Solar physics: Dynamo theory questioned
Nature 535, 7613 (2016). doi:10.1038/535500a
Authors: Paul Charbonneau
Observations of X-ray emission — a diagnostic tool for the mechanisms driving stellar magnetic fields — from four cool stars call into question accepted models of magnetic-field generation in the Sun and stars. See Letter p.526
Planet hunters seek new ways to detect alien life
Nature 535, 7613 (2016). http://www.nature.com/doifinder/10.1038/535474a
Author: Alexandra Witze
Astrobiologists debate which chemical signatures would hint at life on other worlds.
Optics: Human eye sees single photons
Nature 535, 7613 (2016). doi:10.1038/535469b
People can perceive flashes of light as feeble as a single photon.Alipasha Vaziri at the Rockefeller University in New York City and his colleagues asked three volunteers to stare into an optical system in the dark and listen to two sounds, one of which
Planetary science: Revived telescope finds 104 planets
Nature 535, 7613 (2016). doi:10.1038/535468d
Astronomers have spied 104 new worlds in the Milky Way using NASA's Kepler Space Telescope.Part of Kepler broke down in 2013, but engineers managed to repair it and send it on a fresh mission, dubbed K2. This latest discovery, from Ian Crossfield at the
Celebrating its 50th anniversary this year, the TV series "Star Trek" has captured the public's imagination with the signature phrase, "To boldly go where no one has gone before." The Hubble Space Telescope simply orbits Earth and doesn't "boldly go" deep into space. But it looks deeper into the universe than ever before possible to explore the fabric of time and space and find the farthest objects ever seen. This is epitomized in this Hubble image that is part of its Frontier Fields program to probe the far universe. This view of a massive cluster of galaxies unveils a very cluttered-looking universe filled with galaxies near and far. Some are distorted like a funhouse mirror through a warping-of-space phenomenon first predicted by Einstein a century ago.
Relativistic reverberation in the accretion flow of a tidal disruption event
Nature 535, 7612 (2016). doi:10.1038/nature18007
Authors: Erin Kara, Jon M. Miller, Chris Reynolds & Lixin Dai
Our current understanding of the curved space-time around supermassive black holes is based on actively accreting black holes, which make up only ten per cent or less of the overall population. X-ray observations of that small fraction reveal strong gravitational redshifts that indicate that many of these black holes are rapidly rotating; however, selection biases suggest that these results are not necessarily reflective of the majority of black holes in the Universe. Tidal disruption events, where a star orbiting an otherwise dormant black hole gets tidally shredded and accreted onto the black hole, can provide a short, unbiased glimpse at the space-time around the other ninety per cent of black holes. Observations of tidal disruptions have hitherto revealed the formation of an accretion disk and the onset of an accretion-powered jet, but have failed to reveal emission from the inner accretion flow, which enables the measurement of black hole spin. Here we report observations of reverberation arising from gravitationally redshifted iron Kα photons reflected off the inner accretion flow in the tidal disruption event Swift J1644+57. From the reverberation timescale, we estimate the mass of the black hole to be a few million solar masses, suggesting an accretion rate of 100 times the Eddington limit or more. The detection of reverberation from the relativistic depths of this rare super-Eddington event demonstrates that the X-rays do not arise from the relativistically moving regions of a jet, as previously thought.
Origin and implications of non-radial Imbrium Sculpture on the Moon
Nature 535, 7612 (2016). doi:10.1038/nature18278
Authors: Peter H. Schultz & David A. Crawford
Rimmed grooves, lineations and elongate craters around Mare Imbrium shape much of the nearside Moon. This pattern was coined the Imbrium Sculpture, and it was originally argued that it must have been formed by a giant oblique (~30°) impact, a conclusion echoed by later studies. Some investigators, however, noticed that many elements of the Imbrium Sculpture are not radial to Imbrium, thereby implicating an endogenic or structural origin. Here we use these non-radial trends to conclude that the Imbrium impactor was a proto-planet (half the diameter of Vesta), once part of a population of large proto-planets in the asteroid belt. Such independent constraints on the sizes of the Imbrium and other basin-forming impactors markedly increase estimates for the mass in the asteroid belt before depletion caused by the orbital migration of Jupiter and Saturn. Moreover, laboratory impact experiments, shock physics codes and the groove widths indicate that multiple fragments (up to 2% of the initial diameter) from each oblique basin-forming impactor, such as the one that formed Imbrium, should have survived planetary collisions and contributed to the heavy impact bombardment between 4.3 and 3.8 billion years ago.
The possibility of life on other worlds has fueled humankind's imagination for centuries. Over the past 20 years, the explosion of discoveries of planets orbiting other stars has sparked the search for worlds like Earth that could sustain life. Most of those candidates were found with other telescopes, including NASA's Kepler space observatory. NASA's Hubble Space Telescope has also made some unique contributions to the planet hunt. Astronomers used Hubble, for example, to make the first measurements of the atmospheric composition of extrasolar planets.
Embarking on the first attempt at detecting the atmospheres of planets outside our solar system, a team of Cambridge and international researchers discovered that the exoplanets TRAPPIST-1b and TRAPPIST-1c, approximately 40 light-years away, are unlikely to have puffy, hydrogen-dominated atmospheres such as those usually found on gaseous worlds like Jupiter or Saturn.
The lack of a hydrogen-helium envelope increases the Earth-likeliness of these planets and has caused considerable excitement among researchers taking part in the study. The results of their findings are published today in the journal Nature.
“Humanity’s remote exploration of alien environments has truly started,” said Amaury Triaud, a research fellow at Cambridge’s Institute of Astronomy. “It is tantalizing to think that with another ten similar observations, we would start distinguishing whether those planets are more Venus-like, more Earth-like, or if they are radically different.”
Researchers observed the planets in near-infrared light and used spectroscopy to decode a change of light as the planets transited in front of their stars. During transit, starlight shines through a planet’s atmosphere making it possible to deduce its chemical makeup.
Both planets orbit TRAPPIST-1 – an ultracool dwarf star that is much cooler and redder than the sun, and barely larger than Jupiter. TRAPPIST-1 has a mass 8% that of the Sun and is located in the constellation of Aquarius. The planets orbiting the star were discovered in late 2015 through a series of observations by the TRAnsiting Planets and PlanetesImals Small Telescope (TRAPPIST), a Belgian robotic telescope located at ESO’s (European Southern Observatory’s) La Silla Observatory in Chile. The small size of the star TRAPPIST-1 boosts the signal produced by the planets’ atmospheres, easing their study by nearly 100 times compared to similar planets orbiting stars like the Sun.
TRAPPIST-1b completes a circuit around its red dwarf star in 1.5 days and TRAPPIST-1c in 2.4 days. Thanks to the faintness of the star they orbit, and to the planet’s short orbits, it is possible that parts of their surfaces have temperatures similar to the Earth. While it remains unclear whether the planets are habitable, they are the first worlds for which we can determine the existence of a habitable climate.
On May 4, astronomers took advantage of a rare simultaneous transit, when both planets crossed the face of their star within minutes of each other, to measure starlight as it filtered through any existing atmosphere. This double transit, which occurs only once every two years, provided a chance to hasten the atmospheric study of TRAPPIST-1b and TRAPPIST-1c.
The researchers now hope to use Hubble to conduct follow-up observations to search for thinner atmospheres, composed of elements heavier than hydrogen, like those of Earth and Venus.
Observations from future telescopes, including NASA’s James Webb Space Telescope, will help determine the full composition of these atmospheres and hunt for potential biosignatures, such as carbon dioxide and ozone, in addition to water vapor and methane. Webb also will analyze a planet’s temperature and surface pressure – key factors in assessing its habitability.
“Our observations demonstrate that Hubble has the capacity to play a central role,” said lead researcher Julien de Wit, of the Massachusetts Institute of Technology. “It can carry-out an atmospheric pre-screening, to tell astronomers which of these Earth-sized planets are prime candidates for more detailed study with the Webb telescope.”
The TRAPPIST telescope identified these two Earth-sized worlds during a prototype run for a more ambitious venture, called SPECULOOS, which is currently in construction at Cerro Paranal, Chile. SPECULOOS will monitor 1,000 nearby red dwarf stars seeking additional Earth-sized worlds.
Professor Didier Queloz, Professor of Physics at the Cavendish Laboratory, and a founding member of the project, said: “Within the next five years, SPECULOOS will likely detect 20-30 new Earth-sized planets. All of them will have atmospheres that can be investigated by the James Webb.”
Dr Brice-Olivier Demory, a senior research associate at the Cavendish Laboratory, said: “Soon we will have the right targets, and the right telescopes to start investigating rocky planet atmospheres beyond our Solar system. Finding out whether other worlds are indeed Earth-like is only a matter of time.”
The Hubble Space Telescope is a project of international cooperation between NASA and ESA. Goddard manages the telescope and STScI conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington.
Two Earth-sized exoplanets have become the first rocky worlds to have their atmospheres studied using the Hubble Space Telescope.Humanity’s remote exploration of alien environments has truly started.Amaury TriaudNASAArtist's View of Planets Transiting Red Dwarf Star in TRAPPIST-1 System
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