Nature
Water vapor in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b
Tsiaras, et al
In the past decade, observations from space and the ground have found water to be the most abundant molecular species, after hydrogen, in the atmospheres of hot, gaseous extrasolar planets. Being the main molecular carrier of oxygen, water is a tracer of the origin and the evolution mechanisms of planets. For temperate, terrestrial planets, the presence of water is of great importance as an indicator of habitable conditions. Being small and relatively cold, these planets and their atmospheres are the most challenging to observe, and therefore no atmospheric spectral signatures have so far been detected. Super-Earths—planets lighter than ten Earth masses—around later-type stars may provide our first opportunity to study spectroscopically the characteristics of such planets, as they are best suited for transit observations. Here, we report the detection of a spectroscopic signature of water in the atmosphere of K2-18 b—a planet of eight Earth masses in the habitable zone of an M dwarf—with high statistical confidence (Atmospheric Detectability Index = 5.0, ~3.6σ (refs.8,9)). In addition, the derived mean molecular weight suggests an atmosphere still containing some hydrogen. The observations were recorded with the Hubble Space Telescope/Wide Field Camera 3 and analysed with our dedicated, publicly available, algorithms. Although the suitability of M dwarfs to host habitable worlds is still under discussion, K2-18 b offers an unprecedented opportunity to gain insight into the composition and climate of habitable-zone planets.
1909.06748
Planet formation around super massive black holes i the active galactic nuclei
Wada, et al
As a natural consequence of the elementary processes of dust growth, we discovered that a new class of planets can be formed around supermassive black holes (SMBHs). We investigated a growth path from sub-micron sized icy dust monomers to Earth-sized bodies outside the "snow line'', located several parsecs from SMBHs in low luminosity active galactic nuclei (AGNs). In contrast to protoplanetary disks, the "radial drift barrier'' does not prevent the formation of planetesimals. In the early phase of the evolution, low collision velocity between dust particles promotes sticking; therefore, the internal density of the dust aggregates decreases with growth. When the porous aggregate's size reaches 0.1--1 cm, the collisional compression becomes effective, and the decrease in internal density stops. Once 10--100 m sized aggregates are formed, they are decoupled from gas turbulence, and the aggregate layer becomes gravitationally unstable, leading to the formation of planets by the fragmentation of the layer, with ten times the mass of the earth. The growth time scale depends on the turbulent strength of the circumnuclear disk and the black hole mass $M_{BH}$, and it is comparable to the AGN's lifetime ($\sim 10^8$ yr) for low mass ($M_{BH} \sim 10^6 M_\odot$) SMBHs.
Water vapor in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b
Tsiaras, et al
In the past decade, observations from space and the ground have found water to be the most abundant molecular species, after hydrogen, in the atmospheres of hot, gaseous extrasolar planets. Being the main molecular carrier of oxygen, water is a tracer of the origin and the evolution mechanisms of planets. For temperate, terrestrial planets, the presence of water is of great importance as an indicator of habitable conditions. Being small and relatively cold, these planets and their atmospheres are the most challenging to observe, and therefore no atmospheric spectral signatures have so far been detected. Super-Earths—planets lighter than ten Earth masses—around later-type stars may provide our first opportunity to study spectroscopically the characteristics of such planets, as they are best suited for transit observations. Here, we report the detection of a spectroscopic signature of water in the atmosphere of K2-18 b—a planet of eight Earth masses in the habitable zone of an M dwarf—with high statistical confidence (Atmospheric Detectability Index = 5.0, ~3.6σ (refs.8,9)). In addition, the derived mean molecular weight suggests an atmosphere still containing some hydrogen. The observations were recorded with the Hubble Space Telescope/Wide Field Camera 3 and analysed with our dedicated, publicly available, algorithms. Although the suitability of M dwarfs to host habitable worlds is still under discussion, K2-18 b offers an unprecedented opportunity to gain insight into the composition and climate of habitable-zone planets.
1909.06748
Planet formation around super massive black holes i the active galactic nuclei
Wada, et al
As a natural consequence of the elementary processes of dust growth, we discovered that a new class of planets can be formed around supermassive black holes (SMBHs). We investigated a growth path from sub-micron sized icy dust monomers to Earth-sized bodies outside the "snow line'', located several parsecs from SMBHs in low luminosity active galactic nuclei (AGNs). In contrast to protoplanetary disks, the "radial drift barrier'' does not prevent the formation of planetesimals. In the early phase of the evolution, low collision velocity between dust particles promotes sticking; therefore, the internal density of the dust aggregates decreases with growth. When the porous aggregate's size reaches 0.1--1 cm, the collisional compression becomes effective, and the decrease in internal density stops. Once 10--100 m sized aggregates are formed, they are decoupled from gas turbulence, and the aggregate layer becomes gravitationally unstable, leading to the formation of planets by the fragmentation of the layer, with ten times the mass of the earth. The growth time scale depends on the turbulent strength of the circumnuclear disk and the black hole mass $M_{BH}$, and it is comparable to the AGN's lifetime ($\sim 10^8$ yr) for low mass ($M_{BH} \sim 10^6 M_\odot$) SMBHs.
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