Comparative Climates of TRAPPIST-1 planetary system: results from a simple climate-vegetation model
Authors:
Alberti et al
Abstract:
The recent discovery of the planetary system hosted by the ultracool dwarf star TRAPPIST-1 could open new perspectives into the investigation of planetary climates of Earth-sized exoplanets, their atmospheres and their possible habitability. In this paper, we use a simple climate-vegetation energy-balance model to study the climate of the seven TRAPPIST-1 planets and the climate dependence on the global albedo, on the fraction of vegetation that could cover their surfaces and on the different greenhouse conditions. The model allows us to investigate whether liquid water could be maintained on the planetary surfaces (i.e., by defining a "surface water zone") in different planetary conditions, with or without the presence of greenhouse effect.
It is shown that planet TRAPPIST-1d seems to be the most stable from an Earth-like perspective, since it resides in the surface water zone for a wide range of reasonable values of the model parameters. Moreover, according to the model outer planets (f, g and h) cannot host liquid water on their surfaces, even for Earth-like conditions, entering a snowball state. Although very simple, the model allows to extract the main features of the TRAPPIST-1 planetary climates.
Strong XUV irradiation of the Earth-sized exoplanets orbiting the ultracool dwarf TRAPPIST-1
Authors:
Wheatley et al
Abstract:
We present an XMM-Newton X-ray observation of TRAPPIST-1, which is an ultracool dwarf star recently discovered to host three transiting and temperate Earth-sized planets. We find the star is a relatively strong and variable coronal X-ray source with an X-ray luminosity similar to that of the quiet Sun, despite its much lower bolometric luminosity. We find L_x/L_bol=2-4x10^-4, with the total XUV emission in the range L_xuv/L_bol=6-9x10^-4. Using a simple energy-limited model we show that the relatively close-in Earth-sized planets, which span the classical habitable zone of the star, are subject to sufficient X-ray and EUV irradiation to significantly alter their primary and perhaps secondary atmospheres. Understanding whether this high-energy irradiation makes the planets more or less habitable is a complex question, but our measured fluxes will be an important input to the necessary models of atmospheric evolution.
Temperate Earth-sized planets transiting a nearby ultracool dwarf star
Authors:
Gillon et al
Abstract:
Star-like objects with effective temperatures of less than 2,700 kelvin are referred to as ‘ultracool dwarfs’. This heterogeneous group includes stars of extremely low mass as well as brown dwarfs (substellar objects not massive enough to sustain hydrogen fusion), and represents about 15 per cent of the population of astronomical objects near the Sun. Core-accretion theory predicts that, given the small masses of these ultracool dwarfs, and the small sizes of their protoplanetary disks there should be a large but hitherto undetected population of terrestrial planets orbiting them—ranging from metal-rich Mercury-sized planets to more hospitable volatile-rich Earth-sized planets. Here we report observations of three short-period Earth-sized planets transiting an ultracool dwarf star only 12 parsecs away. The inner two planets receive four times and two times the irradiation of Earth, respectively, placing them close to the inner edge of the habitable zone of the star. Our data suggest that 11 orbits remain possible for the third planet, the most likely resulting in irradiation significantly less than that received by Earth. The infrared brightness of the host star, combined with its Jupiter-like size, offers the possibility of thoroughly characterizing the components of this nearby planetary system.
Five transiting hot Jupiters discovered using WASP-South, Euler and TRAPPIST: WASP-119 b, WASP-124 b, WASP-126 b, WASP-129 b and WASP-133 b
Authors:
Maxted et al
Abstract:
We have used photometry from the WASP-South instrument to identify 5 stars showing planet-like transits in their light curves. The planetary nature of the companions to these stars has been confirmed using photometry from the EulerCam instrument on the Swiss Euler 1.2-m telescope and the TRAPPIST telescope, and spectroscopy obtained with the CORALIE spectrograph. The planets discovered are hot Jupiter systems with orbital periods in the range 2.17 to 5.75 days, masses from 0.3MJup to 1.2MJup and with radii from 1RJup to 1.5RJup. These planets orbit bright stars (V = 11-13) with spectral types in the range F9 to G4. WASP-126 is the brightest planetary system in this sample and hosts a low-mass planet with a large radius (0.3 MJup , 0.95RJup), making it a good target for transmission spectroscopy. The high density of WASP-129 A suggests that it is a helium-rich star similar to HAT-P-11 A. WASP-133 has an enhanced surface lithium abundance compared to other old G-type stars, particularly other planet host stars. These planetary systems are good targets for follow-up observations with ground-based and space-based facilities to study their atmospheric and dynamical properties.
