Prospects for Red Dwarf ‘Earths’

by Paul Gilster on March 17, 2009

Most stars in our region of the galaxy are low-mass M-dwarfs, making the investigation of their planetary systems quite interesting. If we learn that stars like these, which comprise over 70 percent of the galactic population, can be orbited by Earth-like planets, then the galaxy may be awash with such worlds. But some models have indicated that Earth-sized planets would be rare around these stars, working on the assumption that scaled-down versions of the Sun’s protoplanetary disk would tend to produce only low-mass planets.

Clearly, we need to know more about the masses of such inner disks, since available mass seems to be a key to the formation of habitable planets. Extrapolate the early nebula from our own Solar System to lower protoplanetary disk masses around M-dwarfs and the terrestrial worlds that form are no larger than Mars — they’re small, dry, worlds unlikely to develop life. Low-mass disks would seem to lead to low-mass planets.

But what if those M-dwarf protoplanetary disks aren’t just scaled-down versions of our Sun’s? Fortunately, we can use accretion simulations to study the possible results of varying these parameters, which is what Gregory Laughlin and Ryan Montgomery recently set out to do. From their paper:

Time and again over the past decade, nature has surprised planet hunters with the sheer diversity of planetary systems. Experience has shown that planet formation under a variety of conditions can be more efficient than the example of our own system would suggest. Our approach in this paper has been to investigate a planet building model that lies at the optimistic, yet still justifiable range of formation scenarios. Should our picture prove correct, then the prospects for discovery of truly earthlike, alarmingly nearby, and potentially habitable planets may lie close at hand.

red_dwarf_star

I like that ‘alarmingly nearby’ phrase! To set about investigating this, Laughlin and Montgomery weigh different disk and star mass scenarios. They study both Io and GJ 876d for clues, looking for reasonable answers to the question of disk density at a close orbital radius from low mass stars. They draw on the formation of the moons of Jupiter as inspiration for their model.

Image: In learning more about how planets form around M-dwarfs, we’ll discover how common Earth-mass planets may be. Credit: David Aguilar/Harvard Smithsonian CfA.

And indeed, the simulated M-dwarf planets that emerge from their simulations are worlds with more similarities to Jupiter’s larger moons than to Earth or Venus. Using an M-dwarf some twelve percent as massive as the Sun as their base, the team simulates the accretion of planetary embryos through the late phases of growth, finding that systems of three to five planets could emerge with masses comparable to Earth’s in stable orbits in or near the habitable zone, along with one to two planets per system with masses comparable to Mars.

The further good news is that a terrestrial world like this around a nearby low-mass M-dwarf should be detectable with radial velocity measurements. Narrowing the sample of local stars to a prime catalog of 169 M-dwarfs, the team tests for detection possibilities from the ground and, with a larger range of targets, from space, the latter simulated by plugging in the characteristics of the proposed TESS (Transiting Exoplanet Survey Satellite) spacecraft. While ground detections are marginal, the simulation showed that a space observatory like TESS would find some seventeen of the simulated planets after a two year survey. It wouldn’t take much, in other words, to put this theory to an early test.

Not only that, but a low-mass star offers up a readily detectable field for a transit of a planet of this size. And because the Laughlin/Montgomery model would produce planets that are poor in volatiles (and thus smaller in radius for a given mass than their volatile-rich counterparts), a Doppler wobble combined with a transit could determine whether this scenario is viable.

All of this used to seem so utterly theoretical, but with both CoRoT and Kepler in space, we’re getting used to the idea that finding Earth-class planets may not take more than another couple of years. Learning that habitable worlds are possible around the numerous red dwarfs in our neighborhood is clearly something we can accomplish, with the added advantage of learning much about how these planets form. The paper is Montgomery and Laughlin, “Formation and Detection of Earth Mass Planets Around Low Mass Stars,” accepted by Icarus and available online.

{ 20 comments }

andy March 17, 2009 at 16:17

What would really help in the field of red dwarf planets detection is high precision RVs at infrared wavelengths. The technology is catching up with visual spectra though.

Infrared RVs would also be very useful to probe planetary systems around brown dwarfs.

James M. Essig March 17, 2009 at 18:00

Hi Folks;

This is an interesting thread. With 70 percent of the stars in the Milky Way being low mass red dwarfs, the opportuinity for a future human civilization to spread throughout the galaxy which can be sustained for over 10 trillion years or more is fantastic.

Simply mastering 0.1C travel opens up the whole Milky Way for colonization and for 0.1C travel out to other galaxies in huge world ships.

With the development of high gamma factor space travel, the prospects of humans traveling out of our local super cluster becomes more managable.

Medical breakthroughs that perhaps offer a chance of virtual immortality can no doubt facilitate colonization of our universe out to atleast the distance of the currently observable universe.

Simply reading this article has greatly renewed my hope for future habitats for our species over truly cosmic time epochs.

One way or another we are going to the stars and I see that travel to other galaxies wherein even after traveling 10 billion or perhaps 100 billion lightyears through space, we will still have essentially 10 trillion more years to dwell of planets around red dwarfs. It is perhaps the mere fact that these long lived Red Dwarfs exist that perhaps we can set up shop all over the currently visible universe.

Thanks;

Jim

Dave Moore March 17, 2009 at 20:57

I want to put in a note of caution about the frequency of Earthlike worlds. There runs the assumption that if we find a planet close to Earth’s mass with an isolation matching ours, that we will get something like today’s Earth with or without the oxygen in the atmosphere.
I would be very surprised if we don’t find a fair number of bodies with Earth’s approximate mass and isolation, but have been constantly surprised by the variety of planets we have discovered so far and Earth with it’s liquid/solid/gas interface presents one of the most complex geochemical states on a planet (this may be why life evolved here).
I suspect that Earth-sized planets in the liquid water zone will come in an enormous variety. If you think of Earth’s history, it could be considered 4 different planets. First there was the mainly oceanic planet with a thick CO2/N2 atm. Then there was the brown Methane planet when the methanogens evolved. And there was the Ice-ball before our version of Earth came into existence. Also we very nearly had a planet with a CO2/N2/H2S atm, if sulfur bacterial had predominated (Ward – Green Skies).
Earth like does not necessarily mean like Earth.

mike shupp March 17, 2009 at 23:21

Actually, given the large numbers of “Super-Jupiters” we’ve spotted so far, the great question seems to be “Why are all of Sol’s planets so puny?”

My gut feeling at this point is that planets of Earth-Venus-Mars size are probably found around ANY star, from class O to M. Granted they may not all be within habitable life zones.

Marvelous, ain’t it! As a 62 year old guy, I can recall when astronomy books for general readers stated that stars with planets were probably extremely rare, then when the books said stars with planets might be reasonably common but that such planets could never be discerned, then when they said it seemed likely to astronomers that extra-solar planets might eventually be detected — maybe half way thru the 21st Century. And now the planetary count is over 300. This is so freaking wonderful!

Administrator March 18, 2009 at 8:44

mike shupp wrote:

As a 62 year old guy, I can recall when astronomy books for general readers stated that stars with planets were probably extremely rare, then when the books said stars with planets might be reasonably common but that such planets could never be discerned, then when they said it seemed likely to astronomers that extra-solar planets might eventually be detected — maybe half way thru the 21st Century. And now the planetary count is over 300. This is so freaking wonderful!

Mike, you’ve captured my feeling exactly. It’s all happened so fast! Truly a golden age of exoplanetary discovery, and we’re lucky to be here to see it.

philw1776 March 18, 2009 at 11:34

Dave Moore says it well. Earth sized and in the HZ opens up a plethora of alternatives quite varied from the ocean & land O2 world that we cannot yet even imagine. Just as we were totally surprised by Hot Jupiters, I’m sure that the infinite variety of possibilities of terrestrial scale planets will amaze us. Add in the strange tidal locked environs of M stars and you’ve got mryiads of speculative scenarios.

And Mike, I totally share your feelings of gratitude and wonderment comparing the late 50s to our emerging knowlege today.

Enzo March 18, 2009 at 17:09

Would a moon around a tidally locked planet be stable ?
My gut feeling is that the answer is no. Does anyone know ?

However, if it was possible, especially for a reasonably fast orbiting one (hence fast rotation), that might allow for a magnetic field to shield it from the flares and atmosphere erosion.
See here :

Of course a moon big enough to be earth like would probably orbit a big giant with a nasty radiation belt of its own.

Paul F. Dietz March 18, 2009 at 20:18

A planet in orbit around a red dwarf in the habitable zone would be moving quite fast. Impacts of comets or asteroids would be correspondingly more energetic which could affect the chance higher forms of life would evolve.

Frank Smith March 19, 2009 at 8:47

I thought flares were fairly common with Red Dwarf stars.

If so, it would be tough for life to survive

Administrator March 19, 2009 at 13:24

Frank, good point, though much depends on the amount and intensity of the flare activity, and of course the possibility of it becoming an evolutionary trigger can’t be ruled out until we know more. For those interested, we wrote up the EV Lacertae flare here:

http://www.centauri-dreams.org/?p=1889

ljk March 19, 2009 at 23:45

Transits of Earth-Like Planets

Authors: L. Kaltenegger, W.A. Traub

(Submitted on 19 Mar 2009)

Abstract: Transmission spectroscopy of Earth-like exoplanets is a potential tool for habitability screening. Transiting planets are present-day “Rosetta Stones” for understanding extrasolar planets because they offer the possibility to characterize giant planet atmospheres and should provide an access to biomarkers in the atmospheres of Earth-like exoplanets, once they are detected.

Using the Earth itself as a proxy we show the potential and limits of the transiting technique to detect biomarkers on an Earth-analog exoplanet in transit.

We quantify the Earths cross section as a function of wavelength, and show the effect of each atmospheric species, aerosol, and Rayleigh scattering. Clouds do not significantly affect this picture because the opacity of the lower atmosphere from aerosol and Rayleigh losses dominates over cloud losses.

We calculate the optimum signal-to-noise ratio for spectral features in the primary eclipse spectrum of an Earth-like exoplanet around a Sun-like star and also M stars, for a 6.5-m telescope in space.

We find that the signal to noise values for all important spectral features are on the order of unity or less per transit – except for the closest stars – making it difficult to detect such features in one single transit, and implying that co-adding of many transits will be essential.

Comments: 17 pages, 3 figures, 6 tables, to appear in ApJ (accepted)

Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP)

Cite as: arXiv:0903.3371v1 [astro-ph.IM]

Submission history

From: Lisa Kaltenegger [view email]

[v1] Thu, 19 Mar 2009 17:22:51 GMT (786kb)

http://arxiv.org/abs/0903.3371

forrest noble March 22, 2009 at 14:42

Hey Jim and all,

Red dwarf “planets” are particularly interesting to me. Even though most likely they have fewer planets in their stellar system, my guess is that maybe 3% have water planets in the proper position in these system. Look at our own solar system; water is a relatively large percentage of it. There are Earth, Venus, Mars, the ice moons of Jupiter and others, and all the water tied up in the outer planets. All of these planets/ moons had the potential of being water planets if they were positioned similarly to the Earth.

According to astronomers, some of these M dwarf stars and their planets could be more than twice the age of the Earth and Sun. I believe that planets of all sizes can exist in these systems and that the universe and our galaxy are also much older than we presently believe. Also small stars evolve very slowly allowing more stable conditions for its planets, moons. Any one or combination of these factors could allow a lot more time for life to get started, evolve, and prosper in such a stellar system. At the least it would seem that there are a lot of possibilities for new homes for us earthlings. around M dwarfs.

kurt9 March 22, 2009 at 16:14

Wouldn’t flares have less effect on oceanic life than land life? Perhaps habitable planets around M-stars have ocean life but no land life.

Enrico Fermi March 23, 2009 at 13:41

So where are they? ;)

James M. Essig March 23, 2009 at 17:23

Hi Forrest;

Thanks for the above comments.

A truely beautiful thing about Earth like planets around Red Dwarfs is that the can in theory permit life, ETI life and human life, to evolve over periods of trillions of years per individual biosphere.

The cores of such planets might convievably be kept warm by fusion powered furness like mechanisms, a process that would permit these planets to have an active magnetosphere which could mitigate loss of atmospheric gases to space.

We see how far we have come in just 150 years. Imagine how advanced we can become over one trillion years of evolution which in itself could be driven by technology.

I can see that Red Dwarf based planets are potentially a happening place. It should be nice to see the results of the Kepler observatory in the next few years.

Regards;

Jim

spaceman March 24, 2009 at 1:48

Hi Jim,

Yes, indeed, it should be nice to see the results from Kepler in the next several years. People often focus on Kelper’s ability to detect an Earth-like planet around a Sun-like star at ~1 A.U., while often losing sight of the fact that Kepler will be able to tell us a lot about the types/numbers of terrestrial planets around M dwarfs. Afterall, wouldn’t an Earth-sized world in the habitable zone of an M dwarf be easily detected via Kepler?

spaceman April 1, 2009 at 13:05

Will Kepler and Corot be able to beat out the ground-based surveys and TESS in terms of finding the terrestrial-sized planets?

Administrator April 1, 2009 at 14:15

It may turn out to be quite a close call. There are some thoughts that a radial velocity detection of a terrestrial world isn’t that far away. In any case, we’ll know within the next two-three years.

ljk April 7, 2009 at 17:25

Life around cool stars may be different than us

It’s amazing to think that just a few years ago we had no clue about planets around other stars. Now we know of over 300, and we’re getting an idea of how they form, where they form, how they behave, and whether there’s a chance of any being like home, our home.

Not only that, we’re learning whether they can form around stars that are different than the Sun: more massive, less massive, hotter, cooler, whatever.

And new Spitzer Space Telescope results show that when conditions to form planets are different, the chemistry is different as well.

Full article here:

http://blogs.discovermagazine.com/badastronomy/2009/04/07/life-around-cool-stars-may-be-different-than-us/

ljk October 9, 2009 at 21:55

On the Method to Infer an Atmosphere on a Tidally-Locked Super Earth Exoplanet and Upper limits to GJ 876d

Authors: S. Seager (1), D. Deming (2) ((1) Mit, (2) NASA/GSFC)

(Submitted on 8 Oct 2009)

Abstract: We develop a method to infer or rule out the presence of an atmosphere on a tidally-locked hot super Earth. The question of atmosphere retention is a fundamental one, especially for planets orbiting M stars due to the star’s long-duration active phase and corresponding potential for stellar-induced planetary atmospheric escape and erosion.

Tidally-locked planets with no atmosphere are expected to show a Lambertian-like thermal phase curve, causing the combined light of the planet-star system to vary with planet orbital phase.

We report Spitzer 8 micron IRAC observations of GJ 876 taken over 32 continuous hours and reaching a relative photometric precision of 3.9e-04 per point for 25.6 s time sampling. This translates to a 3 sigma limit of 5.13e-05 on a planet thermal phase curve amplitude.

Despite the almost photon-noise limited data, we are unable to conclusively infer the presence of an atmosphere or rule one out on the non-transiting short-period super Earth GJ 876d. The limiting factor in our observations was the miniscule, monotonic photometric variation of the slightly active host M star, because the partial sine wave due to the planet has a component in common with the stellar linear trend.

The proposed method is nevertheless very promising for transiting hot super Earths with the James Webb Space Telescope and is critical for establishing observational constraints for atmospheric escape.

Comments: Published in ApJ

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Journal reference: ApJ, 2009, 703, 1884

Cite as: arXiv:0910.1505v1 [astro-ph.EP]

Submission history

From: Sara Seager [view email]

[v1] Thu, 8 Oct 2009 13:55:45 GMT (223kb)

http://arxiv.org/abs/0910.1505

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