How common are earths around small stars?

Jun 03, 2013
How common are earths around small stars?
An artist's conception of a cool M-dwarf star seen with a transiting planet. A new Kepler study of 64 small stars with 95 candidate exoplanets estimates that the closest Earth-sized exoplanet probably orbits an M-dwarf less than 15 light-years away. Credit: MEarth Project, D. Charbonneau

(Phys.org) —The Kepler mission has revolutionized the study of exoplanet statistics by increasing the number of known extrasolar planets and planet candidates by a factor of five, and by discovering systems with longer orbital periods and smaller planet radii than any of the prior exoplanet surveys. There is of course considerable interest in locating Earth-sized planets residing in the habitable zones of their stars, that is, having orbits producing surface temperatures that allow water to remain liquid - a prerequisite for the development of life.

It turns out that small stars, so-called M-dwarfs whose masses are about half a solar-mass and whose are less than about 4000K, are much more numerous than solar-type stars - about twelve times as common. Hunting for Earth-sized planets around M-dwarfs, therefore, is of particular interest. Although the idea of finding around M-dwarfs had been discussed as early as fifty years ago, the possibilities were considered slight because of two concerns about these smaller stars. The first is that because the star is cooler and less luminous than the Sun, the planet needs to be closer for its surface temperature to be suitable, but then gravity will tidally lock it facing the star (much as the Moon is tidally locked facing the Earth). With one face perpetually toward (and one away from) the star, the planet's surface might be either to hot or too cold. The second difficulty was that small stars tend to flare, perhaps affecting a planet's atmosphere.

New research, however, suggests that suitable habitable regions might develop on a planet in either of these cases. Since there are so many more small stars, and since it is so much easier to study their transiting planets because they are closer in and so have shorter orbital periods, a team of Kepler scientists began a focussed study of exoplanets around small stars. CfA astronomers Courtney Dressing and David Charbonneau report their conclusions in this month's Astrophysical Journal. Using Kepler, they identify 64 dwarf stars with 95 candidate (still awaiting confirmation) planets. This sample is large enough to reach some impressive statistical conclusions: on average every six small stars should host an Earth-sized planet in its habitable zone; and to 95% confidence, because small stars are so common, the nearest planet in a habitable zone probably lies within fifteen light-years of Earth.

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GSwift7
3.2 / 5 (11) Jun 03, 2013
they identify 64 dwarf stars with 95 candidate (still awaiting confirmation) planets. This sample is large enough to reach some impressive statistical conclusions


I wonder how this finding deals with the inherent variability of M type stars? That variability would make the transit method very unreliable.

In regard to the tidal locking of earth sized planets, and the flares: A jupiter or super-jupiter planet orbiting in the habitable zone of an M star would not be tidally locked (tidal locking is a result of the ratio of mass between the two objects and the distance, so a larger planet will not be tidally locked at the same distance as a smaller planet.) So, you could have an earth sized moon orbiting a super-jupiter in the habitable zone. Additionally, gas giants seem to have strong magnetic fields, which may protect such a moon from flares. We are a long way from detecting such things, but I'm sure they are there.
philw1776
1.7 / 5 (6) Jun 03, 2013
Were the Jovian to have a Earth sized satellite in orbit the problem due to the proximity to the faint star would be the HUGE tides sweeping across the moon as it orbited the planet. Oceans would scour the planet's landmasses.
MaiioBihzon
2.7 / 5 (14) Jun 03, 2013
G Swift, habitable exomoons almost certainly exist. Are they rare or common? The traditional notion of a habitable zone within a certain range around a star is already being challenged. The obvious example here in our Solar System is Europa, well outside the Sun's "habitable zone," but nevertheless a potential home for life, and is cited in this article: http://phys.org/n...ons.html

René Heller believes habitable exomoons circling red dwarfs are unlikely and rare. But there's still a lot to learn, and he could be wrong. In any case, if you check out the article, the data from Kepler is expected to reveal moons as small as 1/5 Earth-mass. The Kepler mission may be over, but the discovery of exomoons may have just begun.
Czcibor
1.7 / 5 (6) Jun 03, 2013
GSwift7:
In regard to the tidal locking of earth sized planets, and the flares: A jupiter or super-jupiter planet orbiting in the habitable zone of an M star would not be tidally locked (tidal locking is a result of the ratio of mass between the two objects and the distance, so a larger planet will not be tidally locked at the same distance as a smaller planet.)


http://en.wikiped...imescale

May you check this formula? If get it right it should be the opposite - when calculating timescale, the mass of the object is actually in the denominator, so the heavier the satellite, (in this case a gas giant) the quicker tidal lock.
Fleetfoot
5 / 5 (2) Jun 04, 2013
With one face perpetually toward (and one away from) the star, the planet's surface might be either to hot or too cold. The second difficulty was that small stars tend to flare, perhaps affecting a planet's atmosphere.


Actually, these don't necessarily reduce the likelihood of life. Tidal locking would mean that a planet which was too far from the star under normal circumstances will have a warmer and potentially habitable side facing the star. Similarly one that is too close may have a habitable dark side.

In the latter case, being tidally locked also means only the sterile star-facing side would feel the full impact of flares, the habitable zone would be protected. Overall higher levels of radiation may simply promote more rapid evolution and on a locked planet there will be a gradient of exposure from lethal to negligible.

The extreme weather systems would also distribute and mix nutrients very quickly. It would be very different from Earth but not inimical to life.
antialias_physorg
3.4 / 5 (5) Jun 04, 2013
Tidal locking would mean that a planet which was too far from the star under normal circumstances will have a warmer and potentially habitable side facing the star.

At the very least there will be a temperate zone around the terminator (though wind speeds might be pretty high on a tidally locked planet with an atmosphere...but that isn't much of an issue if we count ocean/subterranean life...not all life is dependent on an atmosphere, either)
no fate
3.7 / 5 (3) Jun 04, 2013
The key is stability of the environment, as most of the above posts suggest. The variables have to be just right given the nature of these stars and the decreased size of the habitable zone around them. The offset is the shear quantity of these stars, with so many the odds are good those variables line up more than once.
GSwift7
2.1 / 5 (7) Jun 04, 2013
G Swift, habitable exomoons almost certainly exist. Are they rare or common?


Yep, that's a good question right now, isn't it?

If Jupiter is a good example for what is common (that's a very big IF), it has 3 good sized moons, with ganymede at 2x lunar mass.

Too bad Kepler didn't last another 6 years. You mentioned above that we can detect moons, but that was assuming Kepler continued to work. There's not much hope of doing that with only 3 years of data.
GSwift7
1.7 / 5 (6) Jun 04, 2013
May you check this formula? If get it right it should be the opposite - when calculating timescale, the mass of the object is actually in the denominator, so the heavier the satellite, (in this case a gas giant) the quicker tidal lock


No, if tidal locking increases with mass, then it would be the large object that is tidally locked in stead of the small one.

In the equation you linked to:

The mass of the center object (m sub p) is in the denominator, and the mass of the orbiting object (m sub s) is in the numerator. Note that they are talking about a satellite orbiting a planet, not a planet orbiting a star. In our discussion here, you would place the mass of the planet in the numerator, and the mass of the star in the denominator. So, as I said, less massive objects become tidally locked easier than more massive objects. A sufficiently large planet orbiting an M star in the habitable zone would be less likely to be tidally locked.
GSwift7
1 / 5 (4) Jun 04, 2013
if you check out the article, the data from Kepler is expected to reveal moons as small as 1/5 Earth-mass


As I said, that was before Kepler stopped working. They probably won't get anything that small with the existing data set.

Were the Jovian to have a Earth sized satellite in orbit the problem due to the proximity to the faint star would be the HUGE tides sweeping across the moon as it orbited the planet.


The gravity from the star wouldn't cause much of a tide on the moon. If so, then the moon wouldn't be in a stable orbit in the first place. We aren't talking about being that close, since we're talking about the habitable zone. As with the moons we see around our own gas giants, a highly eliptical orbit is much more likely to cause problems with dangerous tides. One of the Galilean moons has this problem I believe? More circular orbits are better for stable tides on a moon. Keep in mind that the gravity of the planet will be 10000's? of times more than the star's
Fleetfoot
not rated yet Jun 04, 2013
http://en.wikipedia.org/wiki/Tidal_locking#Timescale

May you check this formula? If get it right it should be the opposite - when calculating timescale, the mass of the object is actually in the denominator, so the heavier the satellite, (in this case a gas giant) the quicker tidal lock.


The mass in the denominator is that of the primary body. The mass of the satellite enters via the moment of inertia which is in the numerator, see the definition of "I" a few lines down.
philw1776
1 / 5 (5) Jun 06, 2013
GSwift7 "The gravity from the star wouldn't cause much of a tide on the moon. If so, then the moon wouldn't be in a stable orbit in the first place. We aren't talking about being that close, since we're talking about the habitable zone."

Not so at all, the same extreme tidal force that locks close HZ planets rotation to M star only a few million miles away will drag several tens to hundreds meters high tides from any seas across the satellites' surfaces. These are faint M dwarfs. See Steven Dole's "Habitable Planets for Man" online for tidal formulae.
Torbjorn_Larsson_OM
not rated yet Jun 08, 2013
This is, of course, good news. M stars will have enough energy for oxygenating photosynthesis in the near infrared to red, as shown by chlorophyll f (and likely bacteriochlorophyll g).

As for tidal lock vs climate, as Venus shows even a superrotating upper atmosphere to deal with (near) tidal lock means reasonable winds at surface levels. If the atmosphere is dense enough, and there is of course a GW effect to balance out. But AFAIK models give reasonable climates, hence the claim of the article, with a mildly tighter constraint on the planets that are habitable.

As for flares, the same mechanism would mean the atmosphere stands longer. Ideally you would want a super-Earth for a likelier denser atmosphere and a dynamo field that protects the atmosphere.

I don't understand the moon around giant example. It should be enough to have a giant, tidal locked or not, and then a moon distant enough to not experience tidal lock.
Torbjorn_Larsson_OM
not rated yet Jun 08, 2013
@MaiioBihzon: I wouldn't say the original HZ is challenged as much as complemented. The concept is a filter in a perturbation analysis, it was never meant to be the only examples of habitability but the most likeliest to be habitable (case study: Earth) and the easiest to detect.

It is so useful that the giant tidal zone now is modeled after it as a giant habitable zone. (Cf Enceladus.)

@philw1776: That type of tides are no different from the giant tides the Earth-Moon system started out with. The tidal zone at coasts have been seen as inducive for complex organisms gaining a foothold on land. providing nutrients and so a niche on the way inland.
Modernmystic
2.3 / 5 (3) Jul 23, 2013
This is, of course, good news.


Good news? For one it's highly speculative. Secondly it's neither good or bad unless you have a bias for the data to be one way or the other, in which case your interpretation is slightly suspect...

M stars will have enough energy for oxygenating photosynthesis in the near infrared to red, as shown by chlorophyll f (and likely bacteriochlorophyll g).


Helpful if you're left with an atmosphere not frozen out on the dark side when tidally locked, or not affected by the huge variability (40% lower output via sunspots for months at a time) inherent in these stars. This is also assuming flares common to M type stars don't blow the atmosphere off whether or not a magnetic field is present.

Bottom line is that no matter how rosy the glasses you look through these stars are NOT good candidates for finding planets that develop complex life.

Modernmystic
1 / 5 (2) Jul 23, 2013
double post.