Possible Habitable World in a Six-Planet System

by Paul Gilster on November 8, 2012

At 42 light years from Earth, the star HD 40307 is reasonably within the Sun’s neighborhood, so the news of a potentially habitable planet there catches the eye. HD 40307 is a K-class dwarf, one previously known to be orbited by three super-Earths — with masses between the Earth and Neptune — that are too close to the star to support liquid water on the surface. Now we have the discovery, announced in a new paper in Astronomy & Astrophysics, of three more super-Earth candidates found by digging into data from HARPS (the High Accuracy Radial Velocity Planet Searcher) and HIRES (the High Resolution Echelle Spectrograph).

Mikko Tuomi (University of Hertfordshire) and team put a new software tool called HARPS-TERRA to work on the archival data that allowed them greater precision in filtering out false positives from stellar activity. Says Tuomi:

“We pioneered new data analysis techniques including the use of the wavelength as a filter to reduce the influence of activity on the signal from this star. This significantly increased our sensitivity and enabled us to reveal three new super-Earth planets around the star known as HD 40307, making it into a six-planet system.”

The outermost planet of the six is the one to watch. HD 40307’s habitable zone lies between 0.43 and 0.85 AU. With an orbital semi-major axis of 0.60 AU, candidate planet HD 40307 g is in the sweet spot, receiving about 62 percent of the radiation the Earth receives from the Sun. The paper notes that while that level of radiation may seem low as compared to the Earth, our planet actually lies fairly close to the inner boundary of the Sun’s habitable zone.

Image: An artist’s impression of HD 40307 g. Credit: University of Hertfordshire.

Unfortunately, we aren’t likely to get any help from transits here, as previous attempts to detect the inner candidate HD 40307 b have been unsuccessful, and the likelihood of a transit by HD 40307 g is considered to be about 0.6 percent. But the planet is enticing. From the paper:

As is the case with the previously reported planets in the HZs of nearby stars, additional observational information is necessary – on top of confirming the planetary nature of the three new signals we report in this work – to decide if HD 40307 g indeed can support water on its surface. In the meantime, and given that it would be a rather massive object for a telluric planet, detailed climatic (e.g. Barnes et al., 2012) and planetary interior simulations (e.g. Korenaga, 2010; Stein et al., 2011) can be used to estimate its suitability for supporting liquid water and, perhaps, life. Unlike the other previously reported candidate habitable planets around nearby M dwarfs (e.g., GJ 581 and GJ 667C), HD 40307 g is located farther away from the central star and, like the habitable zone planet candidate Kepler 22b (Borucki et al., 2012), it should not suffer from tidal locking either (Kasting et al., 1993; Barnes et al., 2009b).

HD 40307 g has a minimum mass of about 7 Earth masses, and according to the researchers, it is so deeply nested inside the habitable zone that even with a slightly eccentric orbit, it would remain inside the HZ for a full orbital cycle. In a University of Hertfordshire news release, researcher Guillem Angla-Escude notes that “The star HD 40307, is a perfectly quiet old dwarf star, so there is no reason why such a planet could not sustain an Earth-like climate.” Meanwhile, planet formation theorists will have plenty to work with as they add the system around HD 40307 into the exoplanet mix. Its architecture is different enough from our own — with the tight packing of six planets inside 0.60 AU — that a range of formation possibilities come into play as we ponder how Earth-mass objects eventually emerge in their star’s habitable zones.

Moreover, whereas the interesting and potentially habitable Kepler 22-b is located about 200 pc from Earth, HD 40307 is close enough that the star-planet angular separation should be workable for future missions like ESA’s Darwin or NASA’s Terrestrial Planet Finder, assuming these find their way through budget hurdles and eventually get built. If HD 40307 g can be confirmed, we could be talking about the first habitable planet to be observed at this level.

A Digression on Delayed Missions

Let’s pause on this for a moment as I recall Michael Lemonick’s recent op-ed in the Los Angeles Times. Lemonick, whose new book Mirror Earth is just out (Walker & Company, 2012), runs through what was supposed to have happened with space-based observatories. By the early 2000s, the Space Interferometry Mission was going to bring new precision to exoplanet finding, while the James Webb Space Telescope, then planned as bigger and more powerful than the one under construction, was supposed to be launched and operational by 2007. Terrestrial Planet Finder, designed to image those ‘mirror Earths,’ was banked for 2020.

Says Lemonick:

As of today, however, SIM has been canceled; the smaller, less powerful Webb will launch by 2018 perhaps; and the TPF has been put on the back burner, maybe permanently. These disappointments have partly to do with NASA’s ever-shrinking science budget, but SIM and TPF were also torpedoed by internal squabbling among scientists who disagreed about the best designs and about whether SIM was vital or unnecessary.

To be sure, Lemonick credits the ‘scrappy ingenuity’ that has produced not just Kepler but refinements to Earth-bound radial velocity methods of the kind that produced Centauri B b, not to mention the citizen scientists pooling their transit skills on planethunters.org. We can hope the setbacks are only temporary, but it gives me pause to think how many discoveries we could be making — including not so far off imaging of a planet like HD 40307 g — if missions like these and ESA’s Darwin had a stable line of development.

Comparing Habitable Zone Planets

But back to HD 40307 and its planets. The researchers review current thinking on the few habitable zone planets we’ve found thus far and measure HD 40307 g against them. GJ 581 d and HD 88512 b orbit at the outer edge of the habitable zone of their stars, although mechanisms involving clouds of CO2 or water could create habitable models [NOTE: See andy's comment below, correcting me about HD 85512 b]. Both Kepler-22 b and GJ 667C c orbit well within the habitable zone, with Kepler-22 b possibly a small version of Neptune rather than a rocky planet with a solid surface. The paper continues:

…the precise natures of GJ 667C c and HD 40307 g are unknown and they could also be scaled down versions of Neptune-like planets with thick atmospheres and solid cores. Compared to the candidates orbiting nearby low-mass stars (GJ 581 d, GJ 667C c, HD 85512 b), HD 40307 g is not likely to suffer from tidal-locking – which improves its chances of hosting an Earth-like climate. Like for the other HZ candidates, it is not yet possible to determine its physical and geochemical properties and a direct-imaging mission seems to be the most promising way of obtaining observational information on these properties.

So the work ahead is first to verify the existence of the three new planets and get a better fix on the orbits and minimum masses of each. The paper points out that the dense dynamical packing in this system means that the planet candidates around HD 40307 are likely to have masses close to their minimum values, but additional measurements are clearly in the cards.

The paper is Tuomi et al., “Habitable-zone super-Earth candidate in a six-planet system around the K2.5V star HD 40307,” accepted at Astronomy & Astrophysics, available online.

tzf_img_post

Silvio November 8, 2012 at 13:03

maravilhoso ,

Mike November 8, 2012 at 13:46

HD 40307 and its’ family of planets, especially g, is good and interesting news. I think that with continued improvements in RV instrumentation and continued observations of the local stars that more discoveries will be made.
Hopefully these improved capabilities will allow the detection of more Earth like worlds orbiting in the HZs of local and nearby stars.
I truly hope so because I think ground based RV exoplanet searches of the local stars for Earth-size and Earth-like planets are all we are going to have for the next 2 or 3 decades. Providing the funding is maintained.
Talking of hope and change, the hope is that if something sufficently interesting ( Earth-like ) is discovered nearby ( and no sorry, B b is a Earth mass roaster and doesn’t suffice though ofcourse other planets might be at Alpha Centauri ), then enough “change” will be aquired to fund somekind of exoplanet atmosphere analyser to look for biosigns.
There are few better ways to spend a few billion dollars I believe.

andy November 8, 2012 at 14:46

GJ 581 d and HD 88512 b orbit at the outer edge of the habitable zone of their stars,

HD 88512 is an A-type eclipsing binary star.

HD 85512 b on the other hand is certainly not located in the outer part of the habitable zone, it receives about 86% more insolation than the Earth does, in fact according to several estimates of the habitable zone it is too close to the star. Venus is only slightly more irradiated at 91% Earth insolation.

Ronald November 8, 2012 at 14:51

In itself this is fascinating news, even if it were only because of a system of 6 super-earths (d may just be Neptune class) ranging from about 3 to 10 Me, and in such a compact inner system (at 0.05, 0.08, 0.13, 0.19, 0.25, 0.60 AU).

Another system comes to mind, HD 10180, with 9 planets (all but one being super-earths and Neptunes, one or two may even be earthlike), of which 7 within 0.5 AU, so it is even more densely packed than HD 40307.

It remains intriguing what causes this kind of dense system of super-earths and Neptunes, as we are beginning to learn, probably the most common type of planetary system.
I am inclined to seek the explanation in the direction of metallicity, but HD 40307 has much lower metallicity, about half of solar, whereas HD 10180 has much higher metallicity, about 20% more than solar.
Ok, the planets of the latter are generally larger, even more so considering that their estimated masses are minimum (mass * sin i), due to the RV method used.

With regard to HD 40307 g, at about 7 Me and in the HZ, I expect it to have a dense gaseous envelope, so probably not very earthlike.

FrankH November 8, 2012 at 15:50

@Ronald
I agree – this more of a temperate mini-Neptune than super-Earth.

GaryChurch November 8, 2012 at 15:50

7 times the mass of Earth sounds like 7G’s to me.
Can’t live there.

Enzo November 8, 2012 at 15:51

The most interesting part of these news is that it belongs to a compact system giving hope that such systems can have more habitable planets more difficult to observe.
At >7 Me it is likely to have a really deep atmosphere/ocean and I don’t know how really habitable it could be.

Paul Gilster November 8, 2012 at 16:29

andy writes:

HD 85512 b on the other hand is certainly not located in the outer part of the habitable zone…

Thanks for this — I’ve inserted a note in the text.

Rob Henry November 8, 2012 at 17:52

GaryChurch, not Every planet has Earth’s diameter, and, to me, one 7x it’s mass seems particularly unlikely to. If our own system was typical, and the 5 planets from the mass of Venus to Saturn were used to calibrate the curve then the surface/cloud-top gravity should be no more than a few percent higher than Earth’s. My guess is that a future (floating) human colony there would have the g factor as one of the least of their worries.

ButtonPlay November 8, 2012 at 18:17

A good candidate for direct imageing by the proposed Dittoscope, to decipher its atmospheric spectra, thus be able to tell if its rocky or gassy planet.

Phil November 8, 2012 at 18:30

“…7 times the mass of Earth sounds like 7G’s to me.
Can’t live there…”

You can’t make that assumption without knowing the diameter (which of course without transits, we dont know). This could be a mini-neptune, a water world, or a billiard ball. I agree its unlikely to be a very nice place for humans to visit, but so what?

P

Leonidas November 8, 2012 at 18:48

@GaryChurch: The pull of gravity gets stronger proportionally to the planet’s mass, but it is also inverse square. So, even if the planet is really massive, the bigger it is in diameter, the weaker the pull of gravity will be, because you’ll be farther from the planet’s center. Even on a planet as massive as HD 40307 g, if it’s big enough in size, the surface gravity could be in the order of 1-2g.

Take a look at the planets of our own solar system. Even on Jupiter, the most massive of all, the g-force on the cloud tops, wouldn’t exceed 3g.
If the g-force is 1.0 on the surface of Earth, the g-forces on the surfaces of the planets are:

Mercury = 0.38
Venus = 0.91
Earth = 1.0
Mars = 0.38
Jupiter = 2.6
Saturn = 1.1
Uranus = 0.90
Neptune = 1.1
Pluto = 0.07

(taken from http://www.questacon.edu.au/starlab/weight.html).

Mike November 8, 2012 at 19:22

To Enzo, Yes, it is very interesting that a densely packed inner system with super-Earth/mini Neptune planets has a planet in the HZ. Kepler might give us a firmer idea about the commonality of these type of systems and whether they usually have outer planets.

xcalibur November 8, 2012 at 19:30

7 times the mass of Earth sounds like 7G’s to me.
Can’t live there.

that depends on the volume of the planet. If the diameter is the same as earth, you’d be correct. but it’s likely that the planet is significantly larger – in which case the surface gravity would not increase 1:1, due to the inverse square law.
for example, if this planet is twice as large as earth with 7 times the mass, the surface gravity would be 1.75g. if it were 3 times as large, the surface gravity would only be o.77g.

this is also why the surface gravity of the gas giants in our system is similar to earth’s, except for Jupiter which is 2.5g.

Enzo November 8, 2012 at 20:48

Interesting link to estimate the diameter of a planet from the mass (and, hence, the gravity) with various assumptions for the composition (ice, silicates and iron) :
http://www.sciences.univ-nantes.fr/lpgnantes/index.php?option=com_content&view=article&id=144:modelisation-de-la-structure-interne-des-super-terres&catid=4&Itemid=13&lang=en

If HD 40307 g were made of silicates, its diameter would be ~1.8x Earth’s.
And, with a mass 7-8x Earths, the gravity would be ~2.4 g.
Personally i think it is more likely to have more volatiles and a diameter closer to 2x Earth’s => ~2g

jkittle November 8, 2012 at 21:37

heavier planets have to be further away from the primary star to be be habitable. If mars was 3 earth masses it would retain more heat and be closer to habitable. If this planet is on the inner edge fo habitable AND very heavy… well it may likely be a venus type with runaway greenhouse effect.
Take heart we are still seeing a lot of observational bias .. we will start finding lighter planets at the proper distance from the star when our analysis/ detection systems have the sensitivity and have collected enough data for a long enough time to pull the signal out of the noise. right now such discovery is nearly impossible with our systems.. but as the systems improve we are certainly seeing better and better looking candidates. I also think it will be the stellites of of some of these heavy planets that will be interesting.. what if the ” mass” of HD40307G is actually the mass of two bodies revolving around a common center of gravity?

Michael Simmons November 8, 2012 at 22:01

So, could it have an earth sized habitable moon?

James Pailly November 8, 2012 at 23:52

I’d always assumed a planet X times more massive than Earth would have X times the gravity, but of course you know what they say about the word “assume.” I hadn’t considered how the inverse square law would change the equation. Thanks to Leonidas and xcalibur for explaining this. You guys rock!

Rob Henry November 9, 2012 at 2:43

Excalibur, it is not terribly useful to use the inverse square law to obtain even b.o.t.e. surface gravity estimates either. Much better is the rule that surface gravity is proportional to the simple product of diameter and density. Once we get above Earth’s mass, we tend to find uncompressed densities are very different from actual density, and that rather complicates things.

For an entirely rocky planet of 7 Earth masses I calculate its surface gravity at 2g to 2.7g, and even for a pure iron one 4g based on compressed densities of materials given by Linda Elkins-Tanton
http://iopscience.iop.org/0004-637X/688/1/628/fulltext/74355.text.html
But, it is more likely that a planet of that mass, and forming at that distance (if it indeed formed in situ) would attract just as much ice as rock, thus cutting these figures by at least a third. I can’t see an ocean planet this mass having a surface gravity much greater than 1.5g, and if it is a mini-Neptune, rather than a super Earth, its gravity would be very much less. Since many have given the second interpretation to the Kepler data, it may be more likely that colonists will complain that the gravity is too low there vis a vie Earth.

jkittle November 9, 2012 at 11:04

Michael
I Would think the chances might be good, but realistically we do not have much data on large moons around terrestrial planets.. we have only one example in our solar system. since many people think that the moon has a large degree of responsibility for the evolution of advanced life on earth, there may be an observational bias… that is, our large moon is unusual, but we observe it because it is one of the reasons we are here. It think I anticipated Paul’s next post a bit… I will continue the discussion there…

xcalibur November 9, 2012 at 22:00

Excalibur, it is not terribly useful to use the inverse square law to obtain even b.o.t.e. surface gravity estimates either. Much better is the rule that surface gravity is proportional to the simple product of diameter and density. Once we get above Earth’s mass, we tend to find uncompressed densities are very different from actual density, and that rather complicates things.

If you know the mass and the diameter, inverse square law is a very straightforward means of finding surface gravity. Unless you’re saying that variations in core/mantle distribution would have a notable effect on the surface gravity, which is something I’m not sure about.

In any case, the gravitation of super-earths is far more generous for habitability than one might assume.

Rob Henry November 11, 2012 at 4:03

“Unless you’re saying that variations in core/mantle distribution would have a notable effect on the surface gravity” – xcalibur

No, that is extraordinarily unlikely to play a role of any significance on a planet on low oblateness. On the other hand, in those very fast spinning planets of mixed internal composition, that factor becomes a major consideration, and can result in dramatically different increases in surface gravity with latitude in planets with otherwise identical mass, volume and spin rates.

andy November 11, 2012 at 20:32

I agree – this more of a temperate mini-Neptune than super-Earth.

Even without invoking the mini-Neptune hypothesis the planet is probably not habitable: on a rocky planet there should be a substantial atmosphere from outgassing.

In fact, a couple of recent papers have suggested that the population of low-density inner planets can be explained by outgassed atmospheres on rocky planets: Wu and Lithwick (2012) study the Kepler planets, suggesting that the observed radius trends suggest the super-Earths are rocky cores with hydrogen atmospheres, while Chiang and Laughlin (2012) attempt to derive the “minimum mass extrasolar nebula” under the assumption that the planets formed in situ, again suggesting that the super-Earths are rocky planets that have either primordial or outgassed hydrogen atmospheres (water is not significant in these planets because it does not condense in the inner system). Potentially even the “hot Neptunes” (GJ 436b, etc.) may be rocky planets rather than migrated ice giants.

xcalibur November 12, 2012 at 1:59

I see. Well, exoplanet science is still in its infancy, so there is naturally much more to study and take into account. But I still have reason to be optimistic about terrestrial exoplanets, at least as far as gravity is concerned.
Certainly there will be many differences, some unexpected, as these worlds come into clearer focus. There will need to be many adaptations for future colonists, but I’ll save that line of thought for another thread.

ljk November 12, 2012 at 10:04

Good Night, Exoplanet: Baby Name Book to Raise Science Funds

by SPACE.com Staff

Date: 09 November 2012, Time: 03:31 PM ET

When new planets are discovered beyond the solar system, they often get boring designations such as HD 85512b or Gliese 667Cc. A startup hoping to liven up these names has launched a project to create a Baby Planet Name Book full of more colorful suggestions.

The planet name project is the first official product from Uwingu, a new company that aims to raise money for space research, exploration and education.

Now, for 99 cents apiece, you can nominate any name you like to join the new planet name registry, and you can also vote for your favorites among the current list.

Full article here:

http://www.space.com/18420-exoplanet-name-baby-book-uwingu.html

To quote:

To be clear, Uwingu officials say the names won’t be official, and won’t be attached to particular planets — yet. The only body authorized to officially name celestial objects is the International Astronomical Union (IAU), which hasn’t so far expressed an interest in changing the status quo of planet naming.

[So long as this does not turn into another International Star Registry, where people pay $40 to get some obscure star named after them or a loved one or a pet but means nothing officially. I realize there will soon be way too many exoworlds to label with Roman or Greek mythological names as is tradition, but I also hope we won't see lots of silly, cute, and trite (and obscene) names for alien planets. Then again, human nature and limitations being what they are, that will probably get out of hand like so much else.]

and…

Uwingu will use the money raised from the project to support research efforts like SETI (the Search for Extraterrestrial Intelligence)’s Allen Telescope Array in California, as well as space launches and science outreach. The company has also released a suite of planet-related educational materials for teachers to go along with the new project.

“At Uwingu, we think that it’s important that kids learn, as well as play,” said Uwingu education officer Emily CoBabe. “So we want to make Uwingu a place where teachers can stop by to get the best and most up-to-date space education materials.”

Ronald November 12, 2012 at 14:02

Further to andy and the interesting paper by Wu and Lithwick (2012) on the density of the Kepler planet’;

What I found particularly relevant in this paper, is that super-earths and (hot) Neptunes are probably distinct classes after all (I was inclined to think that they were one continuous class):

“The presence of the two distinct classes suggests that 3Re could be identified as the dividing line between `hot Neptunes’ and `super-Earths’.”

“The data suggest that 3Re is a dividing line between `super-earths’ (largely solid) and `hot Neptunes’ (extensive gaseous envelopes). The atmospheres on mid-sized planets were most likely accreted, while those on compact planets may have come from accretion or outgassing.”

The super-earths (or ‘compact’ planets) are usually denser than water and are largely rocky with a relatively light ( 0.8 solar masses (roughly latest G, earliest K).

The two categories can be similar in mass and occur in similar stellar environments.

“when a mid-sized (Neptune) and a compact (super-earth) planet coexist in a system, the former (the Neptune) is always at least a factor of 2 or more massive than the latter.”

Ronald November 12, 2012 at 16:42

I just noticed my previous comment contains a corruption (which happened more often when I used certain symbols such as =, , together).

The 3rd last paragraph should have read:
The super-earths (or ‘compact’ planets) are usually denser than water and are largely rocky with a relatively light H/He atmosphere (not more than 1% of total mass).

The Neptunes (or mid-sized planets) are usualy lighter than water and they possess a very large mass H/He envelope, which can be up to about half of total mass (the rest is rocky core). These planets are less common than the super-earths and they only occur around stars of masses greater than 0.8 solar mass (roughly latest G, earliest K).

Ronald November 13, 2012 at 7:51

And now that we are talking about different planetary systems, :-) ,
The Extrasolar Planets Encyclopaedia (http://exoplanet.eu/) just published data on their catalog by Lee et al. about the discovery of 3 very large giant planets around giant stars (luminosity class III), two of which are M giants, one a K giant. So, cool giants can have planets too, and remarkable ones:

Their masses are so great (mass sin i varying from 6 to 11 Mj) that I wonder whether we are really dealing with giant planets here or brown dwarfs seen at a ‘small angle’.
Their orbital distances are 1.2, 1.6 and 2.1 AU, but I would not expect habitable moons here, because their stellar luminosities (based on radius and temp) must be very high, well over a hundred (140) times solar for the K giant and several hundred (roughly 500 – 600) times solar for the two M giants, meaning that the HZ is about 12 AU from the K giant and well over 20 to 25 AU from the two M giants.

Ronald November 13, 2012 at 9:49

Further to my previous comment: what makes massive giant planets near those giant stars even more remarkable is their low metallicity: one has rather low metallicity (about 80% of solar), the other two outright low (48 and 56% of solar).
It is now rather well established that main sequence stars seldom have giant planets below about 70-80% of solar metallicity.
Maybe the rules are different for giant stars.

andy November 13, 2012 at 15:41

Maybe the rules are different for giant stars.

Well the rules shouldn’t be different for giant stars because they were main-sequence stars once.

BUT.

Firstly the giant star population in the F-K range contains more massive stars than the main sequence population. The early-type progenitors of these stars are not nearly so amenable to RV surveys. This is the reason there are several campaigns actively targeting G-K giants, as these provide one of the easiest ways to probe the planetary population around intermediate-mass stars, and there do seem to be significant changes in the planet population with stellar mass (even among main-sequence stars: the planetary systems around G-dwarfs and M-dwarfs have rather different characteristics).

Also there is a lot more activity which means you need to get large RV amplitudes to get decent signal-to-noise. Therefore you end up introducing a selection bias towards the freakishly massive planets.

Then again even bearing these in mind there do seem to be a lot of oddities with the planetary population around giant stars that do not entirely appear to make sense at the current time…

Ronald November 14, 2012 at 18:53

To spoil the fun even more with regard to habitable planets and planetary systems, there was a recent study indicating that large close-orbit planets may cause super-flares even in solar type stars. This was already previously suspected in the case of hot Jupiters, but this study suggests that even medium-sized planets (Neptunes, super-earths) in very close orbit may cause super-flares.
http://news.nationalgeographic.com/news/2012/05/120516-superflares-sun-stars-planets-hot-jupiters-nasa-space-science/
referring to a Nature article in May.

I wonder, if such super-flares do indeed occur on a sunlike star with a compact system, would that also have a very destructive effect on the atmosphere of an earthlike planet in an earthlike orbit in the same system, to the extent that it would make an otherwise habitable planet unhabitable and prevent life from arising?

Rob Flores November 16, 2012 at 19:29

I see a planetary formation rule being established that is only broken under
special cimcumstances (our solar system)

I think we are about 2-3 years from Kepler, establishing that.
Low mass terrestrials generally do not form at the orbits of their habitable zones. That is RE .75 – RE1.5 are rare at the HZ distances.
I would bet some money on it. (1-2%) ocurrence IMO

If some program had an ex0 earth-mass candidate and already had a verified double transit, in the HZ , I doubt It could be kept secret for very long. Kepler is getting closer to 4 years of data gathering, we shall know
soon enough.

Ronald November 18, 2012 at 13:28

Rob Flores: yes, I tend to agree, but much more important than my private opinion is the fact that HARPS and Kepler data also seem to point in that direction.
Our kind of system is a minority, perhaps on the order of 5-10% orso, and terrestrial planets (between about 1/3 and 3 Me) in the HZ a (relative) rarity, I am inclined toward the same guesstimate of 1-2% occurrence among solar type stars.

Eniac November 28, 2012 at 23:27

Rob Flores:

Low mass terrestrials generally do not form at the orbits of their habitable zones. That is RE .75 – RE1.5 are rare at the HZ distances.
I would bet some money on it. (1-2%) ocurrence IMO

How do you know this? Consider that planets of this kind are entirely undetectable by today’s methods, so of course we would not find any directly. Do you have indirect evidence that they are not there? Other than speculation based on specific models of planet migration?

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