UFO'S of UAP'S, ASTRONOMIE, RUIMTEVAART, ARCHEOLOGIE, OUDHEIDKUNDE, SF-SNUFJES EN ANDERE ESOTERISCHE WETENSCHAPPEN

However, so far, exomoons have eluded confirmed discovery, said Ji-Wei Xie, an astronomer at Nanjing University in China and a co-author of the new study. He and his colleagues wanted to uncover a potential explanation for why exomoons remain elusive. One possibility is that exomoons are difficult to see and thus simply haven't been detected yet. But another possibility, Xie said, is that, for some reason, exomoons are rare around the exoplanets that astronomers have spotted.

Most of the exoplanets that scientists have detected so far are close to their host stars, because those types of planets are easier to spot using the planet-hunting strategies astronomers have been able to employ. For instance, the radial velocity method looks for repeated wobbles in a star's movements that are signs of a planet's gravitational pull yanking it back and forth, and this quivering is bigger the closer an exoplanet is to its star. In addition, the transit method looks for dips in a star's brightness whenever a planet crosses in front of it, and such darkening is more frequent the nearer an exoplanet is to its star.

Planets that orbit close to their stars are naturally exposed to more high-energy radiation than ones farther away. The authors of the new paper reasoned that, over time, those higher levels of radiation might blast away the atmospheres of exoplanets that are close to their stars; in fact, this idea has popped up in other exoplanet studies as well. Such a "photo-evaporation" process could, in principle, make the orbits of moons around those exoplanets unstable, Xie said.

To see how strong an effect a star might have on an exoplanet's moons, Xie and his colleagues simulated a range of Neptune-like gas giant planets about 14 to 20 times the mass of Earth, each of which orbited a star about 10 times closer than Earth does the sun. The planets started out with up to 500 moons. The scientists simulated what would happen if photo evaporation gradually eroded away the gas planet's atmosphere, reducing the planet's total mass by half over time.

The scientists found that photo evaporation could play a major role in the fate of an exoplanet's moons. As photo evaporation caused the simulated exoplanet to lose mass, the planet's gravitational pull weakened and the orbits of the exomoons grew and became more eccentric, or oval-shaped. Eventually, many of those orbits became unstable, meaning the moons could exit their orbits around those planets altogether. Xie said the simulations could leave few or no moons around the planets.

In the scenario where planets had 500 small moons, the researchers found that only about a quarter of the moons still remained in orbit around their worlds by the end of the simulations. About half of these moons collided with their planets, while a quarter of the moons escaped their planets to become new planet-like objects circling their stars. In the simulations, some of the moons flew away on trajectories that could make them go on to become free-floating "rogue planets" that wander the galaxy.

In scenarios that included larger moons with masses up to twice that of Earth, even fewer moons remained in stable orbits around their evaporating planets. In these scenarios, in addition to escaping from or colliding with their planets, moons also collided with one another, the researchers found.

All in all, these findings "can help guide the future search for exomoons," Xie said. For instance, their findings suggest that exomoon hunters should avoid exoplanets that are very close to their stars. This holds especially true for red dwarf stars, which are luminous with X-rays that enhance photo evaporation, he said.

As for whether habitable exomoons are possible, Xie noted that sun-like stars possess habitable zones that are about as far away from them as Earth is from the sun. At such distances, "the photo-evaporation effect is very limited, and thus our findings will not apply to them," Xie said.

However, red dwarfs, which constitute up to 70 percent of the stars in the cosmos, are up to 50 times fainter than the sun, and so their habitable zones are much closer to them. "Our findings would suggest that there would be less habitable moons around red dwarfs due to the photo-evaporation effect," Xie said.

The scientists detailed their findings online Dec. 1 in The Astrophysical Journal.

Follow Charles Q. Choi on Twitter @cqchoi. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

Credit: NASA

Life on Earth shows up in surprising places. It's been found in high-temperature vents deep undersea and high in the air. But we're still trying to learn more about these so-called "extremophiles." Researchers are now pondering how well can life reproduce in these environments. Also, could microbes of this type be found on other worlds?

In March, a group of University of Houston students — piggybacking on a payload with a prime mission to scope out auroras — will fly a high-altitude experiment from Alaska to see what microbes are in the high atmosphere, between 18 km and 50 km (11 miles and 31 miles) from the ground. The instrument, which looks almost like a small laundry hamper, pops open to collect what's in the atmosphere. Then, as the balloon descends, it shuts closed for researchers to analyze.

Jamie Lehnen, a fourth-year student on the team, says this system could be less open to contamination than pumps and other complicated mechanisms that require servicing on Earth. But it's the first time her group has used it, so isn't sure how well it will function. If it does, however, she's interested in learning about how microbes will react under the stresses of living at high altitudes.

"A lot of times, these microbes when they go up there, they shut down. They are not replicating and they are not metabolically active," she said. "I'm interested in how their stress response is similar to those [microbes] back on Earth's surface."

Some of the earliest high-altitude microorganism experiments did not involve air travel at all — Charles Darwin picked up African dust on his ship while crossing the Atlantic Ocean, while Louis Pasteur made measurements on top of alpine glaciers. Both found microorganisms.

**Researchers commonly search for microbes using high-altitude balloons, but airplanes are also a possibility.**
**Credit: NASA**

That said, microorganism research in the upper atmosphere has been active since the 1930s at least. One of the earliest flights involved Charles Lindbergh, a pilot best known for piloting the Atlantic solo in 1927. Accompanied with his wife, Lindbergh periodically passed the monoplane controls over to her to take samples from the atmosphere around them. The research team found spores of fungi and pollen grains, among other specimens.

Planes still require a substantial amount of atmosphere to fly, so it's with high-altitude balloons and rockets that we can get even higher — to the stratosphere and the mesosphere. According to NASA microbial researcher David Smith, some of the pioneering work in this field was done in the 1970s, particularly in Europe and the Soviet Union. "Everything they did was fascinating, but there hasn't been a lot of follow-up work to validate the results of those collections," he told Seeker.

There are open questions about how valid these early results are, given that contamination protocols may not have been strict. So Smith and other researchers are trying to figure out what kind of microbes live above Earth, and for how long. In May and June, Smith's team will fly with the team from NASA ABoVE (Arctic-Boreal Vulnerability Experiment), which uses a Gulfstream III jet to monitor how climate change affects animals, plants, the environment and infrastructure. In the spring, a vast airstream on the Pacific Ocean moves millions of tons of dust across the ocean, mostly from Asia.

"We want to know what kind of microorganisms are making that leap across the ocean, co-transported with aerosol species," Smith said. "Alaska will allow us an opportunity to test the atmospheric bridge hypothesis, which simply speaking, is continents sneezing on each other."

Smith's team will use a cascade sampler for collection, which passes air through progressively finer impact plates with holes in them, he said. As the air moves through, dust and any microorganisms impact the surface of those plates. A portion of them stick to the surface, allowing researchers to analyze what is there afterwards.

In ultraviolet light (as seen here by the Venus Express mission), Venus has mysterious dark streaks that absorb UV radiation. Some researchers have suggested this could be life in the upper atmosphere, but more research is needed.
**Credit: ESA/MPS/DLR/IDA**

Smith is skeptical that microorganisms are growing or dividing at such high altitudes, because it's so cold and dry up there. But he says that microorganisms may be "persisting", or lingering and not being killed. "Nobody's been able to measure how long microorganisms can stay in the stratosphere. There's works that still needs to be done."

"Virtually all terrestrial and marine surfaces have microorganisms associated with them that can get disattached from the surfaces by wind or other physical disturbances," wrote Aarhus University assistant professor Tina Santl-Temkiv, who has studied microorganisms in hailstones, in an e-mail to Seeker.

"[They] can reach higher levels of troposphere, above around one kilometer, can stay suspended in air for around a week and can travel thousands of kilometers, riding on wind currents. Eventually, they get deposited back to the ground wither through the formation of rain or simply due to gravity."

If Earth's atmosphere is shown to be a great spot for life to divide, however, it could have implications for locations such as Venus. Back in the 1960s, astronomer and science popularizer Carl Sagan suggested that the upper atmosphere of Venus could harbor the descendants of organisms that could have evolved on the surface of the planet when it was cooler.

Even though today the surface can crush and cook unprotected spacecraft, 50 kilometers (31 miles) above is more temperate. Moreover, researchers have found an intriguing substance that blocks ultraviolet light in Venus' clouds. Life hasn't yet been ruled out as a possibility.

"Venus and Earth were similar for 3 billion years [of their evolution] and perhaps as recently as up to about half a billion years ago," said Dr. Lynn Rothschild, a NASA astrobiologist and synthetic biologist that is on Smith's research team. She said this includes liquid oceans, similar atmosphere, and probably the same sorts of minerals and organic compounds as well.

But Venus would be a difficult prospect if the life returns to the surface. The sun got more luminous as the solar system aged, evaporating the water from Venus' oceans. The water vapor, now in the atmosphere, contributed to giving Venus a hellish greenhouse effect on its surface.

It seems that life is hardy, but we don't know if it's tough enough to survive living high above a planetary surface. If it does, however, that could mean that even missions that sample a planet's atmosphere could have to worry about protections against hurting possible life. We'll have to see what these new experiments yield, though, before reaching any conclusions.

WATCH VIDEO: The Mystery Of Venus' Green Glow

Originally published on Seeker.