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Register a new userHabitability in Brown Dwarf Systems
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Authors
Emeline Bolmont
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The very recent discovery of planets orbiting very low mass stars sheds light on these exotic objects. Planetary systems around low-mass stars and brown dwarfs are very different from our solar system: the planets are expected to be much closer than Mercury, in a layout that could resemble the system of Jupiter and its moons. The recent discoveries point in that direction with, for example, the system of Kepler-42 and especially the system of TRAPPIST-1 which has seven planets in a configuration very close to the moons of Jupiter. Low-mass stars and brown dwarfs are thought to be very common in our neighborhood and are thought to host many planetary systems. The planets orbiting in the habitable zone of brown dwarfs (and very low-mass stars) represent one of the next challenges of the following decades: they are the only planets of the habitable zone whose atmosphere we will be able to probe (e.g. with the JWST).
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The habitability of a planet depends on various factors, such as delivery of water during the formation, the co-evolution of the interior and the atmosphere, as well as the stellar irradiation which changes in time. Since an unknown number of rocky exoplanets may operate in a one-plate convective regime, i.e., without plate tectonics, we aim at understanding under which conditions planets in such a stagnant-lid regime may support habitable surface conditions. Understanding the interaction of the planetary interior and outgassing of volatiles with the atmosphere in combination with the evolution of the host star is crucial to determine the potential habitability. M-dwarf stars in particular possess a high-luminosity pre-main sequence phase which endangers the habitability of planets around them via water loss. We therefore explore the potential of secondary outgassing from the planetary interior to rebuild a water reservoir allowing for habitability at a later stage. We compute the boundaries of the habitable zone around M, K, G, and F-dwarf stars using a 1D cloud-free radiative-convective climate model accounting for the outgassing history of CO2 and H2O from an interior evolution and outgassing model for different interior compositions and stellar luminosity evolutions. The outer edge of the habitable zone strongly depends on the amount of CO2 outgassed from the interior, while the inner edge is mainly determined via the stellar irradiation, as soon as a sufficiently large water reservoir has been outgassed. A build-up of a secondary water reservoir for planets around M-dwarf stars is possible even after severe water loss during the high luminosity pre-main sequence phase as long as some water has been retained within the mantle. Earth-like stagnant-lid planets allow for habitable surface conditions within a continuous habitable zone that is dependent on interior composition.
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Christiane Helling Centre for Exoplanetn Science
, University of St Andrews & SRON Netherlands Institute for Spacen Research
2019
The study of the composition of brown dwarf atmospheres helped to understand their formation and evolution. Similarly, the study of exoplanet atmospheres is expected to constrain their formation and evolutionary states. We use results from 3D simulations, kinetic cloud formation and kinetic ion-neutral chemistry to investigate ionisation processes which will affect their atmosphere chemistry: The dayside of super-hot Jupiters is dominated by atomic hydrogen, and not H$_2$O. Such planetary atmospheres exhibit a substantial degree of thermal ionisation and clouds only form on the nightside where lightning leaves chemical tracers (e.g. HCN) for possibly long enough to be detectable. External radiation may cause exoplanets to be enshrouded in a shell of highly ionised, H$_3^+$-forming gas and a weather-driven aurora may emerge. Brown dwarfs enable us to study the role of electron beams for the emergence of an extrasolar, weather-system driven aurora-like chemistry, and the effect of strong magnetic fields on cold atmospheric gases. Electron beams trigger the formation of H$_3^+$ in the upper atmosphere of a brown dwarf (e.g. LSR-J1835) which may react with it to form hydronium, H$_3$O$^+$, as a longer lived chemical tracer. Brown dwarfs and super-hot gas giants may be excellent candidates to search for H$_3$O$^+$ as an H$_3^+$ product.
It is well-known that stars with giant planets are on average more metal-rich than stars without giant planets, whereas stars with detected low-mass planets do not need to be metal-rich. With the aim of studying the weak boundary that separates giant planets and brown dwarfs (BDs) and their formation mechanism, we analyze the spectra of a sample of stars with already confirmed BD companions both by radial velocity and astrometry. We employ standard and automatic tools to perform an EW-based analysis and to derive chemical abundances from CORALIE spectra of stars with BD companions. We compare these abundances with those of stars without detected planets and with low-mass and giant-mass planets. We find that stars with BDs do not have metallicities and chemical abundances similar to those of giant-planet hosts but they resemble the composition of stars with low-mass planets. The distribution of mean abundances of $alpha$-elements and iron peak elements of stars with BDs exhibit a peak at about solar abundance whereas for stars with low-mass and high-mass planets the [X$_alpha$/H] and [X$_{rm Fe}$/H] peak abundances remain at $sim -0.1$~dex and $sim +0.15$~dex, respectively. We display these element abundances for stars with low-mass and high-mass planets, and BDs versus the minimum mass, $m_C sin i$, of the most-massive substellar companion in each system, and we find a maximum in $alpha$-element as well as Fe-peak abundances at $m_C sin i sim 1.35pm 0.20$ jupiter masses. We discuss the implication of these results in the context of the formation scenario of BDs in comparison with that of giant planets.
The recently discovered brown dwarf WISE 0855 presents our first opportunity to directly study an object outside the Solar System that is nearly as cold as our own gas giant planets. However the traditional methodology for characterizing brown dwarfs---near infrared spectroscopy---is not currently feasible as WISE 0855 is too cold and faint. To characterize this frozen extrasolar world we obtained a 4.5-5.2 $mu$m spectrum, the same bandpass long used to study Jupiters deep thermal emission. Our spectrum reveals the presence of atmospheric water vapor and clouds, with an absorption profile that is strikingly similar to Jupiter. The spectrum is high enough quality to allow the investigation of dynamical and chemical processes that have long been studied in Jupiters atmosphere, but now on an extrasolar world.
Magnetospheric processes seen in gas-giants such as aurorae and circularly-polarized cyclotron maser radio emission have been detected from some brown dwarfs. However, previous radio observations targeted known brown dwarfs discovered via their infrared emission. Here we report the discovery of BDR J1750+3809, a circularly polarized radio source detected around 144 MHz with the LOFAR telescope. Follow-up near-infrared photometry and spectroscopy show that BDR J1750+3809 is a cold methane dwarf of spectral type T$6.5pm 1$ at a distance of $65^{+9}_{-8},{rm pc}$. The quasi-quiescent radio spectral luminosity of BDR J1750+3809 is $approx 5times 10^{15},{rm erg},{rm s}^{-1},{rm Hz}^{-1}$ which is over two orders of magnitude larger than that of the known population of comparable spectral type. This could be due to a preferential geometric alignment or an electrodynamic interaction with a close companion. In addition, as the emission is expected to occur close to the electron gyro-frequency, the magnetic field strength at the emitter site in BDR J1750+3809 is $Bgtrsim 25,{rm G}$, which is comparable to planetary-scale magnetic fields. Our discovery suggests that low-frequency radio surveys can be employed to discover sub-stellar objects that are too cold to be detected in infrared surveys.
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