by Nola Taylor Redd

Contributor

27 April 2012
from SPACE Website

 

 

 

Artist's impression of sunrise from the surface of Gliese 581c,

an Earthlike planet in the habitable zone of its red dwarf star.
CREDIT: Karen Wehrstein

 


A new estimate of the number of habitable planets orbiting the most common type of stars in our galaxy could have huge consequences for the search for life.

According to a recent study, tens of billions of planets around red dwarfs are likely capable of containing liquid water, dramatically increasing the potential to find signs of life somewhere other than Earth.

Red dwarfs are stars that are fainter, cooler and less massive than the sun.

 

These stars, which typically also live longer than Class G stars like the sun, are thought to make up about 80 percent of the stars in the Milky Way, astronomers have said.
 

 

 

 

A second look

Red dwarfs generally have not been considered viable candidates for hosting habitable planets.

 

Since red dwarfs are small and dim, the habitable zone surrounding them - the region where an orbiting planet's surface water can remain liquid - is relatively close to them.

"The habitable zone would be very, very small. Consequently, the chances that you would actually find any planet at the right distance from the sun to be attractive to life was likely to be small, too," said Seth Shostak, a senior astronomer at the Search for Extraterrestrial Intelligence Institute in Mountain View, Calif. [The Strangest Alien Planets]

But the study, based on data from the European Space Agency's HARPS spectrograph in Chile, used a sample of 102 red dwarfs to estimate that 41 percent of the dim stars might be hiding planets in their habitable zone.

"The number of habitats might increase by a factor of 8 or 10," Shostak told SPACE.com.
 

 

 

Difficult environments

One of the largest concerns about planets circling red dwarfs is radiation. A red dwarf's habitable zone is generally closer to it than Mercury is to our sun, so a planet there would receive a strong shock of particles when storms erupted on the red dwarf.

"They could essentially give everything on the surface that's exposed to the sky... a heavy dose of radiation," Shostak said. "It could be fatal."

However, if the alien planet had a magnetic field, this could provide some protection. So, too, could an ocean of water. Life that evolved beneath an ocean might be shielded from the brunt of the radiation.

That's not necessarily good news for SETI, which searches for signals from extraterrestrial life.

"We're not sure intelligent life, if under water, will be building radio transmitters and we're going to hear from them," Shostak said. "But it's possible."

Another problem with planets tightly bound to their host star is a phenomenon known as tidal locking, in which one side of the world is perpetually turned toward the sun and receiving almost all of the heat.

But this isn't considered as big of a problem now as it had been.

For one thing, research over the past few years has indicated that the presence of other planets can ease the grip of the parent star, keeping a planet from being perfectly stagnant.

Furthermore, if the planet has an atmosphere, it might also boast wind, which could move the hot atmosphere to the dark side and the cool atmosphere to the sunlit side.

"Clearly, if it's too cold on one side and too hot on the other, somewhere in the middle there's that lovely Goldilocks zone where everybody wants to build their condos," Shostak said.

Even with these challenges, the sheer influx of tens of billions of potentially habitable planets improves the chances of finding alien life.

"SETI is looking for Mr. Right or maybe Ms. Right, depending on your point of view," Shostak said. "It helps to find out that there's 10 times as many candidates as there were before."


 

 

 

 

 

 

 

 


Red Dwarf Stars May Be Best Chance for..

Habitable Alien Planets
by Charles Choi

Astrobiology Magazine Contributor
23 February 2012
from SPACE Website

 

 

 

This artist's concept illustrates a young, red dwarf star surrounded by three planets.

Such stars are dimmer and smaller than yellow stars like our sun,

which makes them ideal targets for astronomers

wishing to take images of planets outside our solar system,

called exoplanets.
CREDIT: NASA/JPL-Caltech


 

Stars known as red dwarfs might have larger habitable zones friendly to ‘life as we know it' than once thought, researchers say.

Red dwarfs, also known as M stars, are dim compared to stars like our sun and are just 10 to 20 percent as massive. They make up roughly three-quarters of the stars in the galaxy, and recently scientists found red dwarfs are far more common than before thought, making up at least 80 percent of the total number of stars.

The fact that red dwarfs are so very common has made astrobiologists wonder if they might be the best chance for discovering planets habitable to life as we know it.

 

More and more planets are getting discovered around red dwarfs - for instance, a potentially habitable "super-Earth" at least 4.5 times the mass of Earth, GJ 667Cb, was recently found orbiting the red dwarf GJ 667C.

"More of these planets are being found, so research is moving from being theoretical and predictive to using actual data from extrasolar planets," said researcher Manoj Joshi, an atmospheric physicist at the University of East Anglia in England.

The habitable zone of a star is defined by whether liquid water can survive on its surface, given that life exists virtually wherever there is liquid water on Earth.

 

Too far from a star, and a world is too cold, freezing all its water; too close to a star, and a world is too hot, boiling all of its water off.

 

 


Orbit of Mercury relative to other planets in our solar system.

Since red dwarfs are so cold compared to our sun,

planets would have to be very close in to be habitable

to any life as we know it - in many cases,

less than the distance between Mercury and our sun
CREDIT: Spacestationinfo.com

 

 

Since red dwarfs are so cold compared to our sun, planets would have to be very close in to be habitable to any life as we know it - in many cases, less than the distance between Mercury and our sun.

 

This closeness actually makes them appealing to hunters of alien worlds - planets near their stars eclipse them more often, making them easier to detect than planets that orbit farther away.

However, being too close to a star can have its disadvantages. For instance, the gravitational pull of the star would cause tides that could wreak havoc on such a world, perhaps leading to a so-called "tidal Venus" scenario where it loses all of its surface water.

 

Also, young red dwarfs less than 3 billion years old may be very active, firing off flares several times per day, causing ultraviolet radiation to jump by 100 to 10,000 times normal levels and potentially sterilizing the surface of a nearby planet or even helping to strip off its atmosphere.

Now scientists find that planets may remain habitable farther away from a red dwarf than once thought. This in turn could mean there is a chance there are far more habitable worlds around red dwarfs than previously suspected.

The habitability of a star depends on how warm or cold it is, which in turn rests in large part on how much starlight it absorbs and reflects.

 

 

 

 

Frozen water such as ice and snow reflects light, which means it helps cool planets, including Earth.

"If a rocky planet forms around an M-star and it has water on it, if it gets cold enough, that'll turn to ice or snow," Joshi said. "As for the odds of rocky planets forming (click above image) around M-stars, Neptune- and sub-Neptune-sized objects have been found, so chances could be good."



A planet's surface ice reflects sunlight,

decreasing the planet's temperature
CREDIT: Glenn Grant/National Science Foundation

 

 

The researchers modeled how reflective ice and snow would be on simulated planets orbiting two real-life red dwarfs. Ice and snow are less reflective against longer, redder wavelengths, while red dwarfs obviously have fairly red light to begin with.

The scientists found that any such planets encircling red dwarf stars would absorb more of their light than previously thought, leading to significantly warmer surfaces.

 

This means the outer edge of the habitable zone around red dwarfs might be 10 to 30 percent farther away from its parent zone than once suggested.

"I was surprised that the effect was as large as it was," Joshi told Astrobiology Magazine. "The zone where liquid water is stable on a planet's surface is farther away from such stars than previously thought."

Joshi cautioned they only looked at the effects of water ice and snow, when other kinds might be important when considering how much energy a planet absorbs and reflects, such as frozen carbon dioxide, nitrous oxide and methane.

 

Also,

"we didn't look at the effects of atmospheric absorption of radiation by gases such as water vapor or carbon dioxide," he added. "That should be done in future."

Joshi and Robert Haberle detailed their findings in the Jan. 23 issue of the journal Astrobiology.

 

 

 

 

 

 

 

 

 


Super-Earth Detected

...in Star's Habitable Zone
Source: University of California, Santa Cruz
February 02, 2012

from Astrobiology Website

 

 

Astronomers have discovered

a potentially habitable super-Earth

orbiting a nearby star.

 

 

 

 


An international team of scientists has discovered a potentially habitable super-Earth orbiting a nearby star.

 

With an orbital period of about 28 days and a minimum mass 4.5 times that of the Earth, the planet orbits within the star's "habitable zone," where temperatures are neither too hot nor too cold for liquid water to exist on the planet's surface. The researchers found evidence of at least one and possibly two or three additional planets orbiting the star, which is about 22 light-years from Earth.

The team includes UC Santa Cruz astronomers Steven Vogt and Eugenio Rivera and was led by Guillem Anglada-Escudé and Paul Butler of the Carnegie Institution for Science.

 

Their work will be published by Astrophysical Journal Letters, and the manuscript will be posted online at arxiv.org/archive/astro-ph.

The host star is a member of a triple-star system and has a different makeup than our sun, with a much lower abundance of elements heavier than helium, such as iron, carbon, and silicon.

 

This discovery indicates that potentially habitable planets can occur in a greater variety of environments than previously believed.
 

 

The Carnegie Planet Finder Spectrograph

at the beginning of Summer 2008, before it was installed at the Magellan II Telescope.

Credit: Carnegie Institute of Washingon

 


The researchers used public data from the European Southern Observatory and analyzed it with a novel data-analysis method.

 

They also incorporated new measurements from the W. M. Keck Observatory's High Resolution Echelle Spectrograph and the new Carnegie Planet Finder Spectrograph at the Magellan II Telescope.

 

Their planet-finding technique involved measuring the small wobbles in a star's motion caused by the gravitational tug of a planet.

The host star, called GJ 667C, is an M-class dwarf star. The other two stars in the triple-star system (GJ 667AB) are a pair of orange K dwarfs, with a concentration of heavy elements only 25 percent that of our sun's.

 

 

The star, GJ 667C.

Credit: Digitized Sky Survey

 

 

Such elements are the building blocks of terrestrial planets, so it was thought to be less likely for metal-depleted star systems to have an abundance of low-mass planets.

"This was expected to be a rather unlikely star to host planets. Yet there they are, around a very nearby, metal-poor example of the most common type of star in our galaxy," said Vogt, a professor of astronomy and astrophysics at UCSC.

 

"The detection of this planet, this nearby and this soon, implies that our galaxy must be teeming with billions of potentially habitable rocky planets."

GJ 667C had previously been observed to have a super-Earth (GJ 667Cb) with a period of 7.2 days, although this finding was never published. This planet orbits so close to the star that it would be too hot for liquid water.

 

The new study started with the aim of obtaining the orbital parameters of this super-Earth.


 

The team also incorporated new measurements

from the High Resolution Echelle Spectrograph at the W.M. Keck Observatory,

located on the island of Hawai'

Image Credit: Pablo McLoud/WMKO

 


But in addition to this first candidate, the research team found the clear signal of a new planet (GJ 667Cc) with an orbital period of 28.15 days and a minimum mass of 4.5 times that of Earth.

 

The new planet receives 90 percent of the light that Earth receives. However, because most of its incoming light is in the infrared, a higher percentage of this incoming energy should be absorbed by the planet.

 

When both these effects are taken into account, the planet is expected to absorb about the same amount of energy from its star that the Earth absorbs from the Sun.

"This planet is the new best candidate to support liquid water and, perhaps, life as we know it," Anglada-Escudé said.

The team found that the system might also contain a gas-giant planet and an additional super-Earth with an orbital period of 75 days. However, further observations are needed to confirm these two possibilities.

"With the advent of a new generation of instruments, researchers will be able to survey many M dwarf stars for similar planets and eventually look for spectroscopic signatures of life in one of these worlds," said Anglada-Escudé, who was with Carnegie when he conducted the research, but has since moved on to the University of Gottingen.