Revived Philae poised to do comet science
Nature 522, 7556 (2015). http://www.nature.com/doifinder/10.1038/522263a
Author: Elizabeth Gibney
Comet lander has enough power to do experiments but needs a better communications link.
The mass of the Mars-sized exoplanet Kepler-138 b from transit timing
Nature 522, 7556 (2015). doi:10.1038/nature14494
Authors: Daniel Jontof-Hutter, Jason F. Rowe, Jack J. Lissauer, Daniel C. Fabrycky & Eric B. Ford
Extrasolar planets that pass in front of their host star (transit) cause a temporary decrease in the apparent brightness of the star, providing a direct measure of the planet’s size and orbital period. In some systems with multiple transiting planets, the times of the transits are measurably affected by the gravitational interactions between neighbouring planets. In favourable cases, the departures from Keplerian orbits (that is, unaffected by gravitational effects) implied by the observed transit times permit the planetary masses to be measured, which is key to determining their bulk densities. Characterizing rocky planets is particularly difficult, because they are generally smaller and less massive than gaseous planets. Therefore, few exoplanets near the size of Earth have had their masses measured. Here we report the sizes and masses of three planets orbiting Kepler-138, a star much fainter and cooler than the Sun. We determine that the mass of the Mars-sized inner planet, Kepler-138 b, is Earth masses. Its density is grams per cubic centimetre. The middle and outer planets are both slightly larger than Earth. The middle planet’s density ( grams per cubic centimetre) is similar to that of Earth, and the outer planet is less than half as dense at grams per cubic centimetre, implying that it contains a greater portion of low-density components such as water and hydrogen.
A permanent, asymmetric dust cloud around the Moon
Nature 522, 7556 (2015). doi:10.1038/nature14479
Authors: M. Horányi, J. R. Szalay, S. Kempf, J. Schmidt, E. Grün, R. Srama & Z. Sternovsky
Interplanetary dust particles hit the surfaces of airless bodies in the Solar System, generating charged and neutral gas clouds, as well as secondary ejecta dust particles. Gravitationally bound ejecta clouds that form dust exospheres were recognized by in situ dust instruments around the icy moons of Jupiter and Saturn, but have hitherto not been observed near bodies with refractory regolith surfaces. High-altitude Apollo 15 and 17 observations of a ‘horizon glow’ indicated a putative population of high-density small dust particles near the lunar terminators, although later orbital observations yielded upper limits on the abundance of such particles that were a factor of about 104 lower than that necessary to produce the Apollo results. Here we report observations of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds. The density of the lunar ejecta cloud increases during the annual meteor showers, especially the Geminids, because the lunar surface is exposed to the same stream of interplanetary dust particles. We expect all airless planetary objects to be immersed in similar tenuous clouds of dust.
Astronomy: A Mars-sized exoplanet
Nature 522, 7556 (2015). doi:10.1038/522290a
Authors: Gregory Laughlin
Analysis of Kepler data has yielded the smallest known mass for an exoplanet orbiting a normal star. Its mass and size are similar to those of Mars, setting a benchmark for the properties of exoplanets smaller than Earth. See Letter p.321
Astronomy: Stars seen forming in a far-off galaxy
Nature 522, 7556 (2015). doi:10.1038/522259a
Astronomers have seen their best glimpse yet of stars forming in the early Universe.The ALMA radio telescope in Chile explored the SDP.81 galaxy, which is 3.6 billion parsecs (11.7 billion light years) away from Earth. Its light was magnified and distorted by the gravitational
The presence of a stratosphere can provide clues about the composition of a planet and how it formed. This atmospheric layer includes molecules that absorb ultraviolet and visible light, acting as a kind of ‘sunscreen’ for the planet it surrounds. Until now, scientists were uncertain whether these molecules would be found in the atmospheres of large, extremely hot planets in other star systems.
The results are published today (12 June) in The Astrophysical Journal.
“Detecting the presence of a stratosphere in an exoplanet and the chemical compound causing it is a major advancement in our ability to study exoplanetary atmospheres,” said co-author Dr Nikku Madhusudhan of the Institute of Astronomy at Cambridge.
In Earth’s atmosphere, the stratosphere sits above the troposphere – the turbulent, active-weather region that reaches from the ground to the altitude where nearly all clouds top out. In the troposphere, the temperature is warmer at the bottom – ground level – and cools down at higher altitudes.
The stratosphere is just the opposite. In this layer, the temperature increases with altitude, a phenomenon called temperature inversion. On Earth, temperature inversion occurs because ozone in the stratosphere absorbs much of the sun’s ultraviolet radiation, preventing it from reaching the surface, protecting the biosphere, and therefore warming the stratosphere instead.
Similar temperature inversions occur in the stratospheres of other planets in our solar system, such as Jupiter and Saturn. In these cases, the culprit is a different group of molecules called hydrocarbons. Neither ozone nor hydrocarbons, however, could survive at the high temperatures of most known exoplanets, which are planets outside our solar system. This leads to a debate as to whether stratospheres would exist on them at all.
“Some of these planets are so hot in their upper atmospheres, they’re essentially boiling off into space,” said Avi Mandell, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and a co-author of the study. “At these temperatures, we don’t necessarily expect to find an atmosphere that has molecules that can lead to these multi-layered structures.”
Using NASA’s Hubble Space Telescope, the researchers have settled this debate by identifying a temperature inversion in the atmosphere of WASP-33b, which has about four-and-a-half times the mass of Jupiter. Team members also think they know which molecule in WASP-33b’s atmosphere caused the inversion – titanium oxide.
“These two lines of evidence together make a very convincing case that we have detected a stratosphere on an exoplanet,” said Korey Haynes, lead author of the study. Haynes was a graduate student at George Mason University in Fairfax, Virginia, and was working at Goddard with Mandell when the research was conducted.
The researchers analysed observations made with Hubble’s Wide Field Camera 3 by co-author Drake Deming at the University of Maryland. Wide Field Camera 3 can capture a spectrum of the near-infrared region where the signature for water appears. Scientists can use the spectrum to identify water and other gases in a distant planet’s atmosphere and determine its temperature.
Haynes and her colleagues used the Hubble observations, and data from previous studies, to measure emission from water and compare it to emission from gas deeper in the atmosphere. The team determined that emission from water was produced in the stratosphere at about 3300 degrees Celsius. The rest of the emission came from gas lower in the atmosphere that was at a temperature about 1650 degrees Celsius.
The team also presented the first observational evidence that WASP-33b’s atmosphere contains titanium oxide, one of only a few compounds that is a strong absorber of visible and ultraviolet radiation and capable of remaining in gaseous form in an atmosphere as hot as this one.
“Understanding the links between stratospheres and chemical compositions is critical to studying atmospheric processes in exoplanets,” said Madhusudhan. “Our finding marks a key breakthrough in this direction.”
Inset image: NASA scientists detected a stratosphere and chemical compounds on WASP-33b by measuring light emitted from the dayside atmosphere of the planet observed as it passed behind its star (top). Temperatures in the stratosphere increase with height (right) because of molecules absorbing radiation from the star entering from the top and reemitting it locally; otherwise, temperatures would cool down at higher altitudes (left). Credit: NASA/GSFC
On a blazing-hot exoplanet known as WASP-33b, a team of astronomers including researchers from the University of Cambridge has detected a stratosphere, one of the primary layers of Earth’s atmosphere.Understanding the links between stratospheres and chemical compositions is critical to studying atmospheric processes in exoplanetsNikku Madhusudhan NASA/GSFCOn a massive planet around a nearby star, NASA’s Hubble Space Telescope has detected a stratosphere, one of the primary layers of the atmospheres of Earth and other planets in our solar system.
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Researchers using NASA's Hubble Space Telescope have detected a stratosphere and temperature inversion in the atmosphere of a planet several times the mass of Jupiter, called WASP-33b. Earth's stratosphere sits above the troposphere, the turbulent, active-weather region that reaches from the ground to the altitude where nearly all clouds top out. In the troposphere, the temperature is warmer at the bottom ground level and cools down at higher altitudes. The stratosphere is just the opposite: There, the temperature rises at higher altitudes. This is called a temperature inversion, and it happens because ozone in the stratosphere absorbs some of the sun's radiation, preventing it from reaching the surface and warming this layer of the atmosphere. Similar temperature inversions occur in the stratospheres of other planets in our solar system, such as Jupiter and Saturn. But WASP-33b is so close to its star that its atmosphere is a scathing 10,000 degrees Fahrenheit, and its atmosphere is so hot the planet might actually have titanium oxide rain.