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Recently verifying its thousandth exoplanet, Kepler data confirm that the most common planet sizes are worlds just slightly larger than Earth. I foun

by pdx 12y ago

Recently verifying its thousandth exoplanet, Kepler data confirm that the most common planet sizes are worlds just slightly larger than Earth. I found this interesting in terms of the rocket equation[1] and the fermi paradox[2] If most planets are larger than we are, their populations will struggle even harder than we do, getting off their rock. [1] https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html https://www.nasa.gov/mission_pages/station/expeditions/exped... [2] no link needed

Udo 12y ago
On the other hand, Earth is a very dense planet (in part due to the accidental formation of the Moon), so bigger planets might still have a lower gravity on average. Also, if they spin faster, getting into orbit will be easier. Maybe Earth is indeed at a comparatively bad place in the gravity spectrum of habitable planets. Getting things up there is difficult enough that we can't have big payloads, but it's still easy enough to justify not building a self-sufficient infrastructure out there.
marcosdumay 12y ago
Only if they insist on using chemical rockets, like we do.
- maaku 12y ago
  And the other options are...? I can think of nuclear thermal, which has its risks, and Orion-like bomb propulsion which no sane civilization would use. What else? Most non-chemical propulsive mechanisms considered for space travel do not have the ISP necessary to launch from the bottom of a deep gravity well. We will probably continue to use chemical rockets to leave this Earth, at least until the first space elevator is built, and that itself will require quite a bit of up mass.
  - a3n 12y ago
    I can't jump from the bottom to the top of the hill. I also can't throw a ball from the bottom to the top of the hill. But I can ride a bike from the bottom to the top, and if I can't ride straight up then I can weave back and forth, taking a shallower and longer path. When I get near the top I can throw the ball to the top of the hill, or maybe even ride my bike all the way to the top. So why can't large planet dwellers ride one optimized vehicle as close to orbit as they can get, and then launch another one or more stage vehicle (of whatever propulsion system) from there?
    - Crito 12y ago
      Humans have launched things like this as well. Orbital Sciences does it with their Pegasus rockets which are launched from high-flying airplanes. They are still chemical rockets though. A key part of getting to space and staying there is going fast. Getting yourself above most of the atmosphere makes it easier to go fast, but you still need to get up to the same speed. Realistically that takes chemical rockets. We know of other options, as mentioned, but they have problems, as mentioned.
    - noselasd 12y ago
      That's what rockets basically do today, use multiple stages optimized for the area they're operating in. The issue with rockets is that they have to use a certain amount of energy to go up, and they have to carry their own fuel which adds weight and means it need to consume even more energy and so on. For certain values of gravity and type of fuel, this becomes hard, to impractical, to impossible.
  - emergencevector 12y ago
    Most non-chemical propulsive mechanisms considered for space travel do not have the ISP necessary to launch from the bottom of a deep gravity well. You don't need much ISP to launch from the bottom of a deep gravity well and thick atmosphere. What you need more of is thrust. Ion engines have orders of magnitude more ISP than chemical rockets, but you can't launch off of many things larger than an asteroid with one.
    - maaku 12y ago
      You are absolutely correct, thank you for the correction.
  - Datsundere 12y ago
    using magnetic repulsion to shoot up a rocket into space. Have you seen how maglev trains work?
    - sliverstorm 12y ago
      Getting shot from a railgun at escape velocity sounds hazardous to your health. A maglev gun sounds equally problematic. The inherent upside to onboard propulsion is you can apply power for the entire duration of escape, reducing both maximum speed and delta-speed.
    - Crito 12y ago
      On our planet, the relatively thick atmosphere makes those sort of solutions very difficult, if not impossible. The forces involved would also be problematic for human life, but those sort of systems could still be useful for raw materials.
  - marcosdumay 12y ago
    Yes, Orion-like bomb propulsion will do it. Looks like explosions can be contained in a steel tube, and the resulting fall-out is almost nill (a giant, nuclear, rocket throwing gun!). Launch-loops may also compose well with chemical rockets. That'll take much more adnvanced tech than simply using rockets, but the laws of physics permit them. (A smaller, electric, rocket throwing gun...) Those civs won't be able to construct a space elevator. It's boderline impossible on Earth already, on any bigger planet it'll be completely impossible.
  - TheOtherHobbes 12y ago
    >And the other options are...? Nobody knows. Consider: 1. We don't know everything about physics. 2. We don't even know how much we don't know about physics. I'd be surprised if alien civs didn't do at least a few things that are hard to imagine on 21st century Earth. Some would be more or less mainstream, like balloon staging: http://www.gizmag.com/zero2infinity-balloon-rocket-launch/34315/ http://www.gizmag.com/zero2infinity-balloon-rocket-launch/34... Others could be much weirder - possibly incomprehensible without a few years of science exchange. I realise this is idle speculation, but it's impossible not to wonder if aliens would be alien, technologically and in other possible ways.
  - x1798DE 12y ago
    Why would no sane civilization use Orion-like bomb propulsion? I imagine the cost-benefit analysis changes when liftoff is nearly impossible with available chemical rockets, though I'm not quite sure what your objection is.
    - TeMPOraL 12y ago
      If it occurs to them at all that the benefits may be greater than costs. I'm extrapolating here a lot from human condition, but we humans have a tendency to first do something and only then begin to understand its potential. It's not that we carefully considered all the benefits of artificial satellite network before inventing the first rocket capable of reaching orbit. So a hypothetical civilization living at the bottom of a deeper gravity well might not even consider spaceflight, and be forever stuck on their world. Kerbals have it easy though. They can tour their entire solar system with the same amount of delta-v we need to reach Low Earth Orbit :/.
      - x1798DE 12y ago
        I was specifically wondering why the Orion-like bomb is so bad that no sane civilization would ever use it. The parent comment seemed to indicate that even if you knew you wanted to go into space and had the idea for an Orion-like system, you wouldn't use it unless you were insane, but I see no justification for that.
  - JoeAltmaier 12y ago
    Superconducting magnet riding the planetary magnetic field?
  - pavel_lishin 12y ago
    > And the other options are...? http://en.wikipedia.org/wiki/Non-rocket_spacelaunch http://en.wikipedia.org/wiki/Non-rocket_spacelaunch
    - Crito 12y ago
      Those are all either solutions that require you to already be in space to set up (like space elevators), or at least require much more advanced technology than the early chemical rockets humans first used. A civilization that used those methods, rather than chemical rockets, would require much more technological advancement before they made it into space.
grey-area 12y ago
If most planets are larger than we are Do you have a source for that assertion? Most discovered planets to date are larger than Earth and closer to the star, but that's probably because it's easier to discover those planets given our current technology... Interesting to think that the size of planets would limit exploration though (from the article): If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport.
- stromgo 12y ago
  > Most discovered planets to date are... It's possible to do better than lump all of the discovered planets together and complain that the set is biased. For example, Kepler can detect a planet iff there is a chance alignment of its orbit with our line of sight, so for each discovered planet, it's possible to compute the probability that its orbit was correctly aligned, and divide by it to compensate. That way it's possible to make unbiased statements about the distribution of planet sizes and orbits, the fraction of stars with planets, etc.
  - grey-area 12y ago
    Wasn't complaining. I'd be interested in a ref for this, most popular science I've seen has skated over the limitations of data collection and made lots of assumptions.
  - mturmon 12y ago
    This appears to be exactly what they're doing. Here's an exchange on HN a couple of years ago on a similar question: https://news.ycombinator.com/item?id=5668324 https://news.ycombinator.com/item?id=5668324 In that submission, astronomers including Mike Brown of Caltech, Steve Vogt of UCSC, and Sara Seager of MIT all make assertions about typical exoplanet composition. A HN reader asked about the biased sampling issue. You can bet they are not just ignoring the possibility of biased sampling.

by pdx 12y ago

Udo 12y ago

On the other hand, Earth is a very dense planet (in part due to the accidental formation of the Moon), so bigger planets might still have a lower gravity on average. Also, if they spin faster, getting into orbit will be easier. Maybe Earth is indeed at a comparatively bad place in the gravity spectrum of habitable planets. Getting things up there is difficult enough that we can't have big payloads, but it's still easy enough to justify not building a self-sufficient infrastructure out there.

marcosdumay 12y ago

Only if they insist on using chemical rockets, like we do.

maaku 12y ago
And the other options are...? I can think of nuclear thermal, which has its risks, and Orion-like bomb propulsion which no sane civilization would use. What else? Most non-chemical propulsive mechanisms considered for space travel do not have the ISP necessary to launch from the bottom of a deep gravity well. We will probably continue to use chemical rockets to leave this Earth, at least until the first space elevator is built, and that itself will require quite a bit of up mass.
- a3n 12y ago
  I can't jump from the bottom to the top of the hill. I also can't throw a ball from the bottom to the top of the hill. But I can ride a bike from the bottom to the top, and if I can't ride straight up then I can weave back and forth, taking a shallower and longer path. When I get near the top I can throw the ball to the top of the hill, or maybe even ride my bike all the way to the top. So why can't large planet dwellers ride one optimized vehicle as close to orbit as they can get, and then launch another one or more stage vehicle (of whatever propulsion system) from there?
  - Crito 12y ago
    Humans have launched things like this as well. Orbital Sciences does it with their Pegasus rockets which are launched from high-flying airplanes. They are still chemical rockets though. A key part of getting to space and staying there is going fast. Getting yourself above most of the atmosphere makes it easier to go fast, but you still need to get up to the same speed. Realistically that takes chemical rockets. We know of other options, as mentioned, but they have problems, as mentioned.
  - noselasd 12y ago
    That's what rockets basically do today, use multiple stages optimized for the area they're operating in. The issue with rockets is that they have to use a certain amount of energy to go up, and they have to carry their own fuel which adds weight and means it need to consume even more energy and so on. For certain values of gravity and type of fuel, this becomes hard, to impractical, to impossible.
- emergencevector 12y ago
  Most non-chemical propulsive mechanisms considered for space travel do not have the ISP necessary to launch from the bottom of a deep gravity well. You don't need much ISP to launch from the bottom of a deep gravity well and thick atmosphere. What you need more of is thrust. Ion engines have orders of magnitude more ISP than chemical rockets, but you can't launch off of many things larger than an asteroid with one.
  - maaku 12y ago
    You are absolutely correct, thank you for the correction.
- Datsundere 12y ago
  using magnetic repulsion to shoot up a rocket into space. Have you seen how maglev trains work?
  - sliverstorm 12y ago
    Getting shot from a railgun at escape velocity sounds hazardous to your health. A maglev gun sounds equally problematic. The inherent upside to onboard propulsion is you can apply power for the entire duration of escape, reducing both maximum speed and delta-speed.
  - Crito 12y ago
    On our planet, the relatively thick atmosphere makes those sort of solutions very difficult, if not impossible. The forces involved would also be problematic for human life, but those sort of systems could still be useful for raw materials.
- marcosdumay 12y ago
  Yes, Orion-like bomb propulsion will do it. Looks like explosions can be contained in a steel tube, and the resulting fall-out is almost nill (a giant, nuclear, rocket throwing gun!). Launch-loops may also compose well with chemical rockets. That'll take much more adnvanced tech than simply using rockets, but the laws of physics permit them. (A smaller, electric, rocket throwing gun...) Those civs won't be able to construct a space elevator. It's boderline impossible on Earth already, on any bigger planet it'll be completely impossible.
- TheOtherHobbes 12y ago
  >And the other options are...? Nobody knows. Consider: 1. We don't know everything about physics. 2. We don't even know how much we don't know about physics. I'd be surprised if alien civs didn't do at least a few things that are hard to imagine on 21st century Earth. Some would be more or less mainstream, like balloon staging: http://www.gizmag.com/zero2infinity-balloon-rocket-launch/34315/ http://www.gizmag.com/zero2infinity-balloon-rocket-launch/34... Others could be much weirder - possibly incomprehensible without a few years of science exchange. I realise this is idle speculation, but it's impossible not to wonder if aliens would be alien, technologically and in other possible ways.
- x1798DE 12y ago
  Why would no sane civilization use Orion-like bomb propulsion? I imagine the cost-benefit analysis changes when liftoff is nearly impossible with available chemical rockets, though I'm not quite sure what your objection is.
  - TeMPOraL 12y ago
    If it occurs to them at all that the benefits may be greater than costs. I'm extrapolating here a lot from human condition, but we humans have a tendency to first do something and only then begin to understand its potential. It's not that we carefully considered all the benefits of artificial satellite network before inventing the first rocket capable of reaching orbit. So a hypothetical civilization living at the bottom of a deeper gravity well might not even consider spaceflight, and be forever stuck on their world. Kerbals have it easy though. They can tour their entire solar system with the same amount of delta-v we need to reach Low Earth Orbit :/.
    - x1798DE 12y ago
      I was specifically wondering why the Orion-like bomb is so bad that no sane civilization would ever use it. The parent comment seemed to indicate that even if you knew you wanted to go into space and had the idea for an Orion-like system, you wouldn't use it unless you were insane, but I see no justification for that.
- JoeAltmaier 12y ago
  Superconducting magnet riding the planetary magnetic field?
- pavel_lishin 12y ago
  > And the other options are...? http://en.wikipedia.org/wiki/Non-rocket_spacelaunch http://en.wikipedia.org/wiki/Non-rocket_spacelaunch
  - Crito 12y ago
    Those are all either solutions that require you to already be in space to set up (like space elevators), or at least require much more advanced technology than the early chemical rockets humans first used. A civilization that used those methods, rather than chemical rockets, would require much more technological advancement before they made it into space.

grey-area 12y ago

If most planets are larger than we are Do you have a source for that assertion? Most discovered planets to date are larger than Earth and closer to the star, but that's probably because it's easier to discover those planets given our current technology... Interesting to think that the size of planets would limit exploration though (from the article): If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport.

stromgo 12y ago
> Most discovered planets to date are... It's possible to do better than lump all of the discovered planets together and complain that the set is biased. For example, Kepler can detect a planet iff there is a chance alignment of its orbit with our line of sight, so for each discovered planet, it's possible to compute the probability that its orbit was correctly aligned, and divide by it to compensate. That way it's possible to make unbiased statements about the distribution of planet sizes and orbits, the fraction of stars with planets, etc.
- grey-area 12y ago
  Wasn't complaining. I'd be interested in a ref for this, most popular science I've seen has skated over the limitations of data collection and made lots of assumptions.
- mturmon 12y ago
  This appears to be exactly what they're doing. Here's an exchange on HN a couple of years ago on a similar question: https://news.ycombinator.com/item?id=5668324 https://news.ycombinator.com/item?id=5668324 In that submission, astronomers including Mike Brown of Caltech, Steve Vogt of UCSC, and Sara Seager of MIT all make assertions about typical exoplanet composition. A HN reader asked about the biased sampling issue. You can bet they are not just ignoring the possibility of biased sampling.