Red Dwarfs: Planets in Abundance

by Paul Gilster on March 5, 2014

Whether or not they’re suitable for life, habitable zone ‘super-Earths’ are seeing increased scrutiny around M-class dwarf stars because the mass ratio of planet to star makes detection easier than around more massive stars. We need radial velocity surveys to help us here because planets on orbits longer than 200-300 days will definitely be out of Kepler’s reach. Moreover, while Kepler targets many K, G and F-class stars, M-dwarfs aren’t bright enough to show up in large numbers in its field of view, making occurrence rates around such stars problematic.

A 2013 paper by Courtney Dressing and David Charbonneau (Harvard-Smithsonian Center for Astrophysics) found that the Kepler sample contains 3897 stars with estimated effective temperatures below 4000K. Out of these, 64 are planet candidate host stars, with 95 candidate planets orbiting them. The researchers deduced from their analysis that about 15 percent of all red dwarfs have an Earth-sized planet in the habitable zone. Ravi Kopparapu (Penn State) recast these results with a revised set of habitable zone parameters. The result: Four out of ten of the nearest small stars are likely to have planets in the habitable zone.

These results are fascinating because they suggest that the nearest habitable planet could be as close as seven light years away. Bear in mind that we know of eight stars within 10 light years of the Sun that fit this definition, so we might find three Earth-sized planets in habitable zones in relatively nearby space. We looked at Kopparapu’s work in Habitable Zone Planets: Upping the Numbers about a year ago, noting that his work on an improved climate model (developed with Penn State’s James Kasting) allows the habitable zone to be moved out further from the host star than it had been before, another finding with promising astrobiological implications.

Now we have word of a new study from Mikko Tuomi (University of Hertfordshire) and colleagues, who have combined data from the HARPS (High Accuracy Radial Velocity Planet Searcher) and UVES (Ultraviolet and Visual Echelle Spectrograph) instruments operated by the European Southern Observatory in Chile. Using Bayesian signal detection criteria and noise models that take into account correlations in the data, the team found three habitable zone super-Earths among eight new planets it discovered orbiting nearby red dwarfs. The stars — GJ 27.1, GJ 160.2, GJ 180, GJ 229, GJ 422, and GJ 682 — are between 15 and 80 light years away, with planetary orbital periods ranging from two weeks to nine years. The researchers were also able to confirm the existence of a companion around GJ 433.

800px-Gliese_433_Hot_SuperEarth

Image: Recent work confirms the existence of a long-period planet around the M-class dwarf GJ 433, about 30 light years from the Sun. Credit: Wikimedia Commons.

But the paper has implications well beyond these new worlds, for Tuomi’s group went on to calculate, using the estimated detection probability function, the occurrence rate of low-mass planets around nearby M-dwarfs. Habitable zone super-Earths, their paper deduces, should orbit at least a quarter of the red dwarfs in the Sun’s neighborhood. The paper on this work compares these results briefly with the Kepler work of Dressing and Charbonneau, but notes a key difference:

…such a comparison is not necessarily reliable because the properties of Kepler’s transiting planet candidates can only be discussed in terms of planetary radii and the radial velocity method can only be used to obtain minimum masses. Because of this, it is not surprising that there are remarkable differences that are unlikely to arise by chance alone.

So we emerge with somewhat different occurrence rates, with Tuomi’s team finding habitable zone super-Earths occurring in a range between Dressing and Charbonneau’s 15 percent and Kopparapu’s 40 percent. Bear in mind that the radius of some of the planet candidates in both the Dressing and Charbonneau paper as well as Kopparapu’s may change later with more accurate observations of the host star, a possibility that would change the occurrence rates from both these studies.

In any case, it makes sense that these estimates might vary. Dressing and Charbonneau, for example, worked with planetary radii between 0.5 and 1.4 times that of Earth, while Tuomi and colleagues made their calculations based on masses between 3 and 10 times that of Earth, and Tuomi points out that his group couldn’t assess the occurrence rates of planets with masses below 3 Earths because they failed to detect any in their sample. The detection methods, transit and radial velocity, differed, and in any case, the relationship between mass and radius is not well established for super-Earths.

The occurrence rate of low-mass planets in general, however, is high:

We find that low-mass planets are very common around M dwarfs in the Solar neighborhood and that the occurrence rate of planets with masses between 3 and 10 M⊕ is 1.08 [+2.83/-0.72] per star. This estimate is likely consistent with that suggested based on the Kepler results for a sample of stars with Teff < 4000 K…, although the comparisons are not easily performed because we could not assess the occurrence rates of companions with periods up to the span of the radial velocity data of a few thousand days. On the other hand, we confirm the lack of planets with masses above 3 M⊕ on orbits with periods between 1-10 days.

Bear in mind that M-class dwarfs are the most common type of star in the galaxy, perhaps comprising up to 80 percent of the total. The new work gives additional weight to the idea that these stars have low-mass planets around them in abundance, and a high probability of at least a super-Earth class world in their habitable zones. Given our ability to detect low-mass planets around cool stars with both transit and radial velocity methods, their stock can only rise as targets for future searches for Earth-sized planets and studies of planetary atmospheres.

The paper is Tuomi et al, “Bayesian search for low-mass planets around nearby M dwarfs. Estimates for occurrence rate based on global detectability statistics,” MNRAS, in press (full text). Ravi Kopparapu’s 2013 paper is “A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs,” Astrophysical Journal Letters Vol. 767, No. 1, L8 (abstract). The Dressing and Charbonneau paper is “The occurrence rate of small planets around small stars,” The Astrophysical Journal Vol. 767, No. 1, 95 (abstract).

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{ 14 comments… read them below or add one }

RobFlores March 5, 2014 at 12:22

The recently developed theory of planetary migration after formation
holds hope for finding interesting solar systems in Red Dwarves.

I would suggest that a midsized Jovian 30-60% JE, if formed at the right
distance to it’s primary, could accrete it’s own Systems of Large moons.
We know Large Gas Giants form large moons. With a migration inwards
it’s moon(s) can lie in the Habitable Zone, with a thick atmosphere 7 day
7 night cycle making it more benign and suitable for the rise of life. Obcourse
flaring would limit life to water bodies, but with an extra thick atmosphere
and the locations moved toward the cooler side of the HZ the possibilities for life look better.
Unfortunately this is the only way large moon could be present around
a planet in the HZ of a Red Dwarf, (the formation of earth’s moon is clearly a very low probability event, and created a dead moon)

Now probably is not high for such a scenario, but with such a cornucopia or
M type stars, it surely would add to the probability of finding ANY type
terrestrial object in HZ of stars overall.

andy March 5, 2014 at 16:37

Gliese 229 already has a claim to fame as the companion of the brown dwarf Gliese 229B, which was the first identified T dwarf.

As for exomoons around M-dwarfs, René Heller finds that serious problems to habitability occur for lower mass stars thanks to tidal heating. Depending on the eccentricity of the satellite orbit, the problems set in below 0.5 solar masses, and 0.2 solar masses as a lower limit for Earthlike moons to avoid entering a tidally-induced runaway greenhouse transition.

Phil March 5, 2014 at 19:40

It might also be worth mentioning, for those who havent read the paper, that this team has found interesting signals, not yet significant enough to claim any discoveries with but certainly worth continuing investigation, around several other nearby low mass stars – one of which is Gj551, otherwise known as Proxima centauri (two long period signals there, so even if they end up being confirmed we are not talking about planets in the Hz).

Watch this space.

p

Michael March 5, 2014 at 22:59

Keep this date Oct 2014 in mind

Proposed micro lensing observation of Proxima Centauri in the search for orbiting planets.

http://arxiv.org/pdf/1401.0239.pdf

Harry R Ray March 6, 2014 at 11:01

Two things come to mind immediately! ONE: Another claim from this study is that almost all M dwarf stars have at least one planet. That means that the odds should be overwhelming for a planet of some kind around Proxima Centauri! Was this star included in his initial search, and, is it possible that one(or more) of the additional ten undocumented planets orbit Proxima! TWO: Can combining the radial velocity data from HARPS and UVES for Glises 667C be used to confirm the earlier claim of additional planets in this system? If anyone knows the answers to the two above questions, please post a comment! Thank you!

Eniac March 6, 2014 at 22:37

andy:

As for exomoons around M-dwarfs, René Heller finds that serious problems to habitability occur for lower mass stars thanks to tidal heating.

How is it that this tidal heating of exomoons cannot be fully compensated for by moving the HZ outwards a bit? Am I wrong in thinking that there isn’t a problem at all, even if Heller’s model is correct?

Michael March 7, 2014 at 14:52

@Eniac March 6, 2014 at 22:37

‘How is it that this tidal heating of exomoons cannot be fully compensated for by moving the HZ outwards a bit?’

You need to increase the mass of the star which increases the width and distance of the HZ from the star allowing the moon to be further away leading to less tidal heating (more room for the moons orbit). That is the way I am reading it. Or do you meaning moving the moon/planet further out so tidal heating keeps the moon warmer as it moves outside the cold end of the HZ and the planet inside the HZ.

Eniac March 8, 2014 at 0:06

Or do you meaning moving the moon/planet further out so tidal heating keeps the moon warmer as it moves outside the cold end of the HZ and the planet inside the HZ.

That is what I meant, of course. It means redefining the HZ to suit the model. The HZ is not a fixed region. It is dependent on your model. It is where your super-Earth/exomoon is at just the right temperature, thick atmosphere and/or tidal warming fully accounted for. Tidal heating does not reduce the number of habitable planets at all, it just moves them further out.

Michael March 8, 2014 at 16:07

@Eniac March 8, 2014 at 0:06

‘Tidal heating does not reduce the number of habitable planets at all, it just moves them further out.’

I think they mean statistical there will be less habitable moons because of the tidal heating effects.

Although tidal heating will be another form of ‘heat’ energy that organisms could use a heavily volcanic world is not ideal one, look at Io around Jupiter. It is constantly been resurfaced over only decades, however if it was a water covered moon that volcanic process may aid life by circulating nutrients from the interior.

Michael March 9, 2014 at 14:26

I liked this informative article about Red Dwarfs, they may have narrow HZ’s but they are stable for billions of years. Any frozen worlds (maybe not tidally locked) further out will eventually get heated up later on again for similar timespans. I also liked that they turn blue as they get much older!

http://www.astroscu.unam.mx/rmaa/RMxAC..22/PDF/RMxAC..22_adams.pdf

‘Another interesting feature in Figure 2 is the track of the star with M = 0:16M . Near the end of its life, such a star experiences a long period of nearly constant luminosity, about one third of the solar value. This epoch of constant power lasts for nearly 5 Gyr, roughly the current age of the solar
system and hence the time required for life to develop on Earth.’

Eniac March 10, 2014 at 22:55

I think they mean statistical there will be less habitable moons because of the tidal heating effects.But that would not be the case. Whatever you lose to tidal heat at the inner edge where the extra heat hurts, you gain at the outer edge where it helps. There is no reason I can think of that more is lost than gained, and if one is given in the paper, it has not been mentioned here.

Eniac March 10, 2014 at 22:56

I think they mean statistical there will be less habitable moons because of the tidal heating effects.

But that would not be the case. Whatever you lose to tidal heat at the inner edge where the extra heat hurts, you gain at the outer edge where it helps. There is no reason I can think of that more is lost than gained, and if one is given in the paper, it has not been mentioned here.

Holger March 13, 2014 at 11:37

Whatever you lose to tidal heat at the inner edge where the extra heat hurts, you gain at the outer edge where it helps. There is no reason I can think of that more is lost than gained, and if one is given in the paper, it has not been mentioned here.

The frequency of planets starts decreasing with larger orbital distance at some point; this could be the reason for the claimed decrease of habitable-zone candidates.

ljk May 15, 2014 at 13:44

Fourteen red dwarf stars that can be viewed by amateur astronomers stuck on Earth:

http://www.universetoday.com/111716/14-red-dwarf-stars-to-view-with-backyard-telescopes/

Includes a nice three-dimensional star chart for interstellar navigation.

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