Astronomers using NASA's Hubble Space Telescope have discovered an immense cloud of hydrogen dubbed "The Behemoth" bleeding off a planet orbiting a nearby star. The enormous, comet-like feature is about 50 times the size of the parent star. The hydrogen is evaporating from a warm, Neptune-sized planet, due to extreme radiation from the star. A phenomenon this large has never before been seen around any exoplanet. It may offer clues to how Super-Earths massive, rocky, versions of Earth are born around other stars through the evaporation of their outer layers of hydrogen. Finding "The Behemoth" could be a game-changer for characterizing atmospheres of the whole population of Neptune-sized planets and Super-Earths in ultraviolet observations.
A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b
Nature 522, 7557 (2015). doi:10.1038/nature14501
Authors: David Ehrenreich, Vincent Bourrier, Peter J. Wheatley, Alain Lecavelier des Etangs, Guillaume Hébrard, Stéphane Udry, Xavier Bonfils, Xavier Delfosse, Jean-Michel Désert, David K. Sing & Alfred Vidal-Madjar
Exoplanets orbiting close to their parent stars may lose some fraction of their atmospheres because of the extreme irradiation. Atmospheric mass loss primarily affects low-mass exoplanets, leading to the suggestion that hot rocky planets might have begun as Neptune-like, but subsequently lost all of their atmospheres; however, no confident measurements have hitherto been available. The signature of this loss could be observed in the ultraviolet spectrum, when the planet and its escaping atmosphere transit the star, giving rise to deeper and longer transit signatures than in the optical spectrum. Here we report that in the ultraviolet the Neptune-mass exoplanet GJ 436b (also known as Gliese 436b) has transit depths of 56.3 ± 3.5% (1σ), far beyond the 0.69% optical transit depth. The ultraviolet transits repeatedly start about two hours before, and end more than three hours after the approximately one hour optical transit, which is substantially different from one previous claim (based on an inaccurate ephemeris). We infer from this that the planet is surrounded and trailed by a large exospheric cloud composed mainly of hydrogen atoms. We estimate a mass-loss rate in the range of about 108–109 grams per second, which is far too small to deplete the atmosphere of a Neptune-like planet in the lifetime of the parent star, but would have been much greater in the past.
Galaxies at redshifts 5 to 6 with systematically low dust content and high [C ii] emission
Nature 522, 7557 (2015). doi:10.1038/nature14500
Authors: P. L. Capak, C. Carilli, G. Jones, C. M. Casey, D. Riechers, K. Sheth, C. M. Carollo, O. Ilbert, A. Karim, O. LeFevre, S. Lilly, N. Scoville, V. Smolcic & L. Yan
The rest-frame ultraviolet properties of galaxies during the first three billion years of cosmic time (redshift z > 4) indicate a rapid evolution in the dust obscuration of such galaxies. This evolution implies a change in the average properties of the interstellar medium, but the measurements are systematically uncertain owing to untested assumptions and the inability to detect heavily obscured regions of the galaxies. Previous attempts to measure the interstellar medium directly in normal galaxies at these redshifts have failed for a number of reasons, with two notable exceptions. Here we report measurements of the forbidden C ii emission (that is, [C ii]) from gas, and the far-infrared emission from dust, in nine typical star-forming galaxies about one billion years after the Big Bang (z ≈ 5–6). We find that these galaxies have thermal emission that is less than 1/12 that of similar systems about two billion years later, and enhanced [C ii] emission relative to the far-infrared continuum, confirming a strong evolution in the properties of the interstellar medium in the early Universe. The gas is distributed over scales of one to eight kiloparsecs, and shows diverse dynamics within the sample. These results are consistent with early galaxies having significantly less dust than typical galaxies seen at z < 3 and being comparable in dust content to local low-metallicity systems.
Astrophysics: Dust-poor galaxies at early times
Nature 522, 7557 (2015). doi:10.1038/522422a
Authors: Veronique Buat
Observations of galaxies that formed early in the Universe's history reveal much lower dust levels than are found in sources from a slightly later era. It seems that galaxies underwent rapid change during a relatively short period. See Letter p.455
Planetary science: Space-rock alert
Nature 522, 7557 (2015). doi:10.1038/522418a
Author: Alexandra Witze
Alexandra Witze watches a pair of films on asteroids — according to many, a vast accident waiting to happen.
Private asteroid hunt lacks cash to spy threats in orbit
Nature 522, 7557 (2015). http://www.nature.com/doifinder/10.1038/522402a
Author: Traci Watson
Foundation fails to raise funds it needs for a space telescope to catalogue near-Earth objects.
Astronomers claim first glimpse of primordial stars
Nature 522, 7557 (2015). http://www.nature.com/doifinder/10.1038/nature.2015.17802
Author: Elizabeth Gibney
Bright galaxy thought to hold stars from generation that seeded rest of Universe.