# Thread: Proxima Centauri b: Terrestrial or Neptunian?

1. ## Proxima Centauri b: Terrestrial or Neptunian?

Read some of the papers on our nearest known neighbor planet, Proxima Centauri b, and did some math.

The attached patchwork table shows the mass of PCb depending on its orbital inclination. Its mass(i) = 1.27 x Earth's mass, last I read. The table was from Excel with formulae to calculate data.

Taking 2 Earth masses as the transition point between terrestrial Earthlike worlds and mini-Neptunes, per:

PROBABILISTIC FORECASTING OF THE MASSES AND RADII OF OTHER WORLDS
Jingjing Chen, David Kipping
https://arxiv.org/pdf/1603.08614.pdf

... it appears there is just over a 55% chance PCb is terrestrial, so you can land on it and maybe colonize it, and just over a 44% chance it has an H/He envelope. If the latter holds but PCb is small enough, you might colonize it anyway with terraforming. Still would have a rocky surface under that gas.

To me, it looks too close to call whether another Earth or mini-Neptune. Another paper gives terrestrial odds as 84%.

ON THE ORBITAL INCLINATION OF PROXIMA CENTAURI b
Stephen R. Kane, Dawn M. Gelino, Margaret C. Turnbull
https://arxiv.org/pdf/1612.02872.pdf

Thoughts?
Last edited by Roger E. Moore; 2018-Jul-25 at 01:23 AM. Reason: add table

2. An assortment of recent papers on Proxima Centauri b. When will they get around to naming it?

https://arxiv.org/abs/1807.01318

The gravitational mass of Proxima Centauri measured with SPHERE from a microlensing event

Zurlo, A.; Gratton, R.; Mesa, D.; Desidera, S.; Enia, A.; Sahu, K.; Almenara, J.-M.; Kervella, P.; Avenhaus, H.; Girard, J.; Janson, M.; Lagadec, E.; Langlois, M.; Milli, J.; Perrot, C.; Schlieder, J.-E.; Thalmann, C.; Vigan, A.; Giro, E.; Gluck, L.; Ramos, J.; Roux, A.
10/2018

Proxima Centauri, our closest stellar neighbour, is a low-mass M5 dwarf orbiting in a triple system. An Earth-mass planet with an 11 d period has been discovered around this star. The star's mass has been estimated only indirectly using a mass-luminosity relation, meaning that large uncertainties affect our knowledge of its properties. To refine the mass estimate, an independent method has been proposed: gravitational microlensing. By taking advantage of the close passage of Proxima Cen in front of two background stars, it is possible to measure the astrometric shift caused by the microlensing effect due to these close encounters and estimate the gravitational mass of the lens (Proxima Cen). Microlensing events occurred in 2014 and 2016 with impact parameters, the closest approach of Proxima Cen to the background star, of 1.6 ± 0.1 and 0.5 ± 0.1 arcsec, respectively. Accurate measurements of the positions of the background stars during the last 2 yr have been obtained with Hubble Space Telescope/Wide Field Camera 3, and with Very Large Telescope/Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) from the ground. The SPHERE campaign started on 2015 March, and continued for more than 2 yr, covering nine epochs. The parameters of Proxima Centauri's motion on the sky, along with the pixel scale, true North, and centring of the instrument detector were readjusted for each epoch using the background stars visible in the IRDIS field of view. The experiment has been successful and the astrometric shift caused by the microlensing effect has been measured for the second event in 2016. We used this measurement to derive a mass of 0.150^{+0.062}_{-0.051} (an error of ˜ 40 per cent) M&sun; for Proxima Centauri acting as a lens. This is the first and the only currently possible measurement of the gravitational mass of Proxima Centauri.

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https://arxiv.org/abs/1711.04685

Exocomets in the Proxima Centauri system and their importance for water transport

Schwarz, R.; Bazsó, Á.; Georgakarakos, N.; Loibnegger, B.; Maindl, T. I.; Bancelin, D.; Pilat-Lohinger, E.; Kislyakova, K. G.; Dvorak, R.; Dobbs-Dixon, I.
08/2018

The scenario and efficiency of water transport by icy asteroids and comets are still amongst the most important unresolved questions of planetary systems. A better understanding of cometary dynamics in extrasolar systems shall provide information about cometary reservoirs and give an insight into water transport especially to planets in the habitable zone. The detection of Proxima Centauri-b (PCb), which moves in the habitable zone of this system, triggered a debate whether or not this planet can be habitable. In this work, we focus on the stability of an additional planet in the system and on water transport by minor bodies. We perform numerous N-body simulations with PCb and an outer Oort-cloud like reservoir of comets. We investigate close encounters and collisions with the planet, which are important for the transport of water. Observers found hints for a second planet with a period longer than 60 days. Our dynamical studies show that two planets in this system are stable even for a more massive second planet (˜12 Earth masses). Furthermore, we perform simulations including exocomets, a second planet, and the influence of the binary Alpha Centauri. The studies on the dynamics of exocomets reveal that the outer limit for water transport is around 200 au. In addition we show that water transport would be possible from a close-in planetesimal cloud (1-4 au). From our simulations, based on typical M-star protoplanetary disks, we estimate the water mass delivered to the planets to be between the extremes 0 and 51 Earth oceans.

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https://arxiv.org/abs/1805.12273

Testing Gravity with wide binary stars like alpha Centauri

Banik, Indranil; Zhao, Hongsheng
07/2018

We consider the feasibility of testing Newtonian gravity at low accelerations using wide binary (WB) stars separated by ≳ 3 kAU. These systems probe the accelerations at which galaxy rotation curves unexpectedly flatline, possibly due to Modified Newtonian Dynamics (MOND). We conduct Newtonian and MOND simulations of WBs covering a grid of model parameters in the system mass, semi-major axis, eccentricity and orbital plane. We self-consistently include the external field (EF) from the rest of the Galaxy on the Solar neighbourhood using an axisymmetric algorithm. For a given projected separation, WB relative velocities reach larger values in MOND. The excess is ≈20} adopting its simple interpolating function, as works best with a range of Galactic and extragalactic observations. This causes noticeable MOND effects in accurate observations of ≈500 WBs, even without radial velocity measurements. We show that the proposed Theia mission may be able to directly measure the orbital acceleration of Proxima Cen towards the 13 kAU-distant alpha Cen. This requires an astrometric accuracy of ≈1 muas over 5 years. We also consider the long-term orbital stability of WBs with different orbital planes. As each system rotates around the Galaxy, it experiences a time-varying EF because this is directed towards the Galactic Centre. We demonstrate approximate conservation of the angular momentum component along this direction, a consequence of the WB orbit adiabatically adjusting to the much slower Galactic orbit. WBs with very little angular momentum in this direction are less stable over Gyr periods. This novel direction-dependent effect might allow for further tests of MOND.

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https://zenodo.org/record/1317470

Unveiling the Secrets of Proxima Centauri's Environment

Berdińas, Zaira M.
07/2018

Proxima Centauri is our closest neighbor. In 2016, we reported the discovery of an exoplanet candidate orbiting in a temperate orbit around this cool star: Proxima b, with an orbital period of only 11.2 days, a semi-major-axis distance of 0.05 au, and a minimum mass of 1.3 Earth masses. This discovery was the main result of the Pale Red Dot campaign in which we combined HARPS radial velocity data with photometric simultaneous observations taken from small-medium size telescopes in different locations. The proximity of Proxima Centauri offers an unique opportunity to apply our knowledge about the Solar system in a new system. In this talk, I will review our knowledge of the Proxima Centauri system from the Proxima b detection to our recent discovery of a dust belt at 1-4 au using ALMA 1.3 mm observations. Such a belt is thought to have a total mass of 0.01 Earth masses, resembling the solar Kuiper belt in a smaller scale. In addition to this Kuiper belt analog, our ALMA images show hints for a warmer and a hotter disk as well as an unknown emission source at a projected distance of about 1.2 arcsec from the star. The possible nature of this source as well as the current and future prospects on the study of the Proxima system will be presented in this talk.

3. https://arxiv.org/abs/1807.09365

New rotation period measurements for M dwarfs in the southern hemisphere: an abundance of slowly rotating, fully convective stars

Newton, Elisabeth R.; Mondrik, Nicholas; Irwin, Jonathan; Winters, Jennifer G.; Charbonneau, David
07/2018

Stellar rotation periods are valuable both for constraining models of angular momentum loss and for under- standing how magnetic features impact inferences of exoplanet parameters. Building on our previous work in the northern hemisphere, we have used long-term, ground-based photometric monitoring from the MEarth Observatory to measure 234 rotation periods for nearby, southern hemisphere M dwarfs. Notable examples include the exoplanet hosts GJ 1132, LHS 1140, and Proxima Centauri. We find excellent agreement between our data and K2 photometry for the overlapping subset. Amongst the sample of stars with the highest quality datasets, we recover periods in 66%; as the length of the dataset increases, our recovery rate approaches 100%. The longest rotation periods we detect are around 140 days, which we suggest represent the periods that are reached when M dwarfs are as old as the local thick disk (about 9 Gyr).

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https://arxiv.org/abs/1806.03856

Computing the minimal crew for a multi-generational space travel towards Proxima Centauri b

Marin, F.; Beluffi, C.
06/2018

The survival of a genetically healthy multi-generational crew is of a prime concern when dealing with space travel. It has been shown that determining a realistic population size is tricky as many parameters (such as infertility, inbreeding, sudden deaths, accidents or random events) come into play. To evaluate the impact of those parameters, Monte Carlo simulations are among the best methods since they allow testing of all possible scenarios and determine, by numerous iterations, which are the most likely. This is why we use the Monte Carlo code HERITAGE to estimate the minimal crew for a multi-generational space travel towards Proxima Centauri b. By allowing the crew to evolve under a list of adaptive social engineering principles (namely yearly evaluations of the vessel population, offspring restrictions and breeding constraints), we show in this paper that it is possible to create and maintain a healthy population virtually indefinitely. A initial amount of 25 breeding pairs of settlers drives the mission towards extinction in 50 +/- 15% of cases if we completely forbid inbreeding. Under the set of parameters described in this publication, we find that a minimum crew of 98 people is necessary ensure a 100% success rate for a 6300-year space travel towards the closest telluric exoplanet known so far.

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https://arxiv.org/abs/1804.02001

The First Naked-eye Superflare Detected from Proxima Centauri

Howard, Ward S.; Tilley, Matt A.; Corbett, Hank; Youngblood, Allison; Loyd, R. O. Parke; Ratzloff, Jeffrey K.; Law, Nicholas M.; Fors, Octavi; del Ser, Daniel; Shkolnik, Evgenya L.; Ziegler, Carl; Goeke, Erin E.; Pietraallo, Aaron D.; Haislip, Joshua
06/2018

Proxima b is a terrestrial-mass planet in the habitable zone of Proxima Centauri. Proxima Centauri's high stellar activity, however, casts doubt on the habitability of Proxima b: sufficiently bright and frequent flares and any associated proton events may destroy the planet's ozone layer, allowing lethal levels of UV flux to reach its surface. In 2016 March, the Evryscope observed the first naked-eye-brightness superflare detected from Proxima Centauri. Proxima increased in optical flux by a factor of ~68 during the superflare and released a bolometric energy of 1033.5 erg, ~10× larger than any previously detected flare from Proxima. Over the last two years the Evryscope has recorded 23 other large Proxima flares ranging in bolometric energy from 1030.6 to 1032.4 erg; coupling those rates with the single superflare detection, we predict that at least five superflares occur each year. Simultaneous high-resolution High Accuracy Radial velocity Planet Searcher (HARPS) spectroscopy during the Evryscope superflare constrains the superflare's UV spectrum and any associated coronal mass ejections. We use these results and the Evryscope flare rates to model the photochemical effects of NO x atmospheric species generated by particle events from this extreme stellar activity, and show that the repeated flaring may be sufficient to reduce the ozone of an Earth-like atmosphere by 90% within five years; complete depletion may occur within several hundred kyr. The UV light produced by the Evryscope superflare would therefore have reached the surface with ~100× the intensity required to kill simple UV-hardy microorganisms, suggesting that life would have to undergo extreme adaptations to survive in the surface areas of Proxima b exposed to these flares.

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https://arxiv.org/abs/1802.04284

A Multi-year Search for Transits of Proxima Centauri. I. Light Curves Corresponding to Published Ephemerides

Blank, David L.; Feliz, Dax; Collins, Karen A.; White, Graeme L.; Stassun, Keivan G.; Curtis, Ivan A.; Hart, Rhodes; Kielkopf, John F.; Nelson, Peter; Relles, Howard; Stockdale, Christopher; Jayawardene, Bandupriya; Pennypacker, Carlton R.; Shankland, Paul; Reichart, Daniel E.; Haislip, Joshua B.; Kouprianov, Vladimir V.
06/2018

Proxima Centauri has become the subject of intense study since the radial-velocity (RV) discovery by Anglada-Escudé et al. of a planet orbiting this nearby M dwarf every ~11.2 days. If Proxima Centauri b transits its host star, independent confirmation of its existence is possible, and its mass and radius can be measured in units of the stellar host mass and radius. To date, there have been three independent claims of possible transit-like event detections in light curve observations obtained by the MOST satellite (in 2014--15), the Bright Star Survey Telescope telescope in Antarctica (in 2016), and the Las Campanas Observatory (in 2016). The claimed possible detections are tentative, due in part to the variability intrinsic to the host star, and in the case of the ground-based observations, also due to the limited duration of the light curve observations. Here, we present preliminary results from an extensive photometric monitoring campaign of Proxima Centauri, using telescopes around the globe and spanning from 2006 to 2017, comprising a total of 329 observations. Considering our data that coincide directly and/or phased with the previously published tentative transit detections, we are unable to independently verify those claims. We do, however, verify the previously reported ubiquitous and complex variability of the host star. We discuss possible interpretations of the data in light of the previous claims, and we discuss future analyses of these data that could more definitively verify or refute the presence of transits associated with the RV-discovered planet.

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Cosmic Rays near Proxima Centauri b

Sadovski, A. M.; Struminsky, A. B.; Belov, A.
05/2018

The discovery of a terrestrial planet orbiting Proxima Centauri has led to a lot of papers discussing the possible conditions on this planet. Since the main factors determining space weather in the Solar System are the solar wind and cosmic rays (CRs), it seems important to understand what the parameters of the stellar wind, Galactic and stellar CRs near exoplanets are. Based on the available data, we present our estimates of the stellar wind velocity and density, the possible CR fluxes and fluences near Proxima b. We have found that there are virtually no Galactic CRs near the orbit of Proxima b up to particle energies 1 TeV due to their modulation by the stellar wind. Nevertheless, more powerful and frequent flares on Proxima Centauri than those on the Sun can accelerate particles to maximum energies 3150 alphabeta GeV ( alpha, beta < 1). Therefore, the intensity of stellar CRs in the astrosphere may turn out to be comparable to the intensity of low-energy CRs in the heliosphere.

4. https://arxiv.org/abs/1712.04483

A Candidate Transit Event around Proxima Centauri

Yiting Li, Gudmundur Stefansson, Paul Robertson, Andrew Monson, Caleb Canas, Suvrath Mahadevan
(Submitted on 12 Dec 2017)

We present a single candidate transit event around Proxima Centauri, found during a blind transit search using a robotic 30\,cm telescope at Las Campanas Observatory. The event lasted 1 hour, with an estimated depth of 5\,mmag, and is inconsistent with the transit window predicted for the recently discovered planet b. We modeled the lightcurve under the assumption that the event was caused by a transiting exoplanet, and our model predicts the planet has a radius R∼1R ⊕ . We encourage continued monitoring of Proxima to elucidate the origin of this event.

5. https://arxiv.org/abs/1808.06819

A simple optimized amplitude pupil mask for attempting to direct imaging of Proxima b with SPHERE/ZIMPOL at VLT

Polychronis Patapis, Jonas Kühn, Hans Martin Schmid
(Submitted on 21 Aug 2018)

Proxima b is a terrestrial exoplanet orbiting in the habitable zone of our closest star Proxima Centauri. The separation between the planet and the star is about 40 mas and this is with current instruments only reachable with direct imaging, using a visual extreme AO system like SPHERE/ZIMPOL. Unfortunately, the planet falls under the first airy ring at 2λ/D in the I band, which degrades achievable contrast. We present the design, optical simulations and testing of an amplitude pupil mask for ZIMPOL that reshapes the PSF, increasing the contrast at r=2λ/D about an order of magnitude. The simple mask can be inserted directly into the current setup of SPHERE.

6. https://arxiv.org/abs/1702.08463

Exploring the climate of Proxima B with the Met Office Unified Model

Ian A. Boutle, Nathan J. Mayne, Benjamin Drummond, James Manners, Jayesh Goyal, F. Hugo Lambert, David M. Acreman, Paul D. Earnshaw
(Submitted on 27 Feb 2017 (v1), last revised 24 Aug 2018 (this version, v2))

We present results of simulations of the climate of the newly discovered planet Proxima Centauri B, performed using the Met Office Unified Model (UM). We examine the responses of both an `Earth-like' atmosphere and simplified nitrogen and trace carbon dioxide atmosphere to the radiation likely received by Proxima Centauri B. Additionally, we explore the effects of orbital eccentricity on the planetary conditions using a range of eccentricities guided by the observational constraints. Overall, our results are in agreement with previous studies in suggesting Proxima Centauri B may well have surface temperatures conducive to the presence of liquid water. Moreover, we have expanded the parameter regime over which the planet may support liquid water to higher values of eccentricity (>= 0.1) and lower incident fluxes (881.7 Wm-2) than previous work. This increased parameter space arises because of the low sensitivity of the planet to changes in stellar flux, a consequence of the stellar spectrum and orbital configuration. However, we also find interesting differences from previous simulations, such as cooler mean surface temperatures for the tidally-locked case. Finally, we have produced high resolution planetary emission and reflectance spectra, and highlight signatures of gases vital to the evolution of complex life on Earth (oxygen, ozone and carbon dioxide).

7. JUST OUT... If indeed there is a SECOND planet orbiting Proxima Centauri, here are the early parameters for its mass and orbit. Looks like ProxCent-c is a Neptunian type, by the math.

LATE ADDITIONS: Read the first part of the paper's Introduction, which includes much recent work performed on constraining PC-b's characteristics. It appears to be terrestrial but not a good pick for habitation.

https://arxiv.org/abs/1809.08210

Dynamical evolution and stability maps of the Proxima Centauri system

Tong Meng, Jianghui Ji, Yao Dong (Submitted on 21 Sep 2018)

Proxima Centauri was recently discovered to host an Earth-mass planet of Proxima b, and a 215-day signal which is probably a potential planet c. In this work, we investigate the dynamical evolution of the Proxima Centauri system with the full equations of motion and semi-analytical models including relativistic and tidal effects. We adopt the modified Lagrange-Laplace secular equations to study the evolution of eccentricity of Proxima b, and find that the outcomes are consistent with those from the numerical simulations. The simulations show that relativistic effects have an influence on the evolution of eccentricities of planetary orbits, whereas tidal effects primarily affects the eccentricity of Proxima b over long timescale. Moreover, using the MEGNO (the Mean Exponential Growth factor of Nearby Orbits) technique, we place dynamical constraints on orbital parameters that result in stable or quasi-periodic motions for coplanar and non-coplanar configurations. In the coplanar case, we find that the orbit of Proxima b is stable for the semi-major axis ranging from 0.02 au to 0.1 au and the eccentricity being less than 0.4. This is where the best-fitting parameters for Proxima b exactly fall. Additional simulations show that the robust stability of this system would favor an eccentricity of Proxima b less than 0.45 and that of Proxima c below 0.65. In the non-coplanar case, we find that mutual inclinations of two planets must be lower than 50 ∘ in order to provide stability. Finally, we estimate the mass of Proxima c to be 3.13 M⊕ ≤m c ≤ 70.7 M⊕ when 1.27 M⊕ ≤m b ≤ 1.6 M ⊕ , if i-mutual ≤ 50∘ and ΔΩ=0∘.
Last edited by Roger E. Moore; 2018-Sep-24 at 03:40 PM.

8. Further thoughts on the previous post: If Proxima Centauri c exists and if it is as potentially massive as math indicates, it is quite a surprise that its effects on its sun have not been noticed before now. I am inclined to think c does not exist, as laid out here.

PC-b, on the other hand, could be the centerpiece for any number of terraforming proposals.

9. A little Excel & paper math. In February of last year, in another thread, I was able after many painful online attempts to calculate the tidal stresses that Proxima Centauri places on Proxima b, given a number of assumptions: 1. Proxima b has 1.1 times the radius of Earth; 2. Proxima b has between 0.0 and 0.29 eccentricity in its orbit; 3. the semi-major axis is 0.0485 AU. This was in line with earlier papers on the topic.

I do not have the original Excel sheet with me, with all the math (will get it later), but I have a copy of the outcome (attached).

A. If the mean tidal stress/force exerted on the Earth by the Sun and Moon = 1.0, then the tidal force exerted on Proxima B, with 0.0 eccentricity, is about 364.09. If the planet is in 3:2 locked rotation, 3 days per 2 years of local time, this could mean quite a large change in land tides across the world as it makes its swift orbit around its sun, slowly rotating all along.

B. If, however, Proxima b has, say, 0.29 eccentricity, the tidal forces compared to that experienced by Earth vary from 169.91 at aphelion to 1,017.26 at perihelion, a startling 1:6 ratio of force. Picture this with 3:2 rotation, too.

If Proxima b has a dead core, like the Moon, there would be no volcanism but there would be severe earthquakes. If there are any seas there (which does not seem likely), then the tides would be stupendous. You might have two seas migrating around the planet's equator, one at the subsolar point and one on the opposite side of the planet, rolling over everything in the landscape along eons-old channels.

Will try to make a new table showing updated figures.

10. More math with the wonderful help of Microsoft Excel: This image shows my calculation of the total tidal acceleration on Earth from Sun and Moon, which is the basis for calculation of the tidal effects on Proxima b from its sun.

I'm better at withstanding public embarrassment now than I was a year ago, so correct away.

The point I am making is that Proxima b might have a lot of earthquakes. More to come.

11. Making an adjustment to the mass of Proxima Centauri based on recent paper:

https://arxiv.org/abs/1807.01318
The gravitational mass of Proxima Centauri measured with SPHERE from a microlensing event
A. Zurlo, et al. (Submitted on 3 Jul 2018)

RESULTS: Mass of Proxima Centauri compared to the Sun is 0.150 +0.062/−0.051 Solar Masses, error of ∼40%. ("This is the first and the only currently possible measurement of the gravitational mass of Proxima Centauri.")

12. And here is the final result. The eccentricity of Proxima b cannot exceed 0.4, according to a recent paper, so the table stretches out to accommodate tidal effects for eccentricities from 0.0 to 0.4.

The tidal effects even with low eccentricity are huge compared to Earth, hundreds of times what we go through. At higher e, you get into the low thousands of times what Earth gets.

Land tides (and stress-caused subterranean heat) should be significant in any event. Recent news: https://link.springer.com/article/10...024-017-1563-5

There, something you did not read in the science papers.
Last edited by Roger E. Moore; 2018-Sep-25 at 05:39 PM. Reason: add link

13. Some bad news from a recent conference....

https://www.hou.usra.edu/meetings/cl...8/pdf/2039.pdf

Evryscope Detection of the First Proxima Superflare: Impacts on the Atmosphere and Habitability of Proxima b

Howard, W. S., et al. Comparative Climatology of Terrestrial Planets III: From Stars to Surfaces, held 27-30 August, 2018 in Houston, Texas. LPI Contribution No. 2065, id.2039 (08/2018)

Proxima b is a terrestrial-mass planet in the habitable zone of Proxima Centauri. Proxima Centauri's high stellar activity however casts doubt on the habitability of Proxima b. Superflares (extreme stellar events with an estimated energy release of at least 10^33 erg) and any associated energetic particles may permanently prevent the formation of a protective atmospheric ozone layer, leading to UV radiation levels on the surface which are beyond what some of the hardiest-known organisms can survive. The Proxima superflare: In March 2016, the Evryscope array of small optical telescopes recorded the first superflare seen from Proxima Centauri. Proxima increased in optical flux by a factor of ~68 during the superflare and released a bolometric energy of 10^33.5 erg, ~10X larger than any previously-detected flare from Proxima. Over the last two years the Evryscope has recorded 23 other large Proxima flares ranging in energy from 10^30.6 erg to 10^32.4 erg; coupling those rates with the single superflare detection, we predict at least 5 superflares occur each year. We use the Evryscope flare rates to model the photochemical effects of NOx atmospheric species generated by particle events from this extreme stellar activity, and show that the repeated flaring may be sufficient to reduce the ozone of an Earth-like atmosphere by 90% within five years; complete depletion may occur within several hundred kyr. Surface UV Environment. Without ozone, the UV light produced by the Evryscope superflare would have reached the surface with ~100X the intensity required to kill simple UV-hardy microorganisms, suggesting that life would have to undergo extreme adaptations to survive in the surface areas of Proxima b exposed to these flares. Assuming full ozone-loss, the surface UV-B flux during the Proxima superflare was an average of 2X higher than that 3.9 Ga ago and 3X higher 2.0 Ga ago, although between flares the UV flux was much lower than Earth's, because late M-dwarfs are far fainter in the UV than solar-type stars. The UV-C superflare flux was 7X higher than that 3.9 Ga ago and 1750X higher than 2.0 Ga ago; again, the UV-C flux potentially reaching Proxima's surface is the critical difference compared to Earth's environment.
Last edited by Roger E. Moore; 2018-Sep-28 at 07:57 PM.

https://arxiv.org/abs/1805.00929

The chemical composition of α Cen AB revisited

Thierry Morel (Submitted on 2 May 2018)

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.

QUOTES: "As we discuss in the following, it is likely that α Cen AB do not host giant planets. There is no conclusive evidence for lower-mass planets either despite theoretical arguments suggesting that terrestrial systems might have formed on stable orbits in the inner pair (e.g. Quintana et al. 2002; Guedes et al. 2008; Quarles & Lissauer 2018). The detection of a transit signal in photometric data requires a very favourable geometrical conﬁguration, while revealing the reﬂex motion of a planet in the sub-Neptune mass regime through RV variations remains extremely challenging in magnetically active, solar-like stars. Any indication, even indirect, of the presence of putative planets in α Cen AB based on stellar abundances is therefore valuable. It is also timely in view of the major observational eﬀorts currently being undertaken to ﬁnd potentially habitable worlds in the system.1 Furthermore, improving the basic parameters of α Cen AB turns out to be relevant for a better characterisation of the properties of Proxima Cen b (see, e.g. Barnes et al. 2016) given that achieving stringent constraints on some fundamental quantities (e.g., chemical composition, age) is fraught with difficulties in late-M dwarfs."

15. Paper tightening up the masses and orbits of worlds in the Proxima Centauri system.

https://arxiv.org/abs/1809.08210

Dynamical evolution and stability maps of the Proxima Centauri system

Tong Meng, Jianghui Ji, Yao Dong (Submitted on 21 Sep 2018)

Proxima Centauri was recently discovered to host an Earth-mass planet of Proxima b, and a 215-day signal which is probably a potential planet c. In this work, we investigate the dynamical evolution of the Proxima Centauri system with the full equations of motion and semi-analytical models including relativistic and tidal effects. We adopt the modified Lagrange-Laplace secular equations to study the evolution of eccentricity of Proxima b, and find that the outcomes are consistent with those from the numerical simulations. The simulations show that relativistic effects have an influence on the evolution of eccentricities of planetary orbits, whereas tidal effects primarily affects the eccentricity of Proxima b over long timescale. Moreover, using the MEGNO (the Mean Exponential Growth factor of Nearby Orbits) technique, we place dynamical constraints on orbital parameters that result in stable or quasi-periodic motions for coplanar and non-coplanar configurations. In the coplanar case, we find that the orbit of Proxima b is stable for the semi-major axis ranging from 0.02 au to 0.1 au and the eccentricity being less than 0.4. This is where the best-fitting parameters for Proxima b exactly fall. Additional simulations show that the robust stability of this system would favor an eccentricity of Proxima b less than 0.45 and that of Proxima c below 0.65. In the non-coplanar case, we find that mutual inclinations of two planets must be lower than 50∘. in order to provide stability. Finally, we estimate the mass of Proxima c (Mc) to be 3.13 M⊕ ≤ Mc ≤ 70.7 M⊕. when 1.27 M⊕ ≤ Mb ≤ 1.6 M⊕., if i-mutual ≤ 50∘. and ΔΩ = 0∘.

16. So far, no transits.

https://arxiv.org/abs/1901.07034

A Multi-Year Search For Transits Of Proxima Centauri. II: No Evidence For Transit Events With Periods Between 1-30 Days

Dax L. Feliz, et al. (Submitted on 21 Jan 2019)

Using a global network of small telescopes, we have obtained light curves of Proxima Centauri at 329 observation epochs from 2006 to 2017. The planet Proxima b discovered by Anglada-Escude et al. (2016) with an orbital period of 11.186 d has an a priori transit probability of ~ 1.5%; if it transits, the predicted transit depth is about 5 millimagnitudes. In Blank et al. (2018), we analyzed 96 of our light curves that overlapped with predicted transit ephemerides from previously published tentative transit detections, and found no evidence in our data that would corroborate claims of transits with a period of 11.186 d. Here we broaden our analysis, using 262 high-quality light curves from our dataset to search for any periodic transit-like events over a range of periods from 1 to 30 d. We also inject a series of simulated planet transits and find that our data are sufficiently sensitive to have detected transits of 5 millimagnitude depth, with recoverability ranging from ~100% for orbital period of 1 d to ~20% for orbital period of 20 d for the parameter spaces tested. The recoverability specifically at the 11.186 d period of Proxima b is ~ 40% for the parameter spaces tested. While we are able to rule out virtually all transits of other planets at periods shorter than 5 d and depths greater than 5 millimagnitudes, we cannot rule out the possibility that transits of Proxima b are present but escaped detection. However, we are able to rule out virtually all transits of other planets at periods shorter than 5 d and depths greater than 3 millimagnitudes.

17. The title fooled me into thinking oxygen had been discovered on Proxima b, but it has not. This is just a technique for finding O2, possibly applicable to Proxima b.

https://arxiv.org/abs/1901.10833

High Resolution Spectroscopy and High Contrast Imaging with the ELT: looking for O2 in Proxima b

George A. Hawker, Ian R. Parry (Submitted on 30 Jan 2019)

We research the requirements of High Contrast Imaging when combined with the cross correlation (CC) of high resolution spectra with known spectroscopic templates for detecting and characterising exoplanets in reflected light. We simulate applying the technique to a potentially habitable Proxima b-like planet and show that the O 2 A-band spectral feature could feasibly be detected on nearby rocky exoplanets using future instruments on the ELT . The technique is then more widely analysed showing that detections of planets and O 2 with signal to noise in the CC function (SNR CC ) > 3 can be obtained when the signal to noise of the simulated planet spectrum (SNR spec ) is from 0.25 to 1.2. We place constraints on the spectral resolution, instrument contrast, point spread function (PSF), exposure times and systematic error in stellar light subtraction for making such detections. We find that accurate stellar light subtraction (with 99.99% removal) and PSFs with high spatial resolutions are key to making detections. Lastly a further investigation suggests the ELT could potentially discover and characterise planets of all sizes around different spectral type stars, as well as detecting O 2 on Super-Earths with habitable zone orbits around nearby M stars.

18. Revised version of a previously noted paper looking for Proxima Centauri planets.

https://arxiv.org/abs/1901.07034

A Multi-Year Search For Transits Of Proxima Centauri. II: No Evidence For Transit Events With Periods Between 1-30 Days

Dax L. Feliz, et al. (Submitted on 21 Jan 2019 (v1), last revised 8 Apr 2019 (this version, v2))

Using a global network of small telescopes, we have obtained light curves of Proxima Centauri at 329 observation epochs from 2006 - 2017. The planet Proxima b discovered by Anglada-Escude et al. (2016) with an orbital period of 11.186 d has an a priori transit probability of ~1.5%; if it transits, the predicted transit depth is about 5 millimagnitudes. In Blank et al. (2018), we analyzed 96 of our light curves that overlapped with predicted transit ephemerides from previously published tentative transit detections, and found no evidence in our data that would corroborate claims of transits with a period of 11.186 d. Here we broaden our analysis, using 262 high-quality light curves from our data set to search for any periodic transit-like events over a range of periods from 1 - 30 d. We also inject a series of simulated planet transits and find that our data are sufficiently sensitive to have detected transits of 5 millimagnitude depth, with recoverability ranging from ~100% for an orbital period of 1 d to ~20% for an orbital period of 20 d for the parameter spaces tested. Specifically at the 11.186 d period and 5 millimagnitude transit depth, we rule out transits in our data with high confidence. We are able to rule out virtually all transits of other planets at periods shorter than 5 d and depths greater than 3 millimagnitudes; however, we cannot confidently rule out transits at the period of Proxima b due to incomplete orbital phase coverage and a lack of sensitivity to transits shallower than 4 millimagnitudes.

19. Last edited by Roger E. Moore; 2019-Apr-15 at 08:06 PM. Reason: more news

20. Are we sure that Proxima Centauri's planets could be habitable? This flare looks bad, probably one of many.

https://arxiv.org/abs/1904.06875

Observation of a possible superflare on Proxima Centauri

John F. Kielkopf, Rhodes Hart, Bradley D. Carter, Stephen C. Marsden (Submitted on 15 Apr 2019)

We report the observation on UT 2017.07.01 of an unusually powerful flare detected in near-infrared continuum photometry of Proxima Centauri. During a campaign monitoring the star for possible exoplanet transits, we identified an increase in Sloan i' flux leading to an observed peak at BJD 2457935.996 that was at least 10\% over pre-flare flux in this band. It was followed by a 2-component rapid decline in the first 100 seconds that became a slower exponential decay with time constant of 1350 seconds. A smaller flare event 1,300 seconds after the first added an incremental peak flux increase of 1% of pre-flare flux. Since the onset of the flare was not fully time-resolved at a cadence of 62 seconds, its actual peak value is unknown but greater than the time-average over a single exposure of 20 seconds. The i' band is representative of broad optical and near-IR continuum flux over which the integrated energy of the flare is 100 times the stellar luminosity. This meets the criteria that established the concept of superflares on similar stars. The resulting implied ultraviolet flux and space weather could have had an extreme effect on the atmospheres of planets within the star's otherwise-habitable zone.

21. The title says it all, but we might still see Proxima b by infrared telescope.

https://arxiv.org/abs/1905.01336

Proxima Centauri b is not a transiting exoplanet

James S. Jenkins, et al. (Submitted on 3 May 2019)

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 μm , 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. Follow-up observations should focus on planetary radio emission, phase curves, and direct imaging. 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.

22. Not looking good for Proxima b

https://arxiv.org/abs/1907.12580

Flaring activity of Proxima Centauri from TESS observations: quasi-periodic oscillations during flare decay and inferences on the habitability of Proxima b
Krisztián Vida, Katalin Oláh, Zsolt Kővári, Lidia van Driel-Gesztelyi, Attila Moór (Submitted on 29 Jul 2019)

We analyze the light curve of the M5.5 dwarf Proxima Centauri obtained by the TESS in Sectors 11 and 12. In the ≈50 day-long light curve we identified and analyzed 72 flare events. The flare rate was 1.49 events per day, in total, 7.2% of the data was classified as flaring. The estimated flare energies were in the order of 10^30−10^32 ergs in the TESS passband. Most of the eruptions appeared in groups. Two events showed quasi-periodic oscillations during their decay phase with a time scale of a few hours, that could be caused by quasiperiodic motions of the emittimg plasma or oscillatory reconnection. From the cumulative flare frequency distribution we estimate that superflares with energy output of 10^33 ergs are expected to occur 3 times per year, while a magnitude larger events (with 10^34 ergs) can occur every second year. This reduces the chances of habitability of Proxima Cen b, although earlier numerical models did not rule out the existence of liquid water on the planetary surface. We did not find any obvious signs of planetary transit in the light curve.

23. Originally Posted by Roger E. Moore
Abstract on discovery of Proxima Centauri c. Looks more like a small Neptune type.

A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 au
Damasso, Mario

By analyzing 17 years of radial velocity data of Proxima Cen collected with the UVES and HARPS spectrographs, we detected a signal of period P 5 years that could be explained by the presence of a second planet, Proxima c, with minimum mass m sin i 6. Earth masses. Together with the low-mass temperate planet Proxima b, this candidate planet would make Proxima the closest multi-planet system to the Sun. We will present the properties of the new RV signal and investigate the likelihood that it is related to a magnetic activity cycle of the star. We will discuss how the existence of Proxima c can be confirmed, and its true mass determined with high accuracy, by combining Gaia astrometry and radial velocities. Proxima c would be a prime target for follow-up and characterization measurements, especially with next generation direct imaging instrumentation due to the large maximum angular separation of ̃1 arcsecond from the parent star. Since the orbit would be beyond the original location of the snowline, Proxima c would challenge the models of the formation of super-Earths. Presently, this study is under review by Science Advances.

24. Originally Posted by Roger E. Moore
That's extra interesting because, if I'm reading the charts correctly and without reading the full referenced paper, it appears the C may be the closest object outside the solar system. And, since it takes 4.3 years for its light to reach us and has a period of 5 years, in frozen time, it's still the closest object to us. [Added: Well, looking closer, it looks like the planet, if we were there, would be more to the back side of Proxima Cent.]

What is the better term for "frozen time", IOW, if you could jump there without delay by any clock and see stuff?
Last edited by George; 2020-Jan-16 at 06:30 PM.

25. Originally Posted by Roger E. Moore
This gets confusing because an earlier report of a planet called Proxima C having a 215 day period, from the 2016 discovery paper of
Proxima b, turned out to be wrong.

26. Originally Posted by Roger E. Moore
This gets confusing because an earlier report of a planet called Proxima C having a 215 day period, from the 2016 discovery paper of
Proxima b, turned out to be wrong.
So, wouldn't that allow for a new Proxima C? I think they show a 3 sigma probability (Discussion section of the paper) so it's not worth betting your house, just your cat.

27. https://www.scientificamerican.com/a...-of-proxima-c/ Good article on the hunt for Proxima Centauri C.

28. Originally Posted by Roger E. Moore
https://www.scientificamerican.com/a...-of-proxima-c/ Good article on the hunt for Proxima Centauri C.

The following is my stab at the position of Proxima C, which does suggest it is both visibly closer to us and physically (Centauri time) closer to us because of the time difference. If I knew the cut-off date of their data set, a more accurate assessment would be easy to make.

Proximab C orbit.jpg
[Earth below the page.]

29. "The UV surface habitability of Proxima b" gives a guarded assessment on the chances that microbes will survive a superflare on Proxima. They had best be hiding under a rock, or underground. https://arxiv.org/abs/2003.00984
Last edited by Roger E. Moore; 2020-Mar-04 at 02:03 PM. Reason: Fix url

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