New Horizons approaches the Pluto system

Surprise Discovery in Comet’s Coma

Scientists from the European Space Agency’s Rosetta mission investigating the surface of comet 67P/Churyumov-Gerasimenko published some surprising results this week in the journal Astronomy and Astrophysics. Examination of the comet’s atmosphere, or coma, revealed a different mechanism for its origin than previously assumed. Rather than photons from the Sun breaking apart water and carbon dioxide on the comet’s surface, it appears that energized electrons close to the surface are actually causing the rapid molecular split and spewing the resulting gas from vents on 67P’s surface.

Basically, heat from the Sun isn’t causing ice on the comet to sublimate directly from the surface to create the coma and part of the tail. Instead, the Sun’s electric field is energizing the molecules’ split through interactions with electrons.

Since last August, the ESA’s Rosetta has orbited within 100 miles of comet 67P, studying its environment with 11 instruments. Among these, the NASA provided Alice spectrograph is examining the chemical composition of the comet’s atmosphere in far-ultraviolet wavelengths, which allows mission scientists to identify highly abundant elements like carbon, nitrogen, oxygen, and hydrogen.

According to Joel Parker of Southwest Research Institute, by examining the spectral emission from hydrogen and oxygen atoms that have broken away from the water molecules, Alice’s data suggests that much of the water and carbon dioxide in the comet's coma actually originates from plumes erupting from its surface.

The breakup of those molecules is similar to that for the plumes on Europa, except that the electrons at the comet’s surface are energized by solar radiation, while the electrons at Europa come from Jupiter’s magnetosphere, explains Dr. Paul Feldman, the paper’s lead author.

"The discovery we're reporting is quite unexpected," said Alan Stern, principal investigator for the Alice instrument. "It shows us the value of going to comets to observe them up close, since this discovery simply could not have been made from Earth or Earth orbit with any existing or planned observatory. And, it is fundamentally transforming our knowledge of comets." –Elise Ricard

Odd Movements of Pluto’s Moons

As the New Horizons mission closes in on Pluto, scientists here on Earth are uncovering more strangeness, further proving that this dwarf planet system is more exception than rule.

First, the Pluto-Charon system may be a more appropriate name for New Horizons’ target since Charon orbits Pluto only slightly more than Pluto orbits Charon. With a “moon” as large as Charon and a “pla-“ sorry, *ahem* “dwarf planet” as small as Pluto, this is considered by many to be a binary system. The evidence lies in the orbits of the outer, newly discovered moons: Styx, Nix, Hydra, and Kerberos. None of these moons truly orbit Pluto, they orbit the center of mass of the Pluto-Charon system which is between the two. These moons seem to have some strange traits as well: Two of them (Nix and Hydra) seem quite bright, like Charon, but Kerberos seems dark and is raising many questions about how such a disparate system might have formed.

Perhaps the most interesting finding (all reported this week in Nature), is that the rotations of Nix and Hydra seem chaotic and are frequently torqued by the gravitational pull of Pluto and Charon. They tumble and wobble! This is very different than the rotations of almost every other moon in the Solar System—most are “tidally locked” as the Moon is to Earth, meaning that we always see the same side of our moon.

A few patterns still hold in this surprising system, though. The moons of Pluto-Charon have a 3:4:5:6 orbital resonance or very close to it. That means that for every rotation of Pluto-Charon, Styx orbits three times, Nix orbits four, Hydra, five and Kerberos, six. This is similar to Jupiter’s major moons Io, Europa and Ganymede, which have a 1:2:4 orbital resonance.

When New Horizons passes by Pluto in a month, we will gain more unprecedented views into the nature of this unusual and important system. The only certainty is that we will find more amazing things to discover! –Josh Roberts

A Star Gets Nasty

In 1867, French astronomers Charles Wolf and Georges Rayet discovered a rare type of star ejecting its outer shell of hydrogen and exposing its hot, helium-burning core. Now referred to as Wolf-Rayet stars, these are thought to be huge stars, at least 20 times the mass of the Sun, producing powerful stellar winds and blowing their hydrogen off before exploding as supernovae.

In 1963, Jason Nassau and Charles Stephenson identified another Wolf-Rayet star, adding to the roughly 500 now known within the Milky Way Galaxy. Recent observations using the Hubble Space Telescope revealed that the star, NaSt1 (a catalog name derived from the first two letters of each discoverer’s name and playfully pronounced “Nasty 1” by scientists), isn’t a typical Wolf-Rayet. Obscured by gas and dust, Nasty 1 isn’t easy to observe, and measurements of its characteristics are difficult, but the study team, led by Jon Mauerhan of the UC Berkeley, found something unexpected.

Mauerhan’s team thought they would detect a twin-lobed structure similar to that associated with the Wolf-Rayet candidate star Eta Carinae. Such structures are thought to be typical of stars that eject gas—from Wolf-Rayets to smaller stars forming planetary nebulae. Instead, the astronomers found that Nasty 1 appears to be surrounded by a flat disk nearly two trillion miles wide, or nearly 1000 times the diameter of our solar system.

Rotating much more slowly than material being ejected by a stellar wind, this disk flies in the face of the previously-held theory. Mauerhan’s group suggests that it may be evidence of a short-lived process in the formation of Wolf-Rayet stars in binary systems, where the companion of a massive star draws off matter from its larger partner. This process of “stellar cannibalism” strips the more massive star of its hydrogen envelope, exposing the helium core. The process isn’t very efficient, however, and some of the mass being exchanged sloppily spills out to form the colossal disk. Since an estimated 70% of large stars belong to binary systems, this proposed mechanism for Wolf-Rayet formation is finding favor among astronomers.

Where the process goes from here is unknown. Due to the mass it has stolen from the Wolf-Rayet star, will the companion explode? Or will the newly-formed Wolf-Rayet star itself explode? Will the two stars merge? Whatever happens, it won’t be something that will be observed soon—according to Mauerhan, the process may last 100,000 years.

The Berkeley team’s findings were published in a recent edition of the Monthly Notices of the Royal Astronomical Society. –Bing Quock

Circular Orbits and Habitability

Digging into exoplanet data collected by Kepler over the past four years, MIT’s Vincent Van Eylen finds that circular orbits, such as the planets’ orbits in our own solar system, could be an indicator of a planet’s habitability.

“Twenty years ago, we only knew about our solar system, and everything was circular and so everyone expected circular orbits everywhere,” he says. “Then we started finding giant exoplanets, and we found suddenly a whole range of eccentricities, so there was an open question about whether this would also hold for smaller planets. We find that for small planets, circular is probably the norm.”

Van Eylen and his colleagues observed data for 28 stars with known Earth-sized planets in their systems, and determined that the orbits of the 74 exoplanets around them were indeed circular.

Ultimately, Van Eylen says that's good news in the search for life elsewhere. Among other requirements, for a planet to be habitable it would have to be about the size of Earth—small and compact enough to be made of rock, not gas. If a small planet also maintained a circular orbit, it would be even more hospitable to life, as it would support a stable climate year-round. (In contrast, a planet with a more eccentric orbit might experience dramatic swings in climate as it orbited close in, then far out from its star.)

“If eccentric orbits are common for habitable planets, that would be quite a worry for life, because they would have such a large range of climate properties,” Van Eylen says. “But what we find is, probably we don't have to worry too much because circular cases are fairly common.” –Molly Michelson

Image: Pluto and its moons, Dan Tell, California Academy of Sciences

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