielded by the overlying atmospheric ozone layer but more importantly by water vapour clouds.

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30

Poveda, A., C. Allen, and M. A. Herrera. "Kinematics and Ages of UV Ceti Stars." International Astronomical Union Colloquium 151 (1995): 57–60. http://dx.doi.org/10.1017/s0252921100034576.

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AbstractThe kinematic properties of 93 UV Ceti stars of the solar neighborhood are studied, based on a list of flares within 25 pc of the Sun (π ≥ 0".04). With updated values for their distances, proper motions and radial velocities (Gliese & Jahreiss 1991) space velocity dispersions are calculated for these stars. It is found that the total velocity dispersion of the flare stars (σ=30±3 km s−1) is similar to that of the F5 V stars from the same catalogue, for which the conventionally estimated mean age is about 3 · 109 years. A number of flare stars are identified as members of the Hyades, Sirius or Pleiades groups. The velocity dispersions found for the nearby flare stars, as well as their scale height and the membership of some of them to young kinematic groups, indicate that they belong to the young disk population. A small number (7) of UV Ceti stars have kinematics corresponding to the thick disk or halo population. Their long-lived chromospheric activity is interpreted as due to coalescence of old contact binaries. The question of the age of Proxima Centauri is examined in the context of our results, and found to be compatible with the ages of a Centauri A and B.

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31

Zahnle, Kevin J., and David C. Catling. "The Cosmic Shoreline: The Evidence that Escape Determines which Planets Have Atmospheres, and what this May Mean for Proxima Centauri B." Astrophysical Journal 843, no. 2 (July 12, 2017): 122. http://dx.doi.org/10.3847/1538-4357/aa7846.

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32

Bonfils, X., N. Astudillo-Defru, R. Díaz, J. M. Almenara, T. Forveille, F. Bouchy, X. Delfosse, et al. "A temperate exo-Earth around a quiet M dwarf at 3.4 parsec." Astronomy & Astrophysics 613 (May 2018): A25. http://dx.doi.org/10.1051/0004-6361/201731973.

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The combination of high-contrast imaging and high-dispersion spectroscopy, which has successfully been use to detect the atmosphere of a giant planet, is one of the most promising potential probes of the atmosphere of Earth-size worlds. The forthcoming generation of extremely large telescopes (ELTs) may obtain sufficient contrast with this technique to detect O2 in the atmosphere of those worlds that orbit low-mass M dwarfs. This is strong motivation to carry out a census of planets around cool stars for which habitable zones can be resolved by ELTs, i.e. for M dwarfs within ~5 parsec. Our HARPS survey has been a major contributor to that sample of nearby planets. Here we report on our radial velocity observations of Ross 128 (Proxima Virginis, GJ447, HIP 57548), an M4 dwarf just 3.4 parsec away from our Sun. This source hosts an exo-Earth with a projected mass m sini = 1.35 M⊕ and an orbital period of 9.9 days. Ross 128 b receives less than 1.5 times as much flux as Earth from the Sun and its equilibrium ranges in temperature between 269 K for an Earth-like albedo and 213 K for a Venus-like albedo. Recent studies place it close to the inner edge of the conventional habitable zone. An 80-day long light curve from K2 campaign C01 demonstrates that Ross 128 b does not transit. Together with the All Sky Automated Survey (ASAS) photometry and spectroscopic activity indices, the K2 photometry shows that Ross 128 rotates slowly and has weak magnetic activity. In a habitability context, this makes survival of its atmosphere against erosion more likely. Ross 128 b is the second closest known exo-Earth, after Proxima Centauri b (1.3 parsec), and the closest temperate planet known around a quiet star. The 15 mas planet-star angular separation at maximum elongation will be resolved by ELTs (>3λ∕D) in the optical bands of O2.

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Gratton, R., A. Zurlo, H. Le Coroller, M. Damasso, F. Del Sordo, M. Langlois, D. Mesa, et al. "Searching for the near-infrared counterpart of Proxima c using multi-epoch high-contrast SPHERE data at VLT." Astronomy & Astrophysics 638 (June 2020): A120. http://dx.doi.org/10.1051/0004-6361/202037594.

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Context. Proxima Centauri is the closest star to the Sun and it is known to host an Earth-like planet in its habitable zone; very recently a second candidate planet was proposed based on radial velocities. At quadrature, the expected projected separation of this new candidate is larger than 1 arcsec, making it a potentially interesting target for direct imaging. Aims. While identification of the optical counterpart of this planet is expected to be very difficult, successful identification would allow for a detailed characterization of the closest planetary system. Methods. We searched for a counterpart in SPHERE images acquired over four years through the SHINE survey. In order to account for the expected large orbital motion of the planet, we used a method that assumes the circular orbit obtained from radial velocities and exploits the sequence of observations acquired close to quadrature in the orbit. We checked this with a more general approach that considers Keplerian motion, called K-stacker. Results. We did not obtain a clear detection. The best candidate has signal-to-noise ratio (S∕N) = 6.1 in the combined image. A statistical test suggests that the probability that this detection is due to random fluctuation of noise is <1%, but this result depends on the assumption that the distribution of noise is uniform over the image, a fact that is likely not true. The position of this candidate and the orientation of its orbital plane fit well with observations in the ALMA 12 m array image. However, the astrometric signal expected from the orbit of the candidate we detected is 3σ away from the astrometric motion of Proxima as measured from early Gaia data. This, together with the unexpectedly high flux associated with our direct imaging detection, means we cannot confirm that our candidate is indeed Proxima c. Conclusions. On the other hand, if confirmed, this would be the first observation in imaging of a planet discovered from radial velocities and the second planet (after Fomalhaut b) of reflecting circumplanetary material. Further confirmation observations should be done as soon as possible.

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Por, E. H., and S. Y. Haffert. "The Single-mode Complex Amplitude Refinement (SCAR) coronagraph." Astronomy & Astrophysics 635 (March 2020): A55. http://dx.doi.org/10.1051/0004-6361/201731616.

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Context. The recent discovery of an Earth-mass exoplanet around the nearby star Proxima Centauri provides a prime target for the search for life on planets outside our solar system. Atmospheric characterization of these planets has been proposed by blocking the starlight with a stellar coronagraph and using a high-resolution spectrograph to search for reflected starlight off the planet. Aims. Due to the large flux ratio and small angular separation between Proxima b and its host star (≲10−7 and ≲2.2λ/D respectively; at 750 nm for an 8 m-class telescope) the coronagraph requires high starlight suppression at extremely-low inner working angles. Additionally, it must operate over a broad spectral bandwidth and under residual telescope vibrations. This allows for efficient use of spectroscopic post-processing techniques. We aim to find the global optimum of an integrated coronagraphic integral-field spectrograph. Methods. We present the Single-mode Complex Amplitude Refinement (SCAR) coronagraph that uses a microlens-fed single-mode fiber array in the focal plane downstream from a pupil-plane phase plate. The mode-filtering property of the single-mode fibers allows for the nulling of starlight on the fibers. The phase pattern in the pupil plane is specifically designed to take advantage of this mode-filtering capability. Second-order nulling on the fibers expands the spectral bandwidth and decreases the tip-tilt sensitivity of the coronagraph. Results. The SCAR coronagraph has a low inner working angle (∼1λ/D) at a contrast of < 3 × 10−5 for the six fibers surrounding the star using a sufficiently-good adaptive optics system. It can operate over broad spectral bandwidths (∼20%) and delivers high throughput (> 50% including fiber injection losses). Additionally, it is robust against tip-tilt errors (∼0.1λ/D rms). We present SCAR designs for both an unobstructed and a VLT-like pupil. Conclusions. The SCAR coronagraph is a promising candidate for exoplanet detection and characterization around nearby stars using current high-resolution imaging instruments.

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Atri, Dimitra. "Stellar Proton Event-induced surface radiation dose as a constraint on the habitability of terrestrial exoplanets." Monthly Notices of the Royal Astronomical Society: Letters, November 4, 2019. http://dx.doi.org/10.1093/mnrasl/slz166.

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Abstract The discovery of terrestrial exoplanets orbiting in habitable zones around nearby stars has been one of the significant developments in modern astronomy. More than a dozen such planets, like Proxima Centauri b and TRAPPIST-1 e, are in close-in configurations and their proximity to the host star makes them highly sensitive to stellar activity. Episodic events such as flares have the potential to cause severe damage to close-in planets, adversely impacting their habitability. Flares on fast rotating young M stars occur up to 100 times more frequently than on G-type stars which makes their planets even more susceptible to stellar activity. Stellar Energetic Particles (SEPs) emanating from Stellar Proton Events (SPEs) cause atmospheric damage (erosion and photochemical changes), and produce secondary particles, which in turn results in enhanced radiation dosage on planetary surfaces. We explore the role of SPEs and planetary factors in determining planetary surface radiation doses. These factors include SPE fluence and spectra, and planetary column density and magnetic field strength. Taking particle spectra from 70 major solar events (observed between 1956 and 2012) as proxy, we use the GEANT4 Monte Carlo model to simulate SPE interactions with exoplanetary atmospheres, and we compute surface radiation dose. We demonstrate that in addition to fluence, SPE spectrum is also a crucial factor in determining the surface radiation dose. We discuss the implications of these findings in constraining the habitability of terrestrial exoplanets.

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