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NASA Space Science

NASA's Kepler Spots Its First Rocky Exoplanet 97

coondoggie writes "NASA today said its star-gazing satellite Kepler has identified its first rocky planet orbiting a sun similar to our own — 560 light years from our solar system. While not in an area of space considered habitable, the rocky planet known as Kepler-10b is never-the-less significant because it showcases the ability of Kepler to find and track such small exoplanetary movements. 'Kepler's ultra-precise photometer measures the tiny decrease in a star's brightness that occurs when a planet crosses in front of it. The size of the planet can be derived from these periodic dips in brightness. The distance between the planet and the star is calculated by measuring the time between successive dips as the planet orbits the star. Kepler is the first NASA mission capable of finding Earth-size planets in or near the habitable zone, the region in a planetary system where liquid water can exist on the planet's surface. However, since it orbits once every 0.84 days, Kepler-10b is more than 20 times closer to its star than Mercury is to our sun and not in the habitable zone.'"
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NASA's Kepler Spots Its First Rocky Exoplanet

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  • Rocky (Score:2, Funny)

    by mcneely.mike ( 927221 ) on Monday January 10, 2011 @09:09PM (#34831108)
    Yo' Adrian!
  • Headline (Score:5, Insightful)

    by ae1294 ( 1547521 ) on Monday January 10, 2011 @09:11PM (#34831130) Journal

    The headline isn't flashy enough.

    Should read:

    NASA's Kepler Spots Hell 560 light years from earth and closing.

    • by gstrickler ( 920733 ) on Monday January 10, 2011 @09:43PM (#34831424)

      I like it. If I had mod points, you would already be at a 5 - funny.

      Of course, if it's tidelocked, there is probably a ring slightly on the dark side of the equator that isn't hellishly hot or cold.

      • by c0lo ( 1497653 ) on Monday January 10, 2011 @11:05PM (#34832016)

        I like it. If I had mod points, you would already be at a 5 - funny.

        Of course, if it's tidelocked, there is probably a ring slightly on the dark side of the equator that isn't hellishly hot or cold.

        Even if the temperature would be bearable, other conditions may be not. Hermian atmosfere [wikipedia.org] suggest that such a ring would probably show metal vapour atmosphere in the terminators line (in Mercury's case: sodium; in Hell-560's case, given a much closer proximity to its start, probably other - refractory elements - would contribute more, as the more volatile ones would be blown [wikipedia.org] away by the solar-wind).

        • by gstrickler ( 920733 ) on Monday January 10, 2011 @11:51PM (#34832294)
          Minor details. I mean, if you can't handle breathing a little metal vapor (or building a respirator to filter it), what kind of space explorer are you?
          • by c0lo ( 1497653 ) on Tuesday January 11, 2011 @12:01AM (#34832356)

            Minor details. I mean, if you can't handle breathing a little metal vapor (or building a respirator to filter it), what kind of space explorer are you?

            Not quite about breathing only. Can you imagine yourself moving when a "thick layer of ice" of "condensed wolfram" forms on the joints of your space suit?

            • by gstrickler ( 920733 ) on Tuesday January 11, 2011 @12:21AM (#34832448)
              Well, if it's a thick layer, that might be difficult. A thin layer on your visor might obscure visibility a bit. Naturally, the solution is to live underground, run pipes over to the hot side for power, and use UV lamps for growing your food in your underground oasis until the planet spirals into it's sun.
              • by c0lo ( 1497653 ) on Tuesday January 11, 2011 @12:32AM (#34832492)

                Well, if it's a thick layer, that might be difficult. A thin layer on your visor might obscure visibility a bit. Naturally, the solution is to live underground, run pipes over to the hot side for power, and use UV lamps for growing your food in your underground oasis until the planet spirals into it's sun.

                What? Closer to that hell of a core, heated to UV-hot by the core tides?

                • by gstrickler ( 920733 ) on Tuesday January 11, 2011 @12:45AM (#34832542)

                  Gotta be better than living in Houston. (old joke: If I owned Hell and Houston, I'd rent out Houston and live in Hell).

                  If it's tidelocked, there shouldn't be any significant tidal heating, and with minimal atmosphere, it should just have a really hot side, and a really cold side, with more moderate temps in between, just like Mercury.

                  • by c0lo ( 1497653 ) on Tuesday January 11, 2011 @01:14AM (#34832638)

                    If it's tidelocked, there shouldn't be any significant tidal heating, and with minimal atmosphere, it should just have a really hot side, and a really cold side, with more moderate temps in between, just like Mercury.

                    Tidal locking doesn't preclude core tides if the core has a fluid component. Here's from a source in CA [nasa.gov] (they may know better than the guys in Houston - even if the californians aren't quite renowned for their sanity).

                    • by gstrickler ( 920733 ) on Tuesday January 11, 2011 @01:17PM (#34837224)

                      That paper is about the moon, it's a 3 body system, the moon, earth, and sun. In a system, where one body is tide locked to it's primary gravitational influence but there are other significant gravitational influences, you can have tides, including core tides. However, in a system in which a body is tide locked to it's primary and has no additional significant gravitational forces (it's not gravitationally bound to, and doesn't have any relatively massive bodies gravitationally bound to it), there are no significant tidal forces.

                      The moon is tidally locked to earth, it's primary gravitational influence. However, the earth-moon system is gravitationally bound to the Sun, and the Sun's gravity has a significant secondary gravitational influence on the moon, therefore, the earth and moon do experience tidal forces due to the Sun. If there is any liquid in the moon, it will flow due to those tidal forces. However, a tide locked planet with no moon in a system with only a single star is not likely to have any other significant gravitational influences. In fact, about the only possible source for another significant gravitational influence would be another massive planet in orbit around the star. How massive it would need to be depends upon the relative masses and distances of the bodies. For instance, Jupiter does not exert a significant tidal effect on Earth because it's too far away.

                      For more information on tidal influences, see Tidal Influences [gsu.edu]

                  • by c0lo ( 1497653 ) on Tuesday January 11, 2011 @01:30AM (#34832714)

                    p>If it's tidelocked, ... , just like Mercury.

                    BTW: Mercury is locked in a 3:2 spin-orbit resonance [wikipedia.org] - if somebody's selling you some real-estate on the Hermian day-night terminators, better buy the Broolyn bridge.
                    If you however decide to go for Mercury, take an insurance: they are saying that Mercury is bound to collide with Venus somewhere in the future (others say that even if you buy the Brooklyn bridge you have some chances to lose: Mercury may collide even with Earth [liberation.fr] - and that's because of Jupiter).

                    • by gstrickler ( 920733 ) on Tuesday January 11, 2011 @05:03AM (#34833622)
                      I didn't say Mercury is tide locked, but I can understand the confusion. Mercury has almost no atmosphere, and even though it's not tide locked and it's very close to the sun, the "dark" side is extremely cold. A tide locked or slowly rotating planet with very little atmosphere will be very cold on the side away from the sun.
      • by c6gunner ( 950153 ) on Wednesday January 12, 2011 @07:03PM (#34855410)

        Of course, if it's tidelocked, there is probably a ring slightly on the dark side of the equator that isn't hellishly hot or cold.

        That's where the devil lives. Duh! Why would he want to spend his free time sweating, when he can go home, relax, and let the night-shift take care of the office?

    • by Anonymous Coward on Wednesday January 12, 2011 @04:21PM (#34853234)

      One should have a look at HD70642 (https://secure.wikimedia.org/wikipedia/en/wiki/HD_70642) which has already been discovered in 2003.

      Wikipedia:
      "The jovian ensures the stability of an Earth-mass planet at 1 AU. This system is one of the most similar in conditions to our solar system than any other currently known planetary systems."

  • by Thorfinn.au ( 1140205 ) on Monday January 10, 2011 @09:13PM (#34831156)
    Not a habitable planet as the gravity would be about 20 m/s and put it in the habitable zone and gravity would get you again as hydrogen would not achieve escape velocity as it does here and thus build up in the atmosphere preventing any accumulation of oxygen
  • by Anonymous Coward on Monday January 10, 2011 @09:15PM (#34831176)
    Molten too, I wonder what a compass would do there (assuming you could stop it melting). And if it is mostly iron what was it originally?
  • by Luke has no name ( 1423139 ) <fox@cyberfo[ ]re.com ['xfi' in gap]> on Monday January 10, 2011 @09:18PM (#34831214)

    That's less than 2 million miles, or .05 AU from the sun.

    Quite toasty.

  • Plane (Score:4, Insightful)

    by mejustme ( 900516 ) on Monday January 10, 2011 @09:23PM (#34831266)
    So this means if a planet orbits a sun in any other plane than the one that happens to line up directly with us, it wont spot anything? Wouldn't that be...most of space?
    • Re:Plane (Score:5, Informative)

      by mopomi ( 696055 ) on Monday January 10, 2011 @09:56PM (#34831540)

      Yes. Mostly. For this (transit photometry) method.

      There are several methods of finding an extrasolar planet.

      Briefly:
      1) Pulsar variations: If a planet orbits a pulsar, the pulsar's timing will vary in a manner that can be detected by us, and we can use 3-D trig to figure out relevant parameters such as mass and radial distance.
      2) Doppler shift of a star's emission lines: If a planet orbits a solar-type star, we can use the star's doppler shift of certain spectra to determine the various parameters of the body (or bodies) orbiting the star.
      3) Gravitational microlensing: If two stars align just right to create a microlensing effect, the star further from us will show up as several images or as an Einstein ring, and its brightness will be amplified. If there's a planet orbiting the star that's closer to us, those mirror images or the ring will change with time, and they will be a bit brighter than without the planet.
      4) Astrometry (measurements of the variation of a star's position relative to the "plane of the sky"): If there's a massive planet with an eccentric orbit, the star will orbit a barycenter that's outside of its mass, causing the star to move relative to the background.
      5) Direct imaging: with certain techniques for processing stellar imagery, we can detect whether or not there's a planet reflecting some of that star's light to us.
      6) Transit photometry: observing the star's brightness decrease as the planet eclipses the star. This works best for planets with a perfect orbital alignment with us, but we can still detect and work out minimum values for the relevant parameters.
      7) Radio flux: Certain jovian-type planets can emit radio fluxes that differ significantly from most stars. These fluxes can be difficult, though not impossible, to detect from the interstellar noise.

      There are more methods...

    • by aXis100 ( 690904 ) on Monday January 10, 2011 @11:08PM (#34832030)

      Yes, but I think that's fairly common due to conservation of angular momentum in the Milky Way and all of the solar systems that formed within it. Our solar system is tilted however.

    • by osu-neko ( 2604 ) on Tuesday January 11, 2011 @01:00AM (#34832586)

      So this means if a planet orbits a sun in any other plane than the one that happens to line up directly with us, it wont spot anything? Wouldn't that be...most of space?

      Yes. This method will only spot a tiny minority of the planets it could potentially spot if the angles were different. The galaxy would have to contain many millions of stars for this to be at all useful. But, as it happens... ;)

    • by Ihlosi ( 895663 ) on Tuesday January 11, 2011 @04:36AM (#34833506)
      So this means if a planet orbits a sun in any other plane than the one that happens to line up directly with us, it wont spot anything? Wouldn't that be...most of space?

      Most of space is actually empty.

      But yes, only a tiny fraction of all stars will have transiting plantes. Kepler makes up for that by looking at lots of start simultaneously. Even if only 0.5% of all stars have transiting planets, you're still likely to find quite a few if you look at thousands of stars.

    • by qazsedcft ( 911254 ) on Tuesday January 11, 2011 @05:42AM (#34833780)
      Yes, but we also know exactly what's the probability of detecting a planet in our plane of sight. The point of the Kepler mission is to examine hundreds of thousands of stars and determine which ones have planets and what kind of planets they are. Then using probability we can then extrapolate how many such planets exist around other stars (not in our plane of sight). The sample size is what makes this extrapolation valid.
  • by Fluffeh ( 1273756 ) on Monday January 10, 2011 @09:31PM (#34831330)
    Seeing as Kepler uses transits to find these planets, I wonder what the expected timeframe is for when they start really pumping out the data. I mean, if it looking at the right place for a year solid, it would expect to see one dimming of our sun from us (if it was pointed at our system from elsewhere). And that is only to find a single transit. Then add another year to get the orbit, probably another year at least to confirm.

    To me it seems that it is going to be a very slow start (apart from these totally hotrock type planets with insanely quick orbit) but then the taps will be turned on and they will start finding exponentially more and more?
    • by arisvega ( 1414195 ) on Monday January 10, 2011 @10:14PM (#34831638)

      And that is only to find a single transit. Then add another year to get the orbit, probably another year at least to confirm.

      Well, probably yes, assuming they're looking for yearly (like Earth's) orbits. Makes a bit of sense, but an Earth-like planet might be closer or further away from its host star, and be perfectly OK for liquid water, life, all that (depending on the host star's energy output). Probably not very different from a year though, it rather depends on the sizes (mass and orbital radius) involved.

      As for the confirmation, it might not get that long; since the dip might be a starspot or a different agent, a Doppler effect study (or astrometry, in the future) might confirm or dismiss it because, to some extent, different methods of detection can be used on the same source for confirmation. Though, on the Kepler mission, I think the confirmation is 'included' and the timeframe is set for 3 years.

      To me it seems that it is going to be a very slow start (apart from these totally hotrock type planets with insanely quick orbit) but then the taps will be turned on and they will start finding exponentially more and more?

      Hopefully yes. For the moment the methods of detection are biased- each of them is capable of locating specific groups of planets based on two parameters, those parameters being the planetary mass and its distance from its host star- there's also gravitational lensing that can 'see' better, but its a one-timer.

      Encouragingly enough, if one plots the findings so far (mass vs orbital distance) it is not hard to imagine that the so far covered areas will start to expand. My point being that, before the Kepler mission, 'hot Jupiters' kept being the majority of bodies discovered, because they are the only ones we had the means detecting- Kepler has been watching the same patch of space, and it should see more than 'hot Jupiters' (provided they're out there and we are going around this the right way).

    • by Anonymous Coward on Monday January 10, 2011 @11:08PM (#34832034)

      I'm kinda hoping that it discovers a small, non-rocky, artificially smooth body orbiting a star.

    • by c0lo ( 1497653 ) on Monday January 10, 2011 @11:27PM (#34832150)

      To me it seems that it is going to be a very slow start (apart from these totally hotrock type planets with insanely quick orbit) but then the taps will be turned on and they will start finding exponentially more and more?

      Given exponential time and budget, I hereby state that "the Kepler programme" is theoretically able to eventually detect all the planets in this and nearby galaxies.
      (seriously... this is to say that "the law of the most restricting factor" will seriously skew the "discovery curve" you mention... just don't hold your breath).

    • It's so very exciting! I'd die to see a real telescopic close-up of the first closely observed exoplanet! Generations would have passed before we reach that stage of discovery though. For now, the artist's concept will do nicely as wallpaper :-)

  • by Anonymous Coward on Monday January 10, 2011 @09:34PM (#34831354)

    I mean really.. Rocky?

  • by xevioso ( 598654 ) on Monday January 10, 2011 @09:36PM (#34831370)
    Im very curious how they know this...how can they determine this is not simply a sunspot if it is that close? It's my understanding all the Kepler measurements are validated on the ground with telescopes checking doppler shifts for the wobble method...for something this close and this small, wouldn't that be a problem?
    • Re:I wonder (Score:3, Insightful)

      by Theotherguy_1 ( 1971460 ) on Monday January 10, 2011 @10:47PM (#34831858)

      Two ways:

      1. The graph of light intensity vs. time has a particular shape for perfectly spherical objects such as planets passing in front of a star.

      2. Those doppler shifts would not be affected by a sun-spot, and measurements of this kind aren't always verified by doppler shift methods.

  • by Skidborg ( 1585365 ) on Monday January 10, 2011 @09:47PM (#34831468)
    So to find a truly earthlike planet, won't they have to focus on a single star for more than a year in order to detect the planet passing the star more than once? What if the planet's orbit never aligns to eclipse the sun? What if there are two or three planets in very similar orbits?
    • by Theotherguy_1 ( 1971460 ) on Monday January 10, 2011 @10:55PM (#34831918)

      So to find a truly earthlike planet, won't they have to focus on a single star for more than a year in order to detect the planet passing the star more than once?

      Yep. And for Jupiter-like planet we'd need to be watching it for hundreds, if not thousands of years if we were to use this method.

      What if the planet's orbit never aligns to eclipse the sun?

      Then we would never detect it via this method.

      What if there are two or three planets in very similar orbits?

      It depends on how well they are aligned. Even if they're perfectly aligned, we're liable to see the first one before the second or third one as it passes in front of the star. If they are even slightly out of phase, they will eventually be in an orbit in which we see all three distinctly. In any case, the radius and shape of the occlusion in front of the star is determined by the shape of the light intensity vs. time graph. Circular disks have a very specific light occlusion shape, while abberant occlusions have different shapes.

  • I don't understand how a planet could be whipping around its primary once every .84 days, in an orbit 20 times closer than Mercury is to the Sun, and not be torn apart by tidal forces.

    What am missing?

  • by MrQuacker ( 1938262 ) on Monday January 10, 2011 @10:11PM (#34831618)
    All these telescopes are looking for planets that orbit in front of their suns, relative to our viewing angle. We only know they are there from the shadow they cast toward us.

    How will we discover planets that are orbiting stars, but that do not cross in front of our field of view?

    • by Theotherguy_1 ( 1971460 ) on Monday January 10, 2011 @10:44PM (#34831834)
      Yes, that is true. Kepler just happens to be looking at a hell of a lot of stars at once, with computer vision algorithms constantly checking each star to look for variations in light intensity. To discover other planets (whose plane of the ecliptic doesn't line up with our field of view), we would need to use other methods, such as gravitational wobble, gravitational lensing, direct imaging, etc.
    • by John Hasler ( 414242 ) on Monday January 10, 2011 @10:56PM (#34831934) Homepage

      How will we discover planets that are orbiting stars, but that do not cross in front of our field of view?

      With difficulty. We can try to detect the slight wobble induced in the star by the planet or attempt to image the planet directly. AFAIK both are beyond current technology for Earth-sized planets.

    • by c0lo ( 1497653 ) on Monday January 10, 2011 @11:31PM (#34832178)

      All these telescopes are looking for planets that orbit in front of their suns, relative to our viewing angle. We only know they are there from the shadow they cast toward us.

      How will we discover planets that are orbiting stars, but that do not cross in front of our field of view?

      Whoa, but this is elementary... just pay them a visit. Until then, discovering them isn't much of a benefit anyway, or is it?

  • by fbumg ( 632974 ) on Monday January 10, 2011 @10:18PM (#34831668)
    "While not in an area of space considered habitable"... Do they just mean because of its proximity to it's sun? Or, is that star considered to be in an area of space that is inhabitable? Not that it makes that much difference I guess, but being 20 miles from the surface of a star makes me think, "no shit sherlock". But if it is the area of space of that star makes me go, "hmm", and then "why are we looking there?".
    • by mopomi ( 696055 ) on Monday January 10, 2011 @10:24PM (#34831692)

      They mean its proximity to the sun, the so-called "habitable zone" that everybody wants to talk about, regardless of the type of planet.

      • I do not profess to know anything, but I never quite understand it when people go on about a planets ability - or lack there of to sustain life.

        Now I (think I) understand the habitable zone, but just because something is too hot or too cold for our liking, does that really mean it would be too hot or cold for whatever may evolve independently of what has evolved here on earth?

        The search for water is often associated with the search for life. Have I watched too much bad SciFi and read too many comics that I fail to see the relevance?

        I have always felt that people put too narrow a view on what life is or could be.

        Feel free to let me know why I am very wrong. .

        • by Anonymous Coward on Monday January 10, 2011 @11:00PM (#34831976)

          Sure, life may exist elsewhere even in our solar system. But suppose humans need an escape hatch because of a mammoth meteroite or some other catastrophe, where could we head out to that would have resources to support long term colonization w/o requiring everyone to wear space suits 24x7?

        • I have always felt that people put too narrow a view on what life is or could be.

          If we assume that life can be anything anywhere how do we decide where to look and how will we know it if we find it? We have one example of a habitable planet. We are narrowing the search by looking for similar places. We have limited resources. Why look for silicon-based life instead of carbon-based life when we don't even know if the former is possible but are an example of the latter?

        • by mopomi ( 696055 ) on Monday January 10, 2011 @11:14PM (#34832064)

          Oh, you're not "very wrong."

          We can only recognize life as we already understand it. A common medical exam question is to define life. A common graduate school exam question is to define life. How do we do that? Based on what we know.

          We know that life (as-we-know-it) requires a few conditions, so we look for planets that could support those conditions.

          Nobody thinks that's the only place to find life, but it's probably the easiest place to find life that we would understand...

        • by osu-neko ( 2604 ) on Tuesday January 11, 2011 @01:30AM (#34832712)

          I have always felt that people put too narrow a view on what life is or could be.

          You're reading too much into it if you're thinking they're limiting their view on what life is or could be. If I tell you I've just arrived in the city and am looking for good Italian restaurants, it does not follow that I am assuming only Italian food exists or could exist. It's just the kind I'm most interested in finding at the moment. The "habitable zone" is the zone that could support all the life we've ever detected. If we detect new forms of life, the zone will get bigger. There's no a priori assumption here that only life as we know it can exist, it's an a posteriori judgement: "this is what we've seen -- where could it survive?"

      • by arisvega ( 1414195 ) on Monday January 10, 2011 @10:52PM (#34831898)

        .. not my favorite term, but a way to derive it in front of astrophysics students is to assume a planetary body, no atmosphere, figure out its surface temperature, and demand it to be 'within liquid water limits'.

        Now, since one may very correctly inquire, "liquid water without atmosphere? Are you on crack? And do your math, some planets are obviously not in it like, well, THE ONE WE'RE STANDING ON", I will have to add that I have been in two conferences so far, and 'habitable zone' seems to be more a popular term than a scientific one.

        There is much talking though on 'expanding' the definition (see 'dwarf planet' for examples on how that works) talking into account atmospheres, orbital characteristics and other stuff.

        Still, given where we stand now regarding exoplanet detection, it's not so bad using what we've got and work our way up from there.

        • by mopomi ( 696055 ) on Monday January 10, 2011 @11:18PM (#34832098)

          .. not my favorite term, but a way to derive it in front of astrophysics students is to assume a planetary body, no atmosphere,

          In my graduate studies, we defined and derived it with and without an atmosphere.

          • by MartinSchou ( 1360093 ) on Tuesday January 11, 2011 @03:38AM (#34833272)

            Question:

            Does the Moon qualify as being in the habitable zone, when you look at it in isolation? With an equator temperature of 100K to 390K and a mean of 220K [wikipedia.org], I would say no.

            A mean temperature that is about the same as that of the interior of Antarctica [wikipedia.org], and extremes that almost dip low enough condense oxygen (at 1 Atm) and high enough to boil water (at 1 Atm).

            Yet, once we add an atmosphere (Earth), we end up with much more reasonable temperatures. Minimum of 184K, mean of about 288K and max of about 330K.

            • by arisvega ( 1414195 ) on Tuesday January 11, 2011 @06:17AM (#34833994)

              equator temperature of 100K to 390K and a mean of 220K, I would say no.

              So would I

              Yet, once we add an atmosphere (Earth), we end up with much more reasonable temperatures.

              My point exactly. Furthermore, if the atmospheres where formed on the same time the planets did, after they 'settle' (i.e. after they stop escaping into space) a 'lid' is formed around the planet that makes it more difficult for heat to escape.

              On the other hand, while the atmospheres are still settling, they are much more optically thick so no cooling occurs (except, of course, from the fraction of the atmosphere escaping to space carrying heat with it). And from what I've read, nearly Earth-sized planets forming in gaseous proto-planetary disks acquire atmospheres comparable in mass with the atmospheres of Earth and Venus, whereas Mars and Mercury-sized planets would be in serious trouble keeping their atmospheres over large timescales.

            • by mopomi ( 696055 ) on Tuesday January 11, 2011 @12:33PM (#34836530)

              Yes, the moon is within the habitable zone, but it's not habitable. If we discover a rocky planet in the habitable zone of another star, the first thing we'll be looking for is an atmosphere (which is quite a bit more difficult than finding the planet, but techniques are being developed and tested). If we discover evidence for an atmosphere, the habitability of that planet jumps into a realm that is much more interesting. Then we start looking for evidence of certain gases in the atmosphere (water vapor, CO2, Nitrogen, etc.).

          • by arisvega ( 1414195 ) on Tuesday January 11, 2011 @06:07AM (#34833934)

            In my graduate studies, we defined and derived it with and without an atmosphere.

            Interesting- how did you do that in the atmosphere case? Multiply it with a factor to inhibit cooling, after/while you get the temperature balance?

            • by mopomi ( 696055 ) on Tuesday January 11, 2011 @11:49AM (#34836070)

              Well, first, let's go into some history.

              A habitable zone around a main sequence star was originally (1959) defined as a (virtual) ring around that star in which at least 10% of the surface of a planet, with an Earth-like atmosphere, in that zone had a mean temperature of between 0 and 30 C with extremes not exceeding -10 and 40 C. This is appropriate for humans to survive.

              The zone was quickly expanded to mean wherever liquid water was stable. The term "biostable" was employed to mean where liquid water was stable and the term "habitable" was restricted to mean a place suitable for humans. Soon, though, "habitable" was expanded to replace "biostable" and to include anywhere that liquid water is stable.

              All (peer-reviewed) models since the original definition have used one type of atmosphere or another, usually an atmosphere chemically similar to Earth's. Most have also considered planetary albedo (surface brightness), solar evolution (as a star moves along the main sequence, the habitable zone changes or disappears, depending on the details), etc.

              Several models have pessimistic estimates to the width and/or lifetime of a habitable zone, most often because an atmosphere like the Earth's is only metastable and it could collapse with only a few % change in solar energy input (distance from or luminosity of the sun, for example can greatly affect the stability of an atmosphere). Other models have included climate stabilization by linking CO2 and surface processes such as the creation/weathering of certain types of rock that remove/add CO2 from/to the atmosphere. There are a lot of these kinds of details that are included in most models of the habitable zone. A lot of the work is in determining which details are more important than others.

              For my graduate work, we had to define the habitable zone around the sun, at the beginning of the solar system (4.556 Ga), and now. To do so, we had to start from the proplyd, condense all of the elements at the right distances from the sun, build the planets (we were allowed to assume that they formed in their current positions unless we wanted to make our work more difficult), allow atmospheres to condense or form, depending on where the planets were, etc., and finally determine which planets were possibly in the habitable zone as the sun evolved (Venus, Earth, and Mars, depending on the details and assumptions), and then determine whether the planets that are here now are in the habitable zone, and why or why not.

              We, of course, used some pretty simple 1-D models for atmosphere, or used published models and argued why they were valid. We used simple models for planetary albedo, didn't evolve the albedo unless the atmosphere collapsed or changed dramatically in some other way (ignored Earth-like clouds, for example), etc. We used simple estimates for the concentrations of radioactive elements that could contribute to the surface temperature, used a simple model for luminosity evolution, etc., etc., etc.

              For these kinds of simple models, the inner edge of the habitable zone is defined by when water will be lost from the atmosphere (through photolysis of the water vapor and escape of the hydrogen) and the outer edge is defined by when CO2 condenses and causes runaway glaciation.

              Technically, the Moon is within the habitable zone, but it's obviously not habitable. Neither are Venus or Mars. This is because they don't have the right atmosphere, and may never have had the right conditions.

              • by arisvega ( 1414195 ) on Wednesday January 12, 2011 @11:23AM (#34848468)

                OK so I see it was a project, not just an extra factor or two. Interesting to hear about the story of the 'habitable zone', and also the distinction between 'habitability' and 'biostability'- it is not hard to envision that a 'biostable' planet can still be colonized (though some science fiction technology is going to be needed!). Just a few points though;

                at least 10% of the surface of a planet, with an Earth-like atmosphere, in that zone

                Well, to get 10% of a planetary surface, that would be some fraction of its radius in terms of distance; on Earth-like planets that would be a couple of thousand miles at most- working with AUs, that's within the error margin. I guess your point is just to define it so you have something to start with?

                Most have also considered planetary albedo

                IMO adding that (and solar evolution) makes the model way more respectful- but still, as far as I know the Earth's albedo is under great dispute, due to its great variability (mainly clouds/no clouds, and also snow) making it computationally very expensive to add to numeric models.

                Since you mentioned 'snowball Earth' and photolysis I think you will probably find the following interesting; several authors are approaching habitability via studying long-term climatic stability via Milankovic cycles and others are studying magnetic field generation through the so-called 'dynamo' mechanism on sub- and super-Earths. The former may be giving opportunities to planetary bodies to escape their snowball status, whereas the latter may produce a planetary magnetic field that would somehow shield the planet's atmosphere from the devastating stellar wind (and, in their case, they find that for super-Earths the dynamo mechanism is inhibited)

                We could go on for hours; the habitability zone means exactly squat if, f.i., the parent star is in the middle of a cluster of erupting supernovae- so then we would need to extend the definition to include stellar environments.

                • by mopomi ( 696055 ) on Wednesday January 12, 2011 @01:41PM (#34850584)

                  I'm not sure what you mean when you talk about 10% of a planetary surface and AUs.

                  Earth's surface area is 5.1x10^8 km^2. 10% of that is, obviously, 5.1x10^7 km^2. The land area of the US is about 9.8x10^6 km^2, so we're talking about 5-times the land area of the US. None of this has anything to do with distance from the star, just to do with the radius of the planet.

                  But, as you say, the point of 10% isn't that it's a special number; it's a starting point. Notice that this definition explicitly excludes any gaseous planets from the get-go. That's not necessarily fair, of course, but we've got to start somewhere, and rocky planets are a LOT simpler to understand w.r.t. possibilities of life.

                  Earth's average albedo isn't really all that controversial or problematic. For example, we can say that the poles probably had such and such an albedo at such and such a time (based on climate models based on core samples), the clouds are difficult for a specific times (decades or so), but again the climate will dictate some average cloud cover that is relatively accurate over long periods over the entire Earth.

                  Going into more detail would require an actual climate model (such as the Hadley model), which doesn't make much sense for extrasolar planets since we know next to nothing about atmospheres (especially their composition) on most extrasolar planets. Of course, we can speculate and use places like Titan, Mars, Venus, Earth, and Triton as jumping-off points for planets that are certain distances from their parent star.

                  Milankovitch cycles are certainly included in long-term habitability or continuous habitability zone research, but we're really limited by not knowing anything about the obliquity/precession cycles of extra-solar planets, which are quite dependent on specific circumstances of those planets.

                  The T-tauri phase of the pre-main sequence stars would strip almost any magnetic field-protected atmosphere from most planets, so we're fairly confident that any planets found orbiting such stars are uninhabitable. In fact, there are a lot of stars that can be ruled out (of having any sort of habitable zone) just by looking at them.

                  Habitability zones are for "well-behaved" stars with well-behaved planets. Some researchers are looking at double or triple star systems, so we'll have a better understanding of the possibility of life in such systems as time goes on.

    • > ...20 miles from the surface of a star...

      Not 20 miles. "20 times closer to its star than Mercury is to our Sun". That would put it somewhere around 1.5 million miles out.

    • by nedlohs ( 1335013 ) on Monday January 10, 2011 @11:09PM (#34832042)

      When referring to planets it always means "where liquid water could exist".

      Because we have an ever so slight bias towards life as we see it on Earth.

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  • by Anonymous Coward on Tuesday January 11, 2011 @04:29AM (#34833494)

    Kepler-10b is more than 20 times closer to its star than Mercury is to our sun and not in the habitable zone

    So if Mercury orbits at between 46 and 69 million kilometers (let's say an average of 57 million km), and Kepler-10b orbits 20 times closer, then that's 20 times 57? 1.1bn kilometers?

    No no of course not. What this messed up math is trying to say is it orbits at one twentieth the distance of Mercury, or about 2.85 million km.

    When did it become the standard in math to say something is X times Y when what is actually meant is Y divided by X? My school math teachers would have kicked me out of class for expressing something like that.

  • by Anonymous Coward on Tuesday January 11, 2011 @11:11AM (#34835672)

    Just wondering, how does Kepler (or any other transit-based planet detection technique) differentiate between an orbiting planet and a sunspot (or starspot, I guess) moving across the star's surface as it rotates?

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