Putting Exoplanets on the Scale

Astronomers have come up with a new technique for measuring an alien planet’s mass, and therefore its composition and potential habitability, even when standard methods don’t work.

You’d never ask a woman her weight. (Actually, you’d never ask a man either.) But for exoplanets, the answer is crucial.

Hot Jupiter

De Wit and Seager (both MIT) have found a new way to measure exoplanets’ mass. For now, the technique is largely only useful for hot Jupiters, but future telescopes will allow them to scale the technique to nearby Earth-sized planets.
C. Carreau / ESA / Nature

Knowing the mass of an alien planet, along with its radius, tells astronomers whether the exoplanet is rocky, watery, or gaseous. In modeling, the mass can even influence whether the planet could host magnetic fields or plate tectonics. In the December 20th Science, Julien de Wit and MacArthur Fellow Sara Seager (both MIT) have found a forward-thinking technique to measure the all-important number.

To measure the mass of an alien planet dozens or hundreds of light-years away, astronomers usually watch for the star’s wiggle in response to the planetary tug. The wiggle comes from the fact that planets don’t actually orbit their parent stars — the star and planet orbit a mutual center of mass. So as the planet orbits, the star orbits too, albeit in a much smaller circle. (The mutual center can be inside the star itself, even if not at the core.)

Stellar wobble is easiest to see when massive planets orbit close to their parent stars, such as with hot Jupiters. But the punier the planet and the farther it orbits, the more weakly the star wobbles. And even if we could measure a star’s slowest sway, which we can’t yet (current telescopes can only measure a star’s velocity down to about 1 mile per hour), starspots and other activity on the star’s surface might cover up a planet-induced signal. That’s the case for hot Jupiter WASP-33, whose parent star pulsates strongly.

A New Way to Weigh Exoplanets

With some creative thinking, de Wit and Seager have found a new way to weigh an exoplanet: by watching it transit across the face of its parent star. When a planet transits, starlight passes through the thin sliver of its outer atmosphere. This passage imprints the starlight with the chemical fingerprints of the atmosphere’s composition. Astronomers collect thistransmission spectrum during transit observations, measuring which wavelengths pass through the atmosphere and which are absorbed.

Transiting exoplanet

When an exoplanet transits across the face of its parent star, starlight passes through a thin sliver of its atmosphere. De Wit and Seager show how measuring the atmosphere’s extent can give the planet’s mass.
NASA Goddard Space Flight Center

De Wit and Seager determined that, because a more massive planet can collect a thicker gaseous shroud around itself, using these transmission spectra to measure the atmosphere’s extent could in theory also reveal the planet’s mass.

It’s not a clear-cut connection: the atmosphere’s extent also depends on its temperature (heat comes from the parent star and the planet itself) and the weight of its molecules. A hot Jupiter will have light molecules in its atmosphere, mostly hydrogen and helium. But a rocky planet’s atmosphere will be composed of heavier stuff such as oxygen, carbon dioxide, and nitrogen. De Wit and Seager show that they can calculate each of these parameters directly from the same transmission spectrum.

This isn’t just a theoretical exercise: the astronomers used the well-studied hot Jupiter HD 189733b to prove the technique works. Radial velocity had already shown the planet has a mass between 1.1 and 1.2 Jupiters; de Wit and Seager’s transmission spectroscopy calculations gave a mass of 1.15 Jupiters.

“This [technique] appears to be very promising,” says exoplanetary scientist Jonathan Fortney (University of California, Santa Cruz). “I think that in practice clouds will confound the technique a bit more than they suggest, but I think it will work a lot of the time.”

Scaling Down

TESS satellite

The TESS satellite will monitor bright, nearby stars for transiting exoplanets. Though its focus is Sun-like stars, it will also catch some 10,000 M dwarfs in its view.
TESS team

For now, hot Jupiters remain the main target for the new technique. Their puffy, extended atmospheres make them good targets for transmission spectroscopy. But de Wit and Seager aim to describe super-Earths and Earth-size planets. Current telescopes can’t take transmission spectra of these smaller words, but the upcoming James Webb Space Telescope could. Scheduled for launch in 2018, it will be able to weigh Earth-size planets orbiting the coolest M dwarf stars up to about 160 light-years away. Its reach will extend twice that far for super-Earths.

Unfortunately, that meansKepler’s 167 confirmed exoplanets and 3,500 candidates will be largely out of reach.

“You wouldn’t be able to do this for stars in the Kepler field, which are too far away and faint,” Fortney explains. “But Kepler wasn’t designed to find planets around bright stars that would be amenable to atmospheric characterization.” Instead a number of upcoming projects, including SPECULOOS and TESS (launching in 2017), will focus on finding planets orbiting nearby stars, whose atmospheres will be easier to study.

The ultimate goal. says de Wit, is to combine all the available data for an exoplanet — including both the wobble of its star and the extent of its atmosphere — to better estimate its mass and understand its atmosphere . The more tools in astronomers’ toolbox, the better, because it’s not enough to just count exoplanets anymore, it’s time to see what they’re made of.

Extract from Sky & Telescope Magazine December 2013
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Curiosity Finds a Once-Habitable Mars

On the floor of Gale crater, NASA’s newest rover is finding that ancient Mars seems to have had an environment quite conducive to microbial life.

The researchers coordinating NASA’s $2.5 billion Mars Science Laboratory have always been careful to note that their beefy Curiosity rover is not searching for life on the Red Planet. Rather, it’s designed to find out whether Mars was ever suitable for life.

After a year of zapping, scratching, sniffing, and tasting rocks and sand near the rover’s landing site, the answer is “yes.” A flurry of findings published in the December 9th issue of Sciencesimultaneously announced at December’s meeting of the American Geophysical Society, provide the best evidence yet that ancient Mars was indeed habitable.

Yellowknife Bay on Mars

Curiosity’s Mast Camera recorded this view of sedimentary deposits inside Gale crater in February 2013. The mudstone ledge at lower right is about 20 cm (8 inches) high. Click here for a larger view.
NASA / JPL-Caltech / MSSS

Curiosity dropped onto the broad floor of Gale crater on August 6, 2012, then spent many months exploring intriguing rocky outcrops in a nearby expanse dubbed Yellowknife Bay. Mission scientists soon realized that much of the terrain was covered in mudstone, silty sediments that settled onto the bottom of an ancient lake.

What’s now clear, as reported by one research team led by project scientist John Grotzinger (Caltech) and a second by David Vaniman (Planetary Science Institute), is that the sediments contain an iron- and sulfur-rich clay called smectite. Moreover, this clay formed in water with a neutral pH and low salinity — just the kind of benign habitat that primitive life forms called chemolithoautotrophs would want. Such microbes derive their energy from the oxidation of inorganic compounds and their carbon from atmospheric carbon dioxide.

A separate analysis by Kenneth Farley (Caltech) and others used isotopic ratios — never measured before by a Martian lander — to estimate the age of a mudstone slab nicknamed Cumberland. It’s between 3.86 and 4.56 million years old, confirming that Gale crater formed very early in Martian history.

But Farley and his team also tested for elemental isotopes produced by the potent cosmic rays that constantly bombard the Martian surface. Cumberland’s “exposure age” is comparatively young, only 60 to 100 million years. Apparently the sediments in Yellowknife Bay spent eons buried under the protective cover of overlying material, which eventually was stripped away by the planet’s incessant winds only in the recent geologic past.

Biologically speaking, this is great news. It means the rover has at least a chance to detect organic matter that might have become trapped in these ancient sediments. In fact, a research team led by Douglas Ming (NASA Johnson Space Center) reports that Curiosity continues to detect chlorinated hydrocarbons in samples of the Martian surface – and that it can’t all be contaminants brought from Earth. Instead, these simple organics might be indigenous to Mars or else hitchhiked there inside meteorites.

In an unusual move for Science, all six of its just-published Curiosity articles are freely available online. You can also watch a press briefingheld during the AGU meeting.

There’s water – is there life?

Hubble Discovers Water Plumes Erupting from Europa

It’s been known since 2005 that Saturn’s 300-mile-wide moon Enceladus has geysers spewing ice and dust out into orbit from deep troughs that rake across its south pole. Now, thanks to the Hubble Space Telescope, we know of another moon with similar jets: Europa, the ever-enigmatic ice-shelled moon of Jupiter.

Read more:http://www.universetoday.com/107144/hubble-discovers-water-plumes-erupting-from-europa/#ixzz2nHZp0ONc