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Astrophysicists May Soon Announce Earth's Twin Around a Distant Star --"Exotic Clouds or Haze a Key" Posted: 30 Oct 2015 09:09 AM PDT
Scientists have cataloged nearly 2,000 exoplanets around stars near and far. While most of these are giant and inhospitable, improved techniques and spacecraft have uncovered increasingly smaller worlds. The day may soon come when astrophysicists announce our planet's twin around a distant star. But size alone is insufficient to judge a globe. Though Earth and Venus are nearly identical in size, the latter's surface is hot enough to melt lead. Astronomers must gather information about an exoplanet's atmosphere, often through observing how the planet scatters or absorbs light from its parent star. But, that information is not always useful -- as is the case with the exoplanet GJ1214b."When an exoplanet passes in front of its star, light can be absorbed at some wavelengths by molecules in the atmosphere, which we can analyze by looking at how light passes through the planet's atmosphere," said Benjamin Charnay, a postdoctoral researcher in the University of Washington Department of Astronomy. "But for this planet, when researchers previously looked with the Hubble Space Telescope, they saw almost no variation with wavelength of light." This "flat spectrum" for GJ1214b indicated that something in the planet's upper atmosphere blocked light, keeping scientists in the dark regarding its atmosphere. Charnay decided to computationally model what its atmosphere could be, based on the planet's temperature and composition. In the process, as he reports in a new paper in Astrophysical Journal Letters, he and his collaborators became the first to simulate three-dimensional exotic clouds in the atmosphere of another world. "It's an important step in characterizing exoplanets," said Charnay. GJ1214b was among the first "mini-Neptune" exoplanets discovered, which are intermediate in size between Earth and Neptune. They're the smallest exoplanets that can be studied with existing technology, and GJ1412b is in an ideal position. "Most of the other mini-Neptunes that have been discovered orbit stars between 100 and 1,000 light years away," said Charnay. "GJ1214b is quite close to Earth, just 42 light years away, and it orbits its star in just 1.6 days." That fast orbit gave scientists the opportunity to record the exoplanet's flat spectrum, ruling out an atmosphere of simple hydrogen, water, carbon dioxide or methane. Instead, something high in the atmosphere was blocking light from penetrating farther down. "There could either be high clouds in the atmosphere or an organic haze -- like we see on Titan," said Charnay. Its atmospheric temperature exceeds the boiling point of water. As a result, if GJ1214b sported clouds, they would probably be some form of salt, said Charnay. But such clouds should form deep in the atmosphere, much lower than the altitude where they are observed. Charnay modeled how the clouds could form in the lower atmosphere and then rise into the upper atmosphere with sufficient circulation. To accomplish this, Charnay, used a climate model developed by his former research group in Paris. He previously used this model for studying Titan and the early Earth, and adapted it for GJ1214b. Charnay ran his three-dimensional cloud model on the UW's Hyak supercomputer. It shows how GJ1214b could create, sustain and lift salt clouds into the upper atmosphere, where they would contribute to the planet's flat spectrum that Hubble detected. His model also makes specific predictions about the effect these clouds will have on the planet's climate and the types of information that future telescopes, like the James Webb Space Telescope, will be able to gather. Charnay would next like to model the other potential cause of the exoplanet's flat spectrum: photochemical haze, which gives Titan its shrouded orange atmosphere and Los Angeles its persistent dome of polluted air. "Light splits chemicals in the atmosphere, creating more complex organic compounds that make the haze," said Charnay. Charnay will have to wait until the James Webb Space Telescope launches later this decade to find out which theory -- clouds or haze -- gives GJ1214b a flat spectrum. In the meantime, in addition to his quest to simulate haze on this exoplanet, Charnay would like to model what the atmosphere was like on Earth before life evolved. "Worlds like Titan (shown at top of page) and this exoplanet have complex atmospheric chemistry that might be closer to what early Earth's atmosphere was like," said Charnay. "We can learn a lot about how planetary atmospheres like ours form by looking at them." The Daily Galaxy via University of Washington Image credit: NASA |
NASA: "Spiral Stars Signal the Presence of Giant Planets" Posted: 30 Oct 2015 08:29 AM PDT
Though astronomers have cataloged thousands of planets orbiting other stars, the very earliest stages of planet formation are elusive because nascent planets are born and embedded inside vast, pancake-shaped disks of dust and gas encircling newborn stars, known as circumstellar disks. The conclusion that planets may betray their presence by modifying circumstellar disks on large scales is based on detailed computer modeling of how gas-and-dust disks evolve around newborn stars, which was conducted by two NASA Hubble Fellows, Ruobing Dong of Lawrence Berkeley National Laboratory, and Zhaohuan Zhu of Princeton University. Their research was published in the Aug. 5 edition of The Astrophysical Journal Letters. "It's difficult to see suspected planets inside a bright disk surrounding a young star. Based on this study, we are convinced that planets can gravitationally excite structures in the disk. So if you can identify features in a disk and convince yourself those features are created by an underlying planet that you cannot see, this would be a smoking gun of forming planets," Dong said. Identifying large-scale features produced by planets offers another method of planet detection that is quite different from all other techniques presently used. This approach can help astronomers find currently-forming planets, and address when, how, and where planets form. Gaps and rings seen in other circumstellar disks suggest invisible planets embedded in the disk. However gaps, presumably swept clean by a planet's gravity, often do not help show location of the planet. Also, because multiple planets together may open a single common gap, it's very challenging to estimate their number and masses. Ground-based telescopes have photographed two large-scale spiral arms around two young stars, SAO 206462 and MWC 758. A few other nearby stars also show smaller spiral-like features. "How they are created has been a big mystery until now. Scientists had a hard time explaining these features," Dong said. If the disks were very massive, they would have enough self-gravity to become unstable and set up wave-like patterns. But the disks around SAO 206462 and MWC 758 are probably just a few percent of the central star's mass and therefore are not gravitationally unstable. The team generated computer simulations of the dynamics of a disk and how the star's radiation propagates through a disk with embedded planets. This modeling created spiral structures that very closely resemble observations. The mutual gravitational interaction between the disk and the planet creates regions where the density of gas and dust increases, like traffic backing up on a crowded expressway. The differential rotation of the disk around the star smears these over-dense regions into spiral waves. Although it had been speculated that planets can produce spiral arms, we now think we know how. "Simulations also suggest that these spiral arms have rich information about the unseen planet, revealing not only its position but also its mass," Zhu said. The simulations show that if there were no planet present, the disk would look smooth. To make the grand-scale spiral arms seen in the SAO 206462 and MWC 758 systems, the unseen planet would have to be bulky, at least 10 times the mass of Jupiter, the largest planet in our solar system. The first planet orbiting a normal star was identified in 1995. Thanks to ground-based telescopes and NASA's Kepler mission, a few thousand exoplanets have been cataloged to date. But because the planets are in mature systems, many millions or a few billion years old, they offer little direct clues as to how they formed. "There are many theories about how planets form but very little work based on direct observational evidence confirming these theories," Dong said. "If you see signs of a planet in a disk right now, it tells you when, where, and how planets form." Astronomers will use the upcoming NASA James Webb Space Telescope to probe circumstellar disks and look for features, as simulated by the modeling, and will then try to directly observe the predicted planet causing the density waves. A computer model image at the top of the page reproduces the two-spiral-arm structure; the "x" is the location of a putative planet. The planet, which cannot be seen directly, probably excites the two spiral arms. The Daily Galaxy via NASA/Goddard Space Flight Center Photo Credit: NASA, ESA, ESO, M. Benisty et al. (University of Grenoble), R. Dong (Lawrence Berkeley National Laboratory), and Z. Zhu (Princeton University) |
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