Getting closer: Super-Earth found in a star’s habitable zone

By Phil Plait | November 8, 2012 6:39 am

Well now, this is an interesting discovery: astronomers have found what looks like a "super-Earth" – a planet more massive than Earth but still smaller than a gas giant – orbiting a nearby star at the right distance to have liquid water on it! Given that, it might – might – be Earthlike.

http://www.eso.org/public/images/eso1134b/

This is pretty cool news. We’ve found planets like this before, but not very many! And it gets niftier: the planet has at least five siblings, all of which orbit its star closer than it does.

Now let me be clear: this is a planet candidate; it has not yet been confirmed. Reading the journal paper (PDF), though, the data look pretty good. It may yet turn out not to be real, but for the purpose of this blog post I’ll just put this caveat here, call it a planet from here on out, and fairly warned be ye, says I.

The star is called HD 40307, and it’s a bit over 40 light years away (pretty close in galactic standards, but I wouldn’t want to walk there). It’s a K2.5 dwarf, which means it’s cooler, dimmer, and smaller than the Sun, but not by much. In other words, it’s reasonably Sun-like. By coincidence, it appears ot be about the age as the Sun, too: 4.5 billion years. It was observed using HARPS, the High Accuracy Radial Velocity Planet Searcher (I know, it should be HARVPS, but that’s harvd to pronounce). This is an extremely sensitive instrument that looks for changes in the starlight as a planet (or planets) orbits a star. The gravity of the star causes the planet to orbit it, but the planet has gravity too. As it circles the star, the star makes a littler circle too (I like to think of it as two kids, one bigger than the other, clasping hands and swinging each other around; the lighter kid makes a big circle and the bigger kid makes a smaller circle). As the star makes its circle, half the time it’s approaching us and half the time it’s receding. This means its light is Doppler shifted, the same effect that makes a motorcycle engine drop in pitch as it passes you.

Massive planets tug on their star harder, so they’re easier to find this way. Also, a planet closer in has a shorter orbit, so you don’t have to look as long to find it. But in the end, by measuring just how the star is Doppler shifted, you can get the mass and orbital period of the planet. Or planets.

In this case, HD 40307 was originally observed a little while back by HARPS, and three planets were found. But the data are public, so a team of astronomers grabbed it and used a more sensitive method to extract any planetary signatures from the data. They found the three previously-seen planets easily enough, but also found three more! One of them is from a planet that has (at least) seven times the mass of the Earth, and orbits with a 198 day period. Called HD 40307g (planets are named after their host star, with a lower case letter after starting with b), it’s in the "super-Earth" range: more massive than Earth, but less than, say Neptune (which is 17 times our mass).

We don’t know how big the planet is, unfortunately. It might be dense and only a little bigger than Earth, or it could be big and puffy. But if its density and size are just so, it could easily have about the same surface gravity as Earth – that is, if you stood on it, you’d weight the same as you do now!

But the very interesting thing is that it orbits the star at a distance of about 90 million kilometers (55 million miles) – closer to its star than is is to the Sun… but that’s good! The star is fainter and cooler than the Sun, remember. In fact, at this distance, the planet is right in the star’s "habitable zone", where the temperature is about right for liquid water to exist!

That’s exciting because of the prospect for life. Now, whenever I mention this I hear from people who get all huffy and say that we don’t know you need water for life. That’s true, but look around. Water is common on Earth, and here we are. We don’t know that you need water for life, but we do know that water is abundant and we need it. We don’t know for sure of any other ways for life to form, so it makes sense to look where we understand things best. And that means liquid water.

Here’s a diagram of the system as compared to our own:

Note the scales are a bit different, so that the habitable zones of the Sun and of HD 40307 line up better (remember, HD 40307g is actually closer to its star than Earth is to the Sun – an AU is the distance of the Earth to the Sun, so HD 40307 is about 0.6 AU from its star). What makes me smile is that the new planet is actually better situated in its "Goldilocks Zone" than Earth is! That’s good news, actually: the orbit may be elliptical (the shape can’t be determined from the types of observations made) but still stay entirely in the star’s habitable zone.

And take a look at the system: the other planets all orbit closer to the star! We only have two inside Earth’s orbit in our solar system… but all five of HD 40307’s planets would fit comfortably inside Mercury’s orbit. Amazing.

So this planet – if it checks out as being real – is one of only a few we’ve found in the right location for life as we know it. And some of those we’ve found already are gas giants (though they could have big moons where life could arise). So what this shows us is that the Earth isn’t as out of the ordinary as we may have once thought: nature has lots of ways of putting planets the right distances from their stars for life.

We’re edging closer all the time to finding that big goal: an Earth-sized, Earth-like planet orbiting a Sun-like star at the right distance for life. This planet is a actually a pretty good fit, but we just don’t know enough about it (primarily its size). So I’m still waiting. And given the numbers of stars we’ve observed, and the number of planets we found, as always I have to ask: has Earth II already been observed, and the data just waiting to be uncovered?

Image credits: ESO/M. Kornmesser; Tuomi et al.


Related Posts:

ALPHA CENTAURI HAS A PLANET!
Kepler confirms first planet found in the habitable zone of a Sun-like star!
A nearby star may have more planets than we do
Exoplanet in a triple star system, smack dab in the habitable zone
Super-Earth exoplanet likely to be a waterworld

CATEGORIZED UNDER: Astronomy, Cool stuff

Comments (53)

  1. rick king

    Would be nice if we had a solar lense telescope to actually look at this planet.

  2. Do we have any readings on the possible size of the planet? it would be interesting to try to figure out what things would be like on the surface, given that it’s 7 times as massive, but the size (and density) would really affect what surface gravity would be like.

    What we do know of planets, at least this would much more likely have a stable magnetic field (liquid mantle with a metalic core and all that), so that could drive up the chances even more! :)

    I look forward to hearing more.

  3. Yeah, well… on a planet with that much mass, I don’t hold out much hope for complex life. It’d be nice to see close-ups though.

    Maybe it has a satellite massy enough to retain an atmosphere convivivial to life… now that would be something…

  4. Andrew Smith

    Phil,

    Could you explain the “clouds” annotations on the diagram and why distance is correlated to cloud formation? Venus is in a 50% cloud zone, but isn’t it 100% covered? And I assume we’re talking water clouds on the inside orbits.

  5. @Al Feersum (#3), that is why I wanted to know about the actual size of the planet. Keep in mind that gravity is inverse square. Even on a planet as large as seven times the mass of the earth, gravity on its surface could be quite close to our 1G environment. Take the list of panets in our own solar system. If they had a surface (for the gas giants), these are your weights:
    Mercury: 0.38
    Venus: 0.91
    Earth: 1.00
    Mars: 0.38
    Jupiter: 2.34
    Saturn: 1.06
    Uranus: 0.92
    Neptune: 1.19
    Pluto: 0.06

    So don’t dismiss the possibility because of the mass. ;) For planets, radius matters a great deal.

  6. Nick

    Interesting, but the fact that there is no outer gas giants (that we know of) in this system, the possibilities of life forming on this planet might be hindered, no?

  7. squawky

    Orbiting that close to the star, it seems likely that this planet is tidally-locked (one side facing towards the star, like the Moon orbiting the Earth) – which doesn’t sound good for habitability… but “life will find a way”, yes?

  8. Hal Clement, where are you when we need you?

  9. Chris

    I did a quick calculation and if the planet has the same density of the Earth, if it had seven times our mass, the gravitational acceleration would only be 19.5 m/s^2, or about twice Earth’s gravity. It’s not 7 times as big because the radius also increases, so the gravitational acceleration is not linear with the mass. Assuming the composition of the atmosphere is the same, the air pressure would be about twice as high. I think it could be potentially livable.

    The interesting thing is how does this effect the plate tectonics. You might think it would be very geologically active, but simulations suggest the higher gravity reduces the mantle convection, makes the crust stronger and reduces plate tectonics.

    Without plate tectonics as an efficient cooling mechanism for the mantle, enhanced internal temperatures may lead to increased mantle melting and heat loss via magma transport through the stagnant lid and extreme surface volcanism, as is the case with Io today [Moore, 2003].

    Furthermore, the recycling of water and CO2 at subduction zones is a crucial regulator for atmospheric greenhouse gas contents and thus surface temperatures. Without any regulation, or sequestration of volcanic CO2, higher temperature would result in higher atmospheric H2O, which is then vulnerable to photo-disassociation in the upper atmospheric and loss to space by sputtering [Bullock and Grinspoon, 2001]. This may have been the fate of Venus’ water, and may well be the ultimate fate of water on planets without plate-tectonics.
    http://www.agu.org/pubs/crossref/2007/2007GL030598.shtml

  10. Andreas H

    We should remember that finding a planet that is actually “earth-like”, as in same size and same distance to it’s “sun-like” star is almost impossible with our current detecting methods. Not because the detection methods are flawed, they are very well thought out, but because they are not quite sensitive enough. This is why we at this point have found such a prevalence of planets that are either orbiting their star very close, or are much more massive than earth.

    The fact that we have found such an enormous amount of planets with our current detecting methods is actually quite astonishing.

    The only depressing thing is that at current rate, our progress in propulsion technology seems very unlikely to result in any possibilities to even launch unmanned probes to alpha centauri in our lifetime. It seems likely that most of us will die knowing about hundreds if not thousands of planets that look promising for life, without any way to actually verify this potential… A very depressing thought…

  11. psweet

    Andreas, that scenario depends in part upon your definition of verification. If we find a planet with an atmosphere bearing lots of methane and O2, would that be good enough? Because that won’t take nearly as long as actually going there.

  12. Orvill aakra

    It!s great to see other planets confirmed within the Gl zone.So when can we expect to get more data on systems like these?Get to the next level so to say.Like get spectrums of the planets while using the transit method,or any other methods existing or new ones around the corner.10years-20-50?

  13. Leonidas

    Ensign, set a course for HD 40307g. Maximum Warp!

  14. @squawky (#7), according to this article: http://www.sciencedaily.com/releases/2012/11/121108073927.htm it does not appear to be tidally locked (emphasis mine).

    This is where the presence of liquid water and stable atmospheres to support life is possible and, more importantly, the planet is likely to be rotating on its own axis as it orbits around the star creating a daytime and night-time effect on the planet which would be better at creating an Earth-like environment.

    As to other criteria (i.e. gas giants, etc.) we just don’t know given the paucity of data we have regarding conditions required for life. :) We only have a datapoint of one.

  15. Artor

    We don’t know that carbon & liquid water is necessary for life, but it’s a pretty good guess. Carbon has the benefit of being able to form complex compounds in combination with traces of many other elements. There are other elements that can do this, but to my limited knowledge, none that do it as enthusiastically as carbon. Liquid water is important for a couple of reasons; one is that it provides a reactive medium for carbon-based molecules to interact with soluble minerals. Another is that it delineates a temperature range that is neither too cold for life-sustaining chemical activity, nor too hot to break down those complex carbon chains.
    None of this is to say that life can’t exist in some other format, but we know that this one works, and there are good reasons for it to do so. Perhaps biologists can model other chemical blends that have a strong potential for life too, and we can look for those signs as well, but until then, the old carbon & liquid water game is as good as we can expect to find. Bear in mind that I’m nothing more than an amateur here, so take this with a grain of salt, but I’d like to hear from any biologists if they have anything to add.

  16. MaDeR

    Geee, so pesymistic.

    I have story for you. Long ago one certain philosopher tried to think up example of something that we will never know about. He thought up stars. They are too far and all that jazz.

    Ironically, spectroscopy was invented not long after.

    Now back to our planets… nice inteferometer or two and sudenly you know way, way more about it! So yes, we are on border of technological capability to verify and even directly look at closer planets in our lifetime.

  17. It would be great to finally discover & prove another Earth like planet exists. Like most people I’m very skeptical. The distance invoved 42 light years from Earth and even with our best instruments make’s It impossible to verify.
    I alway’s hoped that we would discover another planet with life but I’m afraid that day is well well off

  18. Randy A.

    Chris (comment #9) says: “if the planet has the same density of the Earth…” That assumption is good for a back of the envelope calculation, but it’s unlikely to be true. The greater mass would result in a larger lower mantle with more high density silicates. This would make the over-all density of the planet higher.

    Of course, we can postulate a planet with some odd composition. But such a planet might not be conducive to life.

    For example, a much smaller iron core would lower the over-all density. But the planet would lack a magnetic field, and be exposed to more radiation from its sun.

    It’s fun to think about the possibilities. What about a planet with LOTS of water? An ocean deep enough so the tide wasn’t always a shallow water wave? What would that be like?

  19. Jim Johnson

    “… we don’t know you need water for life.”

    We know you need water for all forms of life we’ve ever seen.

  20. Wzrd1

    @ psweet, “If we find a planet with an atmosphere bearing lots of methane and O2, would that be good enough? ”
    Not really, with lots of methane and O2, one has an explosive atmosphere. As in 5-15% methane at STP is explosive.

  21. Tara Li

    That’s an awfully small sample size, though, Jim Johnson. Several basic systems have already been considered where water does not play any kind of central role – systems outlined to fit other bodies we already know about, such as hydrocarbon cycles for Titan, silicon/fluorine systems for Venus & Mercury, etc.

    The critical factor to look for, really, is a system that is thermodynamically out of equilibrium. That’s one of the reasons sniffs of methane on Mars are considered so important – methane is destroyed quickly under those conditions, so there’s a possibility life may be making more of it. Note – it’s a matter of *those conditions*, as methane spotted on Uranus or Neptune is really expected, and relatively stable under *those* conditions.

    Thanks for a great science-oriented post, Dr. Plait!

  22. theoncomingstorm

    This is cool, wish I had a hand in it but that ship sailed a long time ago.

  23. Don´t try to make contact, because if it is a super-earth, there might be superhumans living there.
    *crawls back under his bed with the friendly demons*

  24. theoncomingstorm

    Do you think we found the real supermans home planet.

  25. Doug

    I love that diagram. Please include a similar one for every planetary discovery.

    And maybe scale the size of the planet with its estimated size (where possible, and it might have to be logarithmic to make any sense).

  26. Gregory

    I have issues with the adjective “Earthlike”. Are we talking about Earth during the Hadean, when the planet was still cooling and was being heavily bombarded with left-overs from the Solary System’s formation, and when the churning of the crust probably did not permit continents? The Archaean, marked by massive volcanoes and a highly reductive atmosphere? The Proterozoic, an eon of 2 billion years that saw the excruciatingly slow build up of atmospheric oxygen, without which surface life was not possible?

    The fact is that Earth itself has been “earthlike” for only 1/9th of its existence, and even so, there were periods during this time when modern life would have been very hard pressed to survive.

  27. Jeff S.

    Life is a type of biochemical process, and if you want to generalize biochemical processes to call them life, then maybe life does exist outside earth. We don’t know yet, but it is possible.

  28. amphiox

    re Wzrd1 @20;

    An atmosphere with high levels of methane and oxygen would be almost definitive for being inhabited precisely because the mixture is potentially explosive. In the presence of each other oxygen and methane rapidly react and consume each other. If you find large amounts of both it means that something must be constantly producing them, and the only known processes that can do this fast enough are biological.

    Though an atmosphere subject to repeated spontaneous detonations would be an interesting place to try to live. (More likely though would be for the two gases to reach an equilibrium ratio that would not be explosive.)

  29. amphiox

    The size/density thing actually means that Super-Earths are the class of planet that would on average have the highest surface gravities.

  30. andy

    Well unfortunately it is currently looking (judging by the statistics coming out of the Kepler mission) like planets in this kind of mass range have very substantial atmospheres apart from the extremely-irradiated examples of Kepler-10b and CoRoT-7b. These may be either primordial, accreted from the nebula (mini-Neptunes), or perhaps the result of outgassing from the planetary interiors (perhaps such a planet can be thought of as a super-Venus, though the implied atmosphere mass fractions are substantially higher) – either way very different from the thin atmosphere of Earth.

    Even the optimistic case of a massive “super-Earth” telluric planet would have challenges: higher gravity and lower surface area/volume ratio probably mean such planets would be covered with global oceans. This would prevent the silicate/carbonate cycle negative feedback that provides long-term stability of the Earth’s climate, making the climate much more vulnerable to either collapse into a snowball or alternatively a runaway greenhouse condition.

  31. andy

    Could you explain the “clouds” annotations on the diagram and why distance is correlated to cloud formation? Venus is in a 50% cloud zone, but isn’t it 100% covered? And I assume we’re talking water clouds on the inside orbits.

    Basically it assumes that you can set the cloud fraction independently of the greenhouse properties of the atmosphere. Since clouds are reflective, increasing the cloud coverage turns down the temperature of the planet as the energy is reflected into space.

    So Earth has a Bond albedo of 0.306 but Venus with its 100% cloud cover has a Bond albedo of 0.90 – it is more reflective. Mars which has a mainly rocky surface (few clouds, not much ice either) has a Bond albedo of 0.250, less reflective than the Earth. Mercury is even less reflective than Mars, with a Bond albedo of 0.068.

    This has a corresponding impact on the temperatures of the planets. Solving for the energy balance of the planets, assuming the heat is evenly spread around the planet and there is no greenhouse effect, the effective temperatures come out as follows:

    Mercury: +167°C
    Venus: -89.0°C
    Earth: -18.9°C
    Mars: -63.2°C

    As you can see, Venus by virtue of being so reflective ends up with by far the coldest temperature of the terrestrial planets in our solar system. Or to put it another way, in terms of the energy budget of the planet’s climate, the reflective clouds more than compensate for it being closer to the Sun than the Earth or Mars.

    This logic leads to the setting of the extended inner habitable zone for cloudy planets. Unfortunately it utterly neglects the question of what kind of atmosphere would support this amount of reflective clouds. E.g. if you want water clouds then you probably need a lot of water vapour in the atmosphere and water is a really good greenhouse gas.

    Venus’s atmosphere is not water-rich, in fact it is incredibly dry – the clouds are droplets of sulphuric acid and the bulk of the atmosphere is carbon dioxide. The reason for the hot conditions on the surface of Venus are not because it absorbs more sunlight than Earth – it actually absorbs less because it reflects so much back into space – it is an energy-efficient hellworld. (The increased sunlight at Venus’s location may on the other hand be responsible for the planet evolving into its current state).

    This kind of thing leads me to be really sceptical of this proposed inward extension of the habitable zone.

  32. Fascinating,wonderful discovery but so many uncertainties.

    Great write -up, cheers BA. :-)

    @32. andy : Great explanation of that cloud notion and speculated HZ extension. Much appreciated. :-)

    @24. theoncomingstorm asked : “Do you think we found the real supermans home planet.”

    Wrong star, that’d have to be orbiting LHS 2520 ared dwraf in Corvus as the BA discussed a few days ago. ;-)

    Linked to my name here or /& see : DC comics pins Krypton to the star map posted by the BA here on Guy Fawkes day – November 5th, 2012 at 9:31 a.m.

    PS. Please Phil Plait can you let us all know if this blog will be totally disappearing or whether we can still refer back to old items and threads here (or better yet have it still running in parallel with the new one, pretty please?) after your move to slate magazine?

  33. If folks are interested, the space-dot-com website has an article and good graphic on this discovery as well which is linked to my name here or see : “Super-Earth Planet: Potentially Habitable Alien World Explained (Infographic)” by Karl Tate posted 7th November 2012 at 07:01 PM.

    @16. MaDeR :

    Geee, so pesymistic. I have story for you. Long ago one certain philosopher tried to think up example of something that we will never know about. He thought up stars. They are too far and all that jazz. Ironically, spectroscopy was invented not long after.

    Yep. Good true story. :-)

    That philospher was Auguste Comte in 1835.
    As found when reading Timothy Ferris’ superb book ‘Seeing the Dark’ :

    “Groping for an example of knowledge permanently beyond human ken – always a dangerous presumption – Comte declared that while humans might eventually learn the shapes, distances, sizes and motions of celestial bodies, “never, by any means, will we be able to study their chemical composition.” Comte’s assertion was refuted just a few years after his death when spectroscopes were trained on the Sun and stars by the physicists Joseph Fraunhofer, Gustav Kirchoff and Robert Bunsen revealing their composition and ushering in the new science of astrophysics.”

    – Brackets added, P.237, Ferris, ‘Seeing in the Dark’, Simon & Schuster, 2002.

    ***

    This find also has me wondering what planets may lie beyond HD 40307 g if its possible equivalent to our solar system’s Edgeworth-Kuiper cometary belt is similarly shifted inwards?

    Imagining some Pluto, Eris and Haumea like worlds at the outer edge of the HZ there – a possibility maybe?

  34. Incidentally, thinking possibly habitable exoplanets has anyone heard any further news on the question of whether Gliese 581 g exists or not?

    See linked BA bog post in my name – ‘Does Gliese 581g exist?’ posted by the BA on the 18th of January 2011 at 2:46 in the arvo.

  35. Anders

    Phil, or anyone really… If there are no other planets outside this super-Earth, wouldn’t that make it more vulnerable to asteroids?

  36. Nick L

    Two questions.

    1. Can this technique distinguish between a planet orbiting a star and a planet with moons orbiting a star?

    2. Has anyone ruled out looking for chlorophyll using absorption spectroscopy when hunting for earth like planets? I haven’t heard if anyone has checked just how detectable the plant life on earth is and I’ve often wondered if it could be used to confirm if there is life on another planet.

  37. Christian Treczoks

    I’m wondering about the orbits of the inner planets. If there are several planets in relatively dense orbits, how stable are these orbits then? What is the chance of e.g. a planetary alignment situation throwing one of them out of its place and hurling it across its solar system?

    Or astroids: It’ll be a hell of a ride through all those gravity wells when going through its periphel point – no one knows where and in which direction it will get out of that chaos.

    Maybe it is not the best place to stay if the skyfall forecats is more important than the weather forcasts…

  38. ctj

    Phil, where did you get that diagram, and how was it calculated?

    the biggest problem i have with any discussion of a stellar system’s “habitable zone” is that, quite frankly, i have not seen any convincing evidence that the earth is in the sun’s HZ. no, really! the moon is supposedly within the HZ, yet we know for a fact that liquid water – essential for life as we know it – cannot exist on its surface, the single most important characteristic of an HZ. this diagram puts mars closer to the center of the HZ, and we have the same problem: liquid water does not (not sure i can safely say “cannot”) exist on its surface.

  39. Fascinating! Of course one obvious question that comes up when dealing with potentially habitable planets is surface gravity, but as others have pointed out, density matters a lot. I think it’s interesting that Saturn’s gravity at the cloud-tops is nearly identical to ours, yet the planet is over 95 Earth masses!

    That brings me to another question: what’s the cut-off between “super earth” and “gas giant”? In our own solar system, of course, there’s a very big gap between the size of the rocky planets and the gas giants, but obviously there’s a whole universe out there with all kinds of wacky size distributions. A very large “super earth” is probably going to have an atmosphere, and quite possibly one much denser than ours. If you had a terrestrial planet that was, say, 7 Earth masses, but had an atmospheric composition more like Venus, with a surface pressure of many hundreds of atmospheres, would this be a very small gas giant or just a “super-venus”? I understand it’s possible for gas giants to have rocky cores… Is the difference that gas giants have a point at which the critical pressure is such that there’s a smooth transition between gas and liquid? Or is that not always a defining feature of gas giants? How about a super-earth-sized planet with a CO2 atmosphere that has a pressure and temperature profile such that the curve passes through the triple point of CO2, transitioning smoothly from gas to liquid with a solid high-pressure dry-ice crust over a rocky core?

  40. randy k

    Spectrum analysis and pointing SETI toward a 44 light yr planet, should yield many clues however if we look at only half that planet then we could

  41. randy k

    Only looking at half the planet could yield false results. if its their pacific ocean side of that planet……the water vs land parts could yield results ..different characteristics .

  42. Nigel Depledge

    Artor (15) said:

    Carbon has the benefit of being able to form complex compounds in combination with traces of many other elements. There are other elements that can do this, but to my limited knowledge, none that do it as enthusiastically as carbon.

    Carbon is the only element known to form long chains of itself, which makes it a much better candidate as a basis for life than, say, boron, silicon, nitrogen or bismuth.

    Liquid water is important for a couple of reasons; one is that it provides a reactive medium for carbon-based molecules to interact with soluble minerals. Another is that it delineates a temperature range that is neither too cold for life-sustaining chemical activity, nor too hot to break down those complex carbon chains.

    True, but there’s a third reason that might be even more important. Water is a bloody good solvent. It can dissolve salts, sugars, organic acids, alcohols, pyrroles and a whole range of other stuff, while at the same time not dissolving anything that is too oily. I’m not aware of any other solvent (not even liquid ammonia or supercritical CO2) that can dissolve as wide a range of different types of stuff as water.

  43. Nigel Depledge

    Randy A (18) said:

    An ocean deep enough so the tide wasn’t always a shallow water wave? What would that be like?

    We don’t need to imagine, because tidal range does not depend on depth of water.

    The tidal range depends on the strength of the tidal forces that cause it. Shortly after the Earth and Moon had solidified, the moon orbited very much closer to Earth than it does now, and the tides were several metres (or was that tens of metres) high.

  44. Nigel Depledge

    Jeff S (27) said:

    Life is a type of biochemical process, and if you want to generalize biochemical processes to call them life, then maybe life does exist outside earth.

    This is a bit tricky, because biochemistry means “the chemistry of life”. We need a definition of life that does not risk entering a circular argument.

  45. MaDeR

    @ctj: problem is that some definitions of habitable zone does not take atmosphere in account. Obviously, it is hard to take it in account, as we do not have full knowledge about full range of possible atmospheres for all possible worlds. Knowledge about atmosphere is critical to know about HZ of given planet. For example, without accounting for atmosphere, we would be a little outside HZ (minus 17C or something like that).

    This leave only experimental science. In other words we have to detect atmosphere and its chemical composition, temperature, pressure etc empirically. Thankfully, in next decade or two out technology should be up to this challenge.

  46. ctj

    MaDeR @46:

    that’s my whole point. i keep hearing astronomers talking about stellar habitable zones as if it’s something we actually know.

  47. Andrei

    That’s a hell of a HDR illustration for this article…. And usually, the “artist’s conceptions” with these topics are all done the same – the planet is seen as a crescent or maybe as a third quarter phase, illuminated by another light source than it’s primary (point observed also by others on this very blog), a dark sky full of stars, half the times with the Milkyway in the background, and a spherical star, without diffraction spikes or light bleeding. And half the times with the Milkyway behind it. And la cerise sur le gateau – star spots or maybe planetary transits! The dynamic range in the picture is more than millions (more than 6 orders of magnitude) but yet, somehow, we see all this in a picture with a dynamic range less than 500 – well, about half of that – i.e. 8 bits of intensities….
    I know this illustration is just CGI, but unfortunately, the digital photo revolution flooded the web with HDR photos that mimic the appearance of CGIs (and viceversa) that HDRs lost their original artistic appearance and became hip and kitsch. And viceversa, the CGIs are now only kitsch and hip….

  48. andy

    Incidentally, thinking possibly habitable exoplanets has anyone heard any further news on the question of whether Gliese 581 g exists or not?

    Well the last two papers to come out on arXiv both came to the conclusion that it doesn’t exist:

    Baluev (arXiv:1209.3154) investigates the system taking into account red noise and comes to the conclusion that only three planets are well-supported (b,c and e)

    Tuomi & Jenkins (arXiv:1211.1280) come to the conclusion that the system only contains 4 planets using Bayesian analysis, they also consider the issue of red noise.

    My own investigations using the Systemic console seem to show that the 32-day period (“planet g”) disappears when the HARPS and HIRES datasets are combined, furthermore it is only present if the planetary orbits are assumed to be circular. Note that the latest claim by Vogt et al. for the planet’s existence (arXiv:1207.4515) only takes the HARPS dataset into account rather than the combination of HARPS and HIRES.

    The claimed period of planet g (32.129 days) also matches the following alias relationship suspicously well:

    1/(claimed period g) ≈ 1/(lunar month) – 1/(1 year)

    Both of the frequencies on the right hand side are relevant to the observation schedules. Taking this into account and also considering the apparent disappearance of the period in the combined dataset, I regard “Gliese 581 g” as a systematic effect in the HARPS data rather than a genuine planet candidate.

  49. andy

    Incidentally @Phil Plait I don’t know how much control you have over the spam filter here, but if possible it might be worth whitelisting a few sites so that they don’t get held up in the moderation queue: being able to link arXiv and ADS abstracts without incurring the moderation queue penalty would be really useful! Maybe something to consider in the blog’s new home?

  50. Peter Davey

    As the scientists, Jack Cohen (who has provided aid and advice to a number of science fiction writers) and Ian Stewart, have said: “Life exists everywhere it can, and everywhere it can’t.”

    As Arthur C Clarke once pointed out, if it turned out that there is life on Titan, and that the life reached the point of intelligence, and developed a religion, it would probably thank its Deity for creating such a hospitable world.

    Each to their own – whether their style of clothing. or choice of atmosphere.

  51. @49. andy : Thanks for that. :-)

    @36. Anders asked : “Phil, or anyone really… If there are no other planets outside this super-Earth, wouldn’t that make it more vulnerable to asteroids?”

    Plus comets, yeah, I’d imagine it could well do so although it would depend on whether and what sort of asteroid and cometary belts /disks / clouds the host star(s) have, naturally.

    @ The Bad Astronomer, Dr Phil Plait : Please can you answer the question I’ve asked in comment 33 here? Please?

  52. Matt B.

    @4 Andrew Smith – I believe it means a planet can be habitable with that much cloud cover, not that it does have that much.

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