Many billions of rocky planets in the habitable zones around red dwarfs in the Milky Way

Mar 28, 2012
This artist's impression shows a sunset seen from the super-Earth Gliese 667 Cc. The brightest star in the sky is the red dwarf Gliese 667 C, which is part of a triple star system. The other two more distant stars, Gliese 667 A and B appear in the sky also to the right. Astronomers have estimated that there are tens of billions of such rocky worlds orbiting faint red dwarf stars in the Milky Way alone. Credit: ESO/L. Calçada

(PhysOrg.com) -- 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 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, 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 where 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 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 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 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 Stephane 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 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 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.

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hb_
5 / 5 (4) Mar 28, 2012
Wow! This probably means that life of some sort - bacteria at least - is inevitable.

As for the flare, this may make it hard for large organisms, but bacteria and fungus should be able to evolve a very high level of radiation hardness.
Jack_J_Smith
1.5 / 5 (13) Mar 28, 2012
who cares, we can't even establish a permanent moon base
TychoCraterCafe
4.8 / 5 (4) Mar 28, 2012
Well I guess that means the Fermi Paradox just went critical. And, Jack, I too am totally appalled at the state of human space activities, acknowledged or otherwise. Doesn't anybody wonder what the hell Lockheed, Northrop, SAIC et al have been doing out in the deserts of the southwest for all these years? Frankly, I'm beginning to believe some of the rumors...
villhest
4.7 / 5 (13) Mar 28, 2012
Kevintrs:

While it is true that red dwarf stars are more common than our own, it is not true that our star is "very, very special". In fact, roughly 1 out of 13 stars are "G" type stars (our star is G2V).
Modernmystic
1 / 5 (7) Mar 28, 2012
...and all are, most probably tidally locked around small red stars. Which makes the word "habitable" intellectually dishonest...

So there are many billions of rocky planets with double or more Earth's gravity in habitable zones of small stars which are almost certainly uninhabitable...WOW....
Modernmystic
2 / 5 (13) Mar 28, 2012
Kevintrs:

While it is true that red dwarf stars are more common than our own, it is not true that our star is "very, very special". In fact, roughly 1 out of 13 stars are "G" type stars (our star is G2V).


Which makes them pretty rare...less than 10%. If you're in the top ten percent of your class you're considered distinguished. Honestly this groping to make the principle of mediocrity fit all situations gets silly after a while.

Let's not even get into metallicity, galactic habitable zones, age, and the nasty problems of shifting habitable zones in stars even mere percents off in mass of ours one way or the other...
Jordian1
3.3 / 5 (7) Mar 28, 2012
This is a fairly inaccurate title.

The closer a planet is to a star, the more likely it is to be tidally locked. The habitable zone for Red Dwarfs and smaller stars seen in this study is much further in compared to larger stars. So it could be that even if there were the right conditions and make up, one side of the planet would always be facing the star. Thus making it uninhabitable, unless it had a thick atmosphere. Even then, it would turn out like Venus. Not a good thing.

Also, the composition of the stellar disk the planets are formed from is a huge deal. Not all stars and planets are made equal, and Earth had a protoplanetary disc that was imbued with a ton of heavy elements, including lots of water. Most Red Dwarfs are older stars with lower metal content (formed when the Universe had fewer heavy elements). Meaning that any planets around them will be resource poor in the elements for life. Carbon, Iron, Silica etc need for life would not exist in any great quantities.
nkalanaga
4.7 / 5 (12) Mar 28, 2012
Earth would still be habitable if it was tidally locked. A 1 bar atmosphere can distribute heat efficiently enough to prevent water from freezing over most of the dark side, and oceans will further spread the heat. The subsolar point may be too hot for humans, but it won't boil, and certainly native life can thrive on such a world.

crack.seismo.unr.edu/ftp/pub/gillett/joshi.pdf
Simulations of the Atmospheres of Synchronously Rotating ...
M. M. Joshi, R. M. Haberle, and R. T. Reynolds. Space Sciences Division, NASA Ames Research Center, MS 245-3, Moffett Field, California 940351000
malapropism
5 / 5 (7) Mar 28, 2012
On this side of the coin someone honestly and openly admits the problem.
"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."

On this side of the coin it would seem in direct contradiction to the above statement.
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,

There is no contradiction in these statements: that a planet might be bathed in UV or subjected to intense solar flare activity making the existence of life on it less probable, or even if it is indeed devoid of life, does not preclude studying its atmosphere and searching for life or make the search any less worthy of doing. You don't know until you look, is what they are saying.
nkalanaga
5 / 5 (3) Mar 28, 2012
If the planet can hold air and water, which some simulations say it could in spite of "normal" red dwarf flares, marine life would be well shielded. Even if there is no surface life, chemosynthetic, and possibly photosynthetic, life would have no problems in the oceans.

Most of the radiation from flares wouldn't get through an atmosphere, and life can deal with UV in several ways. If it's warm enough for liquid water, UV photodissociation will eventually produce enough oxygen to form an ozone layer. That's why ozone isn't considered a good indicator of life on a planet.
Jitterbewegung
5 / 5 (1) Mar 29, 2012
Have any astronomers looked at the Alpha Centauri system yet?
Torbjorn_Larsson_OM
3.7 / 5 (3) Mar 29, 2012
Very encouraging for astrobiologists of course. We know chemical evolution is very easy, since it happens all over the universe and it very rapidly originated biological evolution here. The remaining part of the puzzle were habitable terrestrials. Even if some of these could be neptunes, many will be terrestrials.

Notable is that while Gliese 667 Cc is the current confirmed planet best matching Earth (at an ESI of 0.85), there are 23 better candidates in the HEC pipeline up to 0.99 similarity. [ http://phl.upr.ed...log/data ]

@ hb:

The problem with tough environments isn't for matured cell populations with many traits but for the chemical to biological evolution transition. Shostaks selfassembled cells are fragile.

That said, superEarths with a modicum of volatiles have well shielded environments; a massive atmosphere and gravity well would balance increased solar activity attrition, water and crust shield against UV et cetera.
Torbjorn_Larsson_OM
3 / 5 (4) Mar 29, 2012
@ Jack Smith:

Who cares if anyone doesn't care, these are very fruitful and exciting research areas for planetary researchers and astrobiologists!

@ kevintrs:

Our sun is quite typical. It is slightly calmer than the average, which is why Kepler need more observation time due to more noise. But only slightly.

Also, IIRC ~ 40 % of M stars are calm when mature.

@ TychoCraterCafe:

This has nothing to do with the poorly constrained Fermi question. For one thing, most biologists would put the evolution of complex multicellulars to very unlikely.

@ Jitterbewegung:

I believe I heard of long and intense scrutiny. That aside, it is really uninteresting for astrobiologists. We will never visit other solar systems, unless we want to take the expense of colonization. Good and useful research is much more cheap from here.
Torbjorn_Larsson_OM
3.4 / 5 (5) Mar 29, 2012
@ ModernMystic:

There is no inherent general problem with tidal lock vs habitability. Modeling shows that thick atmospheres compensate for thermal differences, closeness to active M stars isn't a problem for superEarths (see above), and the same goes for plate tectonics/magnetic fields vs tidal locked superEarths since they will have both anyway under loose constraints.

Sadly it is you who promote an intellectually dishonest position.

@ ModernMystic, Jordan1:

You confuse the valid research strategy of constrained sensitivity analysis (looking for Earth analogs as a known starting point for "surface" biospheres) with the invalid "Rare Earth" strategy of using unconstrained bayesian betting to make the bet potentially infinitely expensive.

Every planet will be an individual, that doesn't mean habitable planets won't exist. (Or you could use the same bet to make individual organisms 'unlikely' to exist.)
Modernmystic
1 / 5 (1) Mar 29, 2012
Indeed there are many problems with tidal lock vs habitability.

http://www.ncbi.n...17407406

http://www.exocli...planets/

Constant extreme hurricane force winds to maintain thermal equilibrium, weak magnetic fields, problems with photosynthesis and hence getting an oxygen rich atmosphere, the list is looooong indeed.
Modernmystic
1 / 5 (1) Mar 29, 2012
Our sun is quite typical.


Incorrect. It is, in point of fact, representative of less than ten percent of all stars. It is also metal rich compared to most stars. It is just the right mass to allow a planet to remain for billions of years in a migrating habitable zone. It resides in a part of the galaxy which is not bathed in constant hard radiation AND also allows for some higher metallicity rates. It's quite unique in many respects...

For one thing, most biologists would put the evolution of complex multicellulars to very unlikely.


Well not most of the ones who are in the media anyway :)
Jitterbewegung
not rated yet Mar 29, 2012
"We will never visit other solar systems, unless we want to take the expense of colonization."

It's only about 9 years traveling time if you travel at the speed of artificial gravity for 179 days then coast for 8 years then reverse engines for 179 days.

Even the Moon is only a couple of hours away at g and Mars a day or two.
Modernmystic
1 / 5 (1) Mar 29, 2012
"We will never visit other solar systems, unless we want to take the expense of colonization."

It's only about 9 years traveling time if you travel at the speed of artificial gravity for 179 days then coast for 8 years then reverse engines for 179 days.

Even the Moon is only a couple of hours away at g and Mars a day or two.


Cool website...

http://www.conver...tor.html
nkalanaga
5 / 5 (2) Mar 29, 2012
There have been searches for planets at Alpha Centauri, but so far no results. The orbit of the A and B stars is significantly tilted relative to our line of sight, so if planets orbit in the same plane, we wouldn't see transits. Simulations show that orbits in the habitable zones of both A and B are stable, but those in the "ice zone" aren't, so there likely wouldn't be any Jovians. Earth-mass planets are still a little too small to detect by radial velocity.

Proxima has no Neptune-mass or larger planets within or anywhere near its habitable zone, and no transiting planets. Again, Earth-mass planets would be hard to detect. Further, it's a very active flare star, making it less habitable than calmer red dwarfs.
Jitterbewegung
not rated yet Mar 30, 2012
"Cool website..."

It's a shame they don't know the difference between c and C.

Ps. I calculated to half of c in the belief that traveling at this speed would save me from the radiation burns of traveling closer to c. Can anyone tell me if this idea would work or is half c still dangerous?
nkalanaga
5 / 5 (1) Mar 30, 2012
It wouldn't be nearly as dangerous as higher speeds, but it would still be quite dangerous. The radiation wouldn't be the issue so much as dust and gas. Even gas at that speed would erode the front of your ship, and dust particle impacts would, gram for gram, produce more energy than high explosives.

Ober
1 / 5 (1) Mar 31, 2012
Also as you get closer to light speed, you experience more interaction with the quantum vacuum. Not too sure what affect matter-anti matter quantum foam would do to you!!!!
barakn
not rated yet Mar 31, 2012
Also as you get closer to light speed, you experience more interaction with the quantum vacuum. Not too sure what affect matter-anti matter quantum foam would do to you!!!!

So you're saying the quantum foam has a preferred rest frame which is somehow tied to our galaxy's rotation? Thump, thump. That's the sound of Einstein rolling over.
RealScience
4 / 5 (4) Apr 01, 2012
@MM - even if a super-earth around a red dwarf is tidally locked to its sun, a moon around that planet would be most likely be locked to its planet, and thus NOT to the red dwarf.
So large moons of planets around red dwarfs are a very likely place for habitability.
The first one could be called 'Pandora' after the inhabited moon in the movie of the same name.
rwinners
not rated yet Apr 02, 2012
I like red tinted sun-glasses, but I wonder if I wouldn't be choosing a different shade given that red light!
rwinners
not rated yet Apr 02, 2012
"We will never visit other solar systems, unless we want to take the expense of colonization."

I agree. And we will never visit another solar system until we learn to live, long term and comfortably, in space. We have a long way to get there.
Modernmystic
1 / 5 (1) Apr 02, 2012
@MM - even if a super-earth around a red dwarf is tidally locked to its sun, a moon around that planet would be most likely be locked to its planet, and thus NOT to the red dwarf.
So large moons of planets around red dwarfs are a very likely place for habitability.


Actually they're very unlikely to be habitable, because, as they're tidally locked they will have virtually no magnetic field. Moreover they'll be so small they won't have plate tectonics...another important feature to habitability.

The first one could be called 'Pandora' after the inhabited moon in the movie of the same name.


Wasn't the name of the movie Avatar? Also science fiction is good for what it is...entertainment. Best not to let toooo much of it bleed into what you actually think might be out there...
Modernmystic
1 / 5 (1) Apr 02, 2012
Also as an aside Avatar was horrendously BAD science fiction...

Pocahontas in space...Wow...THAT story hasn't been done to death has it?

Unobtainium....really???
RealScience
5 / 5 (2) Apr 02, 2012
@MM - Good catch on the movie - that should be "the moon of the same name in the movie Avatar".

But the comment specifically says 'large moons', which can be shielded from radiation in several ways:

On the far edge of the habitable zone a moon only a bit bigger than Mars could retain an atmosphere much denser than earth's. A dense atmosphere helps keep a moon warm if it is at the far edge of the habitable zone (and with the right oxygen level humans can survive an atmosphere many times denser than earth's).

Large enough moons would retain liquid cores, which would generate magnetic fields (and plate tectonics) even without 'daily' rotation. The moon diameter necessary is inversely proportional to the concentration of long-lived radioactive isotopes, which is likely to vary considerably from star system to star system.

We don't know the range of red-dwarf-planet moon sizes, but even if 1% are suitable that's hundreds of millions in our galaxy.
Modernmystic
1 / 5 (1) Apr 02, 2012
On the far edge of the habitable zone a moon only a bit bigger than Mars could retain an atmosphere much denser than earth's.


I don't know about "much denser", but if it did it would rely on plate tectonics to do so...see below...

A dense atmosphere helps keep a moon warm if it is at the far edge of the habitable zone (and with the right oxygen level humans can survive an atmosphere many times denser than earth's).


Photosynthesis (as we know it) is going to be a problem on ANY body orbiting a red dwarf...hence so will oxygen levels.

Large enough moons would retain liquid cores, which would generate magnetic fields (and plate tectonics) even without 'daily' rotation.


They'd probably have to be larger than Mars (as it's core is dead already), and since the ratio of Earth to it's moon is WAY off this seems unlikely even on super Earths...
RealScience
5 / 5 (2) Apr 02, 2012
@MM - the movie certainly had some bad science (the floating mountains / unobtainium being the worst offense).

But good science fiction has been a tolerable predictor of future technology - atom bombs, geostationary telecom satellites, the 'communicator' from star trek, etc.

And it has some relevance to the current subject - in exoplanets one has even been nicknamed 'Tatooine' because it was the first found orbiting two suns as did its namesake in Star Wars.
RealScience
5 / 5 (2) Apr 02, 2012
Photosynthesis (as we know it) is going to be a problem on ANY body orbiting a red dwarf...hence so will oxygen levels.


Even with our sun's spectral peak in the yellow/green, there are earthly photosynthesizers that use red and even into the infrared. Life evolving near a red dwarf would have no problem using red (and might even have three-stage H2O splitting to use longer wavelengths).

They'd probably have to be larger than Mars (as it's core is dead already), and since the ratio of Earth to it's moon is WAY off this seems unlikely even on super Earths...


We do not know how big the moons of super-earths around red dwarfs get. The highest mutually-orbiting-major-body ratio in our solar system is Charon/Pluto, with Earth/Moon being second. The Charon/Pluto ratio applied to a super-earth would give a much-bigger-than-Mars-sized moon.

A bit larger than Mars and a bit richer in heavy elements may not be rare.
nkalanaga
5 / 5 (2) Apr 02, 2012
The light wouldn't look red if you were there, except at sunset. Red Dwarfs are hotter than the filament in a 100W lightbulb, so would produce whiter light than the bulb. Such a bulb looks white when there's no outside light to compare it to, even though it's obviously orange/yellow in sunlight.

And, as RealScience says, there are photosynthesizers that use red and infrared, including some oddities that can use the infrared from hot rocks at deep sea vents.

Big moons aren't a problem for gas giants. In systems where they've migrated inward they could easily collect planets larger than Earth.
Modernmystic
1 / 5 (1) Apr 02, 2012
Re photosynthesis and red dwarfs:

http://en.wikiped...ynthesis

We do not know how big the moons of super-earths around red dwarfs get.


That's true we don't, but looking at the TERRESTRIAL planets in our system the outlook is bleak if we're applying the principles of isotropy and mediocrity objectively. Venus and Mercury have NO moons and Mars likely didn't form one either but simply captured two bits of debris from the belt. Sadly the Earth/Moon system is likely very rare indeed. We simply don't know.

Big moons aren't a problem for gas giants.


Well they are in our system....

In systems where they've migrated inward they could easily collect planets larger than Earth.


I think you use the word easily too easily :)
nkalanaga
5 / 5 (1) Apr 02, 2012
No, I've read the published papers, based on computer simulations, of migrating jovians. Based on the systems we know so far, ours is quite unusual, although I agree that most rocky planets won't have moons large enough to have life. The Moon is only about 1% Earth's mass, so to have an Earth-like moon would require a planet the size of Saturn, which would not be a rocky planet.

Captured moons are another matter. There probably were no large rocky objects in the outer Solar System to be captured. On the other hand, jovians are unlikely to form close to a star, where we've found a lot. They almost had to migrate through the habitable zone to get there. If there were planets in that zone, simulations show that they could be captured. If the jovian then stopped migrating in the HZ, you'd have a potentially habitable moon.
Modernmystic
1 / 5 (1) Apr 03, 2012
If there were planets in that zone, simulations show that they could be captured. If the jovian then stopped migrating in the HZ, you'd have a potentially habitable moon.


Agreed, however I don't think I'd apply the word easy to a migrating Jovian capturing a "large" rocky planet. I agree it COULD happen, and I also agree that I COULD have won the lottery last week :)
nkalanaga
5 / 5 (1) Apr 03, 2012
By "easy" the papers seem to mean "a statistically significant chance", as opposed to "an event too rare to seriously consider". In most cases the encounter would probably eject the smaller world from the system, but if even one or two percent were captured, there would be quite a few such worlds in the galaxy. The odds are better than winning the lottery, though far from a sure bet.

Part of the reason it could happen is that a migrating jovian would have its own accretion disk. Collisions with matter falling into that disk, and tidal interactions with the disk itself, would provide the braking needed to slow the planet enough for capture. A bare jovian, like Jupiter today, would be VERY unlikely to capture a large object.

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