Habitable Zone Planets: Upping the Numbers

by Paul Gilster on March 14, 2013

Whether we’re planning to go to the stars on a worldship or with faster transportation, the choice of targets is still evolving, and will be for some time. Indeed, events are moving almost faster than I can keep up with them. It was in early February that Courtney Dressing and David Charbonneau (Harvard-Smithsonian Center for Astrophysics) presented results of their study of 3897 dwarf stars with temperatures cooler than 4000 K, revising their temperatures downward and reducing their size by 31 percent. The scientists culled the stars from the Kepler catalog, and their revisions had the effect of lowering the size of the 95 detected planets in their data.

They went on to deduce that about 15 percent of all red dwarf stars have an Earth-sized planet in the habitable zone. [PG note: The 15% figure is a revised estimate that I've just learned about from Ravi kumar Kopparapu. Dressing and Charbonneau call attention to this change at the end of their paper. See citation below].

That would make the nearest Earth-like planet in the habitable zone about 13 light years away (I’m still holding out for the much closer Proxima Centauri when it comes to M-dwarfs). But Ravi kumar Kopparapu (Penn State) has revisited Dressing and Charbonneau’s work because of a key fact: The latter used habitable zone limits based on a 1993 study by James Kasting which Kopparapu believes are not valid for stars with effective temperatures less than 3700 K. The scientist was in the news almost as recently as Dressing and Charbonneau with his study of habitable zones around main sequence stars, where he presented an improved climate model developed with Kasting and other colleagues that allowed him to move the habitable zone boundaries out a bit further from their stars than they had been before (see Habitable Zones: A Moving Target for more).

Rory Barnes (University of Washington) called the work of Kopparapu and colleagues ‘the new gold standard for the habitable zone,’ and in a paper just accepted by Astrophysical Journal Letters, Kopparapu now uses his habitable zone revisions to estimate the rate of occurrence of terrestrial-sized planets in the habitable zone of M-dwarfs. His new paper was based on the Harvard team’s data and used the same calculation method. But with the new habitable zone parameters worked in, the number of habitable planets is greater than previously thought. Four out of ten of the nearest small stars should have potentially habitable planets.

habitable planet

Image: The graphic shows optimistic and conservative habitable zone boundaries around cool, low mass stars. The numbers indicate the names of known Kepler planet candidates. Yellow color represents candidates with less than 1.4 times Earth-radius. Green color represents planet candidates between 1.4 and 2 Earth radius. Planets with “+” are not in the habitable zone. Credit: Penn State.

Kopparapu’s new work would place the average distance to the nearest habitable planet at around 7 light years. Given that there are eight stars within 10 light years of the Sun that fit this model, we could expect to find perhaps three Earth-sized planets in the habitable zones there.

M-dwarfs are becoming increasingly important in the search for terrestrial-class worlds. Because their orbits are close to the parent star, habitable worlds in such systems would transit often and produce a stronger transit signal than a similar planet around a G-class star like the Sun. That makes M-dwarfs good Kepler targets and also suggests that future space-based missions may find this class of star an extremely useful target. Given that M-dwarfs may comprise as much as 80 percent of all stars in the galaxy, it could turn out that most life-bearing planets orbit small red stars, assuming life can indeed develop around them.

The paper is Kopparapu, “A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs,” accepted at Astrophysical Journal Letters (preprint). The Dressing and Charbonneau paper is “The Occurrence Rate of Small Planets Around Small Stars,” to be published in The Astrophysical Journal (draft version online).

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{ 24 comments }

Harry R Ray March 14, 2013 at 10:01

WHAT DID I TELL YOU! In my PREVIOUS comment on this subject, I KNEW this would happen! I am a bit perplexed about one thing, though. In the Charboneau/Dressing paper, KOI2626.o1 was listed at 1.7 Re, and KOI2626.01 was listed at 1.4 Re. Now, BOTH are yellow in the above illustration, indicating that Kapparopu believes them to be LESS than 1.4 Re This means that KOI1422.01′s radius May also be revised downward, perhaps BELOW the level of habitability. If anyone who comments here regularly has more detailed information on this, please post a comment.

andy March 14, 2013 at 17:17

Eight? I get seven:

Proxima Centauri (V645 Centauri): flare star, quite an active one too.

Barnard’s Star (V2500 Ophiuchi): quiet, but still can spring surprises. GCVS lists it as a BY Draconis star.

Wolf 359 (CN Leonis): flare star, quite a high flare rate.

Lalande 21185 (NSV 18593): not sure about this one, SIMBAD lists it as a flare star but this is not mentioned in its NSV entry, which lists it as a possible BY Draconis star. I wonder if anyone’s checked those astrometric planet claims yet…

BL Ceti: flare star, in a binary with…

UV Ceti: it’s UV Ceti. ‘Nuff said.

Ross 154 (V1216 Sagittarii): yet another flare star.

Am I missing one?

Astronist March 14, 2013 at 17:39

Andy, Paul may be including Luhman’s Star, a binary brown dwarf only just discovered this month (and currently referenced as WISE J and then a long string of digits).

Stephen

Tarmen March 14, 2013 at 18:05

Andy, these dwarfs are like islands in the sea, good raw matter and energy for our kind. We can use ‘em somehow. I can imagine colonists desperate for fuel, drifting into a flare star system as a last resort. Many of our colonial ships will surely freeze in the dark. UV Ceti may serve as a’ last chance for 200 miles’ gas station.

Ronald March 14, 2013 at 19:00

Although I like the optimism, a few words of caution;

The author uses very generous assumptions for defining a habitable planet in his most optimistic estimate: a radius for earthlike planets of 0.5 – 2 Re, and a habitable zone (HZ) from 0.75 – 1.77 AU (in our solar system). In doing so, he comes to an estimate of about 0.5 – 0.6 habitable planets per M-dwarf star!
With regard to the HZ, in particular the inner edge at 0.75 AU (based on a younger Venus limit) is very optimistic for a long-term HZ.

Even in his most conservative estimate, using a planet radius of 0.5 – 1.4 Re and a HZ from 0.97 – 1.7 AU (as per Kopparapu, Ramirez, Kasting et al., 2013) and after applying another correction (see Discussion in the paper), the fraction of habitable planets is still 0.41 per star.

Well, according to that recent re-estimate of the HZ (Habitable Zones Around Main-Sequence Stars: New Estimates; 2013), which is an update of the landmark work on HZ definition and estimation by Kasting (1993), the HZ (in our solar system) should be defined as from 0.97 – 1.70 AU.
Also, according to Kepler definitions, an R of 0.8 – 1.3 (corresponding to a mass of roughly 0.5 to just over 2 Me) would be more realistic for defining an earthlike planet.

If we took the more conservative, but still generous, HZ (0.95 – 1.7 AU) and a planet R range from 0.8 – 1.3 Re, their lower estimate would again be reduced. By how much? After checking the Kepler data (KOI) in the paper, I noticed that, by leaving out planets between 0.5 – 0.8 and 1.3 – 1.4 Re, the fraction would be reduced by about 20-25%, i.e. to about 0.3 per M-dwarf. Remarkably, there are very few planets 0.5 – 0.7 Re, but that may partly be observational bias. Even if we cut their most conservative result by half, that is still about 0.2 per M-dwarf.

Quite impressive, also in comparison with recent estimates of the fraction of habitable earthlike planets around solar type stars. Maybe that estimate will also go up as more data come in.

Of course, with red dwarfs (M-dwarfs) you have the disadvantages of a very narrow HZ, tidal locking and often flare stars, as andy also points out above.

Ronald March 14, 2013 at 19:14

andy: I also came to 7 instead of 8 M-dwarfs within 10 ly. But maybe they included Ross 248 (HH Andromedae) at 10.3 ly.

Ronald March 14, 2013 at 19:19

Ross 248 is also a flare star, btw.

coolstar March 14, 2013 at 20:04

I haven’t read this recent work yet but it seems not, given the info above, to be consistent with Charbonneau’s own work on the Mearth project. And, it’s worth remembering that where these planets FORMED is likely much more important than where they end up……

Horatio March 14, 2013 at 20:37

@andy

List of stars within 5 parsecs from Sol

http://en.wikipedia.org/wiki/List_of_nearest_stars#List

Horatio March 14, 2013 at 20:45

@andy

You’re understandably missing the very recently discovered binary brown dwarf WISE 1049-5319.
http://en.wikipedia.org/wiki/WISE_1049-5319

lepton March 15, 2013 at 4:09

The 8th maybe Ross 248 at 10.322ly, rounded to closest integer is 10.
The next M star Lacaille 9352 is at 10.742ly, that will round to 11.

Didac March 15, 2013 at 5:24

Andy, Ross 248 (HH Andromedae) is only slightly above 10 light-years.

Paul Gilster March 15, 2013 at 10:24

Astronist writes:

Actually, I was pulling that figure from (if memory serves) Dr. Kopparapu’s paper, though I don’t recall the stars being listed.

Jer March 15, 2013 at 10:41

In reading the last several articles, it seems we are focussing on the big picture: ‘solar system’ layout and constituent star and planet characteristics, to determine how habitable/ colonizable /survivable /visitable the system is. However, perhaps we need to consider the small picture as well. As someone without any astrophysics nor exo-biology background, i am only guessing, but:
http://news.ncsu.edu/releases/tpleeanthropic/
“Foundations of Carbon-Based Life Leave Little Room for Error” seems important. Especially the start of the second paragraph: “Both carbon and oxygen are produced when helium burns inside of giant red stars. Carbon-12, an essential element…”

Rob Flores March 15, 2013 at 12:37

Are we really going to send a very costly colonization effort to
Red Dwarfs? Is that the plan? surveing the Real Estate.

Tidelocked = Narrow Band of habitablity.
Flares = Cavern Living, foods stuffs also grown indoors.
Surface operations = Difficult due to hostile surface weather.
Low Orbit Launch/Ops = lots of obstacles.
Plate tectonics = Subduction on the wetside of planet, not on dry. Chaotic.
Can’t be terraformed.

This sounds like a dead end to me. Until we find a very Earthlike planet it’s
better to hold up in our own solar system.

Also even if propusionl reaches a hight percent of Speed of light, is there not a practical limit to relativistic speeds. Unless you cap your speed at 45% of C, I don’t see how you could evade substatial sized debris alogng your flight parth, which would be a mortal threat.

andy March 15, 2013 at 13:51

I left WISE J104915.57-531906.1 off the list because neither of them are M-dwarfs. (I also left out the Sun, Alpha Centauri A/B and Sirius A/B for the same reason.)

WISE 1049-5319A = L8±1
WISE 1049-5319B = L/T

Rob Henry March 15, 2013 at 17:35

I find the idea that the equivalent HZ for tidally locked planets will turn out to be the same as for free-rotating ones ludicrous.

I realise that much of its atmosphere will freeze out on its night side under many conditions, but for these purposes we have to assume that its heat circulation or atmospheric rate of return from the dark side is sufficient, so we are left with a typical HZ planetary hemisphere with twice the average daylight for its distance from their sun, and a sub solar point with PI times as much sunlight as the hottest time for the tropics (that is during the equinox) of a freely rotating planet.

To me, the habitable zone HAS to extend out further here. If not, what am I missing?

Daniel March 15, 2013 at 18:09

Luyten 726-8 A and B at 8.72 light years is M-Dwarf binary System that you people missing I believe.
However binary orbital period is 26.5 years and Orbital Eccentricity is 0.62 which make the distance of both stars between 2.1 and 8.8 AUs , that may cause orbital instability in the possible planets in this stars Habitable zone

Well how knows maybe still a chance for HZ terrestrial planets on this binary system

Daniel March 15, 2013 at 18:17

Oh Yeah I see UV Ceti is Luyten 726-8 system sorry my mistake.

But well still good to point out the star characteristics and still the missing star that you talking about could be the B star of this system

Ronald March 15, 2013 at 19:58

(Something came through distorted in my previous comment; Paul, please ignore that one, here is the correct one)

Ok, I checked the nearest 48 M-dwarfs up to 5 parsec (16.3 ly);

Of those 48 M-dwarfs:
- At least 24, i.e. half, are flare stars.
- 12, i.e. a quarter, are at least (very) variable, mostly of the BY Draconis type.
- 12, i.e. also a quarter, are non-variable, but 5 0f these are extremely dim (less than 0.1 % of solar).

That leaves 7 M-dwarfs out of 48, less than 15%, as non-variable with reasonable luminosity (greater than 0.1%), being:
Luyten’s Star, Kruger 60A, Wolf 1061, Gliese 687, Gliese 674, Gliese 412A, Gliese 832.

GaryChurch March 16, 2013 at 12:30

“-is there not a practical limit to relativistic speeds. (?)

The most commonly proposed solution is a “deflector beam” that is projected ahead of the Starship to deflect particles out of the flight path. There is a large power curve at about .3c that goes up steeply and may be a limiting factor. But even at .1c Starflight becomes practical with suspended animation….and even without suspened animation for a generation ship. I think freezing people will be the way it happens though.

The only concept I have read about that does not require unobtanium is the small singularity engine. The incredible energy needed to manufacture these tiny black holes will not be availabe for another century at the least. This kind of power may be able to project a deflector beam and travel above .99c where time dilation makes trips to almost anywhere only a matter of a few years of shipboard time. Even other galaxies.

andy March 16, 2013 at 18:27

@Daniel: The count of 7 M-dwarfs within 7 light years includes both stars of the Luyten 726-8 system:

Luyten 726-8 A = BL Ceti
Luyten 726-8 B = UV Ceti

andy March 16, 2013 at 18:28

*10 light years

ljk March 20, 2013 at 9:09

Universe Today via PhysOrg, 3/20/2013:

“Closest exoplanet deserves a ‘real’ name, says Uwingu”

“It’s time to ‘get real’ about naming exoplanets, says Uwingu CEO and
scientist Dr. Alan Stern. And so the latest project from the space
funding startup company is a contest to name the nearest exoplanet,
currently known as Alpha Centauri Bb.

‘Let’s face it,’ Stern told Universe Today, ‘the current names
astronomers use for exoplanets are boring. The public is really
excited about all the planets that are being found around other stars,
but the names do nothing to help fuel that excitement. We’re giving
the public the chance to name the closest exoplanet.’”

[snip]

“‘The IAU has had ten years to do something about this and they
haven’t done anything,’ said Stern. ‘What we’re doing might be
controversial, but that’s OK. It’s time to step up to the plate and do
something.’”

http://phys.org/news/2013-03-closest-exoplanet-real-uwingu.html

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