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Thread: Proxima Centauri b: Terrestrial or Neptunian?

  1. #1
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    Question 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?
    Attached Images Attached Images
    Last edited by Roger E. Moore; 2018-Jul-25 at 01:23 AM. Reason: add table

  2. #2
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    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

    Berdiñas, 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.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    — Mark Twain, Life on the Mississippi (1883)

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    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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    http://adsabs.harvard.edu/abs/2018AstL...44..324S

    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.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    — Mark Twain, Life on the Mississippi (1883)

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    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.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    — Mark Twain, Life on the Mississippi (1883)

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