ESO: Billions of Planets in Habitable Zone around Red Dwarfs

[bystander]

Many Billions of Rocky Planets in the Habitable Zones around Red Dwarfs in the Milky Way
European Southern Observatory | 2012 Mar 28

[i]Artist’s impression of sunset on super-Earth Gliese 667 Cc (Credit: ESO/L. Calçada)[/i]

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A new result from ESO’s HARPS planet finder shows that rocky planets not much bigger than Earth are very common in the habitable zones around faint red stars. The international team estimates that there are tens of billions of such planets in the Milky Way galaxy alone, and probably about one hundred in the Sun’s immediate neighbourhood. This is the first direct measurement of the frequency of super-Earths around red dwarfs, which account for 80% of the stars in the Milky Way.

This first direct estimate of the number of light planets around red dwarf stars has just been announced by an international team using observations with the HARPS spectrograph on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile. A recent announcement (eso1204), showing that planets are ubiquitous in our galaxy used a different method that was not sensitive to this important class of exoplanets.

The HARPS team has been searching for exoplanets orbiting the most common kind of star in the Milky Way — red dwarf stars (also known as M dwarfs). These stars are faint and cool compared to the Sun, but very common and long-lived, and therefore account for 80% of all the stars in the Milky Way.

“Our new observations with HARPS mean that about 40% of all red dwarf stars have a super-Earth orbiting in the habitable zone where liquid water can exist on the surface of the planet,” says Xavier Bonfils (IPAG, Observatoire des Sciences de l'Univers de Grenoble, France), the leader of the team. “Because red dwarfs are so common — there are about 160 billion of them in the Milky Way — this leads us to the astonishing result that there are tens of billions of these planets in our galaxy alone.”

The HARPS team surveyed a carefully chosen sample of 102 red dwarf stars in the southern skies over a six-year period. A total of nine super-Earths (planets with masses between one and ten times that of Earth) were found, including two inside the habitable zones of Gliese 581 (eso0915) and Gliese 667 C respectively. The astronomers could estimate how heavy the planets were and how far from their stars they orbited.

By combining all the data, including observations of stars that did not have planets, and looking at the fraction of existing planets that could be discovered, the team has been able to work out how common different sorts of planets are around red dwarfs. They find that the frequency of occurrence of super-Earths [3] in the habitable zone is 41% with a range from 28% to 95%.

On the other hand, more massive planets, similar to Jupiter and Saturn in our Solar System, are found to be rare around red dwarfs. Less than 12% of red dwarfs are expected to have giant planets (with masses between 100 and 1000 times that of the Earth).

As there are many red dwarf stars close to the Sun the new estimate means that there are probably about one hundred super-Earth planets in the habitable zones around stars in the neighbourhood of the Sun at distances less than about 30 light-years.

"The habitable zone around a red dwarf, where the temperature is suitable for liquid water to exist on the surface, is much closer to the star than the Earth is to the Sun," says Stéphane Udry (Geneva Observatory and member of the team). "But red dwarfs are known to be subject to stellar eruptions or flares, which may bathe the planet in X-rays or ultraviolet radiation, and which may make life there less likely."

One of the planets discovered in the HARPS survey of red dwarfs is Gliese 667 Cc. This is the second planet in this triple star system (see eso0939 for the first) and seems to be situated close to the centre of the habitable zone. Although this planet is more than four times heavier than the Earth it is the closest twin to Earth found so far and almost certainly has the right conditions for the existence of liquid water on its surface. This is the second super-Earth planet inside the habitable zone of a red dwarf discovered during this HARPS survey, after Gliese 581d was announced in 2007 and confirmed in 2009.

“Now that we know that there are many super-Earths around nearby red dwarfs we need to identify more of them using both HARPS and future instruments. Some of these planets are expected to pass in front of their parent star as they orbit — this will open up the exciting possibility of studying the planet’s atmosphere and searching for signs of life,” concludes Xavier Delfosse, another member of the team (eso1210).

The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample - X. Bonfils et al

• arXiv.org > astro-ph > arXiv:1111.5019 > 21 Nov 2011 (v1), 23 Nov 2011 (v2)

The HARPS search for southern extra-solar planets XXXV. Super-Earths around the M-dwarf neighbors Gl433 and Gl667C - X. Delfosse et al

• arXiv.org > astro-ph > arXiv:1202.2467 > 11 Feb 2012

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[Céline Richard]

Many Billions of Rocky Planets in the Habitable Zones around Red Dwarfs in the Milky Way European Southern Observatory | 2012 Mar 28
(...) "there are probably about one hundred super-Earth planets in the habitable zones around stars in the neighbourhood of the Sun at distances less than about 30 light-years. "The habitable zone around a red dwarf, where the temperature is suitable for liquid water to exist on the surface, is much closer to the star than the Earth is to the Sun," says Stéphane Udry (Geneva Observatory and member of the team). "But red dwarfs are known to be subject to stellar eruptions or flares, which may bathe the planet in X-rays or ultraviolet radiation, and which may make life there less likely."
One of the planets discovered in the HARPS survey of red dwarfs is [and confirmed in 2009.
(eso1210).

If a super-Earth has a magnetic field, if a magnetosphere forms around such a planet, maybe life could develop. Maybe a magnetosphere could enable life to develop on a super-Earth, despite of X-rays or ultraviolet radiation, due to the red dwarf's eruptions.
The magnetosphere protects life on Earth from dangerous radiations of the solar wind. Is a magnetosphere unlikely to form around a super-Earth? Why stellar eruptions from a red dwarf may make life there less likely, while on Earth, there is life despite of the solar wind ? Are red dwarf's eruptions more harmful for life than solar eruptions?

[Ann]

It would seem that red dwarfs, at least in their youth, have flares that are similar in size to the flares of the Sun. But the red dwarfs themselves are not similar to the Sun in size or in average energy output. A planet in the habitable zone of a red dwarf must be much, much closer to its star than we are to the Sun. Imagine being very close to a red dwarf, and suddenly it has a major flare. The total light output of the red dwarf might actually double, or more than double, during the flare. Imagine what it would have been like if the Sun had suddenly erupted and become twice as bright as normal in the Earth's sky!

Check out this page. GALEX, Galaxy Evolution Explorer, NASA's instrument for ultraviolet light, serendipitously happened to witness a dramatic ultraviolet flare coming from a red dwarf. If there were habitable planets in orbit around that dwarf, the flare must have killed any life on the surfaces of them.

But as for the flares, my understanding is that red dwarfs get less and less active in this respect as they age. And they do get to live very long lives.

Ann

[Chris Peterson]

Check out this page. GALEX, Galaxy Evolution Explorer, NASA's instrument for ultraviolet light, serendipitously happened to witness a dramatic ultraviolet flare coming from a red dwarf. If there were habitable planets in orbit around that dwarf, the flare must have killed any life on the surfaces of them.

Not necessarily. There's no reason life couldn't evolve to be highly resistant to UV- we certainly see that with some life here on Earth. And of course, on any planet with oceans, it is very possible for life (as we know it) to survive even if the parent star is producing a huge amount of UV and even ionizing radiation.

[bystander]

Tidal heating shrinks the 'goldilocks zone'
Nature News | Richard A. Lovett | 2012 May 08

Overlooked factor suggests fewer habitable planets than thought.

[i]Planets orbiting close to M-class dwarf stars may experience too much tidal heating to be able to maintain liquid water. (Credit: NASA, ESA, and D. Aguilar (CfA))[/i]

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A previously little-considered heating effect could shrink estimates of the habitable zone of the Milky Way’s most numerous class of stars — ‘M’ or red dwarfs — by up to one half, says Rory Barnes, an astrobiologist at the University of Washington in Seattle. That factor — gravitational heating via tides — suggests a menagerie of previously undreamt-of planets, on which tidal heating is a major source of internal heat. Barnes presented the work yesterday at a meeting of the American Astronomical Society’s Division on Dynamical Astronomy in Timberline Lodge, Oregon.

The habitable zone is the orbital region close enough to a star for a planet to have liquid water, but not so close that all of the water evaporates. For our Sun, the zone extends roughly from the inner edge of the orbit of Mars to the outer edge of that of Venus. For smaller, cooler stars, such as M-class dwarfs, the zone can be considerably closer to the star than Mercury is to the Sun. And because close-in planets are easier to spot than more distant ones, such stars have been a major target for planet hunters seeking Earth-like worlds.

There's just one problem with finding habitable planets around such stars, says Barnes. Because tidal forces vary dramatically with the distance between a planet and its star, closer orbits also result in massively larger tidal forces.

Since planets do not have perfectly circular orbits, these tidal forces cause the planet to flex and unflex each time it moves closer to or further from its star; kneading its interior to produce massive quantities of frictional heat. Substantial heat can be produced, he added, with even slight deviations from a perfectly circular orbit. And, Barnes notes, other factors — such as the rate of the planet's rotation and its axial tilt — can also influence heat production.

A similar tidal process makes Jupiter's moon Io the most volcanic body in the Solar System. "I’m just scaling that Io–Jupiter system up by a factor of 1,000 in mass," Barnes said at the meeting. "It's the same process, on steroids."

Hot effects

Barnes added that this process can also change a planet's orbit, making it more circular. This would reduce the amount of heat being generated. But even if a planet winds up in a near-circular orbit, Barnes says, his calculations show that it could easily have already undergone enough heating to lose all of its water into space. "Terrestrial planets can be permanently sterilized," he said.

Such planets could also wind up in what is thought to be the habitable zone, looking to exoplanet hunters like Earth-like worlds. "It sounds like what we're looking for," he said. "But is it an Earth twin, or a 'habitable-zone' Venus?"

For the smallest red dwarfs, Barnes added, tidal heating could reduce the habitable zone by about 50% of present estimates, but brighter suns have farther-flung habitable zones where this is no longer an important factor. "At one-quarter of solar mass, the effect goes away," Barnes says.

But even for planets not ‘sterilized’ by the process, there can be major effects. "This is a source of energy that doesn’t exist on Earth," he says.

If the effect is strong, it could produce a planet-sized Io. Or, if the planet would otherwise be frozen, tidal heating could maintain a liquid ocean beneath a planet-girdling ice cap, like a planet-sized version of Jupiter's moon Europa, which has an ocean beneath a thick layer of ice, kept liquid by tidal heat.

"Nobody's put serious work into this before," says Douglas Hamilton, an astronomer at the University of Maryland in College Park.

The next step, adds Sylvio Ferraz Mello, an astronomer at the University of Sao Paulo, Brazil, is to look at the geological consequences of comparatively mild tidal heating on otherwise Earth-like worlds — a factor that could have unknown, but possibly major ramifications for plate tectonics.

Hamilton warns, however, that his findings are still theoretical. And even if there are ways to determine whether a terrestrial planet circling in the habitable zone of an M-class star is an Earth twin or a tidal Venus, he adds, "that's well beyond any [space telescope] missions we have on the drawing board."

Tidal Heating Could Kill 'Habitable' Alien Worlds
Discovery News | Ian O'Neill | 2012 May 10