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> We have cataloged planets in hundreds of other solar systems. We see ones with only rocky planets. Ones with only gas giants. But we're the only one we know o

by mbell 8y ago

> We have cataloged planets in hundreds of other solar systems. We see ones with only rocky planets. Ones with only gas giants. But we're the only one we know of with both gas giants and rocky planets. The only one where the rocky planets wind up protected from most of dinosaur killer kinds of impacts. Not true: https://en.wikipedia.org/wiki/Kepler-90 https://en.wikipedia.org/wiki/Kepler-90

btilly 8y ago
Thank you for that correction. I am having trouble finding the reference that I found that claim in. But note that Kepler-90 does not have rocky planets that look like the Earth. It has super-earths, and gas giants. A gas giant will do a worse job of protecting a super-earth than it will of protecting small rocky planets. So Jupiter still seems rare. Incidentally while looking for the article that I saw before, I stumbled across https://www.reddit.com/r/askscience/comments/4ohkkn/how_unique_is_our_solar_system/ https://www.reddit.com/r/askscience/comments/4ohkkn/how_uniq... which pointed out something I had not thought of. Our Sun is an unusual combination of old and metallic. It is likely that a neutron star collision happened near the stellar nursery where the Sun is born. This is important for a couple of reasons. The first is that metallic stars give rise to more planets. The second is that metallic stars can give rise to planets with richer chemistry. This is another way in which we are weird, which could be potentially important for the development of intelligent life.
- cgriswald 8y ago
  Jupiter is hard to find. It’s orbit is ~11 years making it harder to measure and more time consuming. It’s pull on the sun is a lot harder to measure than a hot Jupiter, making the radial velocity method more difficult. We aren’t finding average systems. We are finding systems that are easy (and quick) to find using the methods we have available.
  - btilly 8y ago
    Harder but not beyond us. Per https://physics.stackexchange.com/questions/230973/how-can-we-see-planets-thousands-of-light-years-away-but-dont-know-if-there-are https://physics.stackexchange.com/questions/230973/how-can-w... at this point we would be able to pick up Jupiter and Saturn around another star.
    - cgriswald 8y ago
      Yes, but I don’t see how that is relevant. Your claim is that we aren’t seeing Jupiter/Earth type systems and so they are rare. The counterargument is that we aren’t looking for them, so we don’t see them. We don’t know whether they are rare or not.
    - sliken 8y ago
      Keep in mind that there's many more orbits that don't occlude a star than those that do. So there's some luck required, and likely at least 2 orbital occlusions (22 years) to be sure.
- joshvm 8y ago
  Statistics suggest that there is at least 1 planet per star. Multi-planetary systems are the norm, not a rarity. There is a huge amount of observational bias in exoplanet research and given that we've "only" found a few thousand planets, it's naive to think our Solar System is unique. It's practically certain that with better instruments we'll find new planets in currently known systems. For example HD_10180 (7 planets currently) also has two potential gas giants. See also HD 219134. Also: stats of 100 billion to one, those are reasonable odds. That would make two species in the Milky Way, let alone the other billions-trillions of galaxies in the Universe.
  - btilly 8y ago
    I suspect that you almost exactly reversed the point. The point of the Fermi paradox is that an advanced technological system should be able to colonize the whole galaxy in a geologically short time scale using Von Neumann machines. So if it had happened anywhere else in the galaxy, we'd expect to know about it because they would be already here. But they are not here. So we're probably the first in this galaxy. Given that there are around 250 billion stars in the galaxy, if the odds were above 1/100 billion per star, the observed outcome would be unlikely. So it is probable that the real odds are below 1/100 billion. I was offering potential requirements under which real odds on the order of 1/trillion could be justified. These odds are low enough that the observation "we're the first technological civilization in the Milky Way" is not a surprise and the Fermi paradox stops being a paradox. Neither the Fermi paradox nor my argument suggests that there aren't lots of other galaxies which contain advanced technological civilizations. The time for an advanced galactic civilization to cross intergalactic space is NOT a geological eye-blink. And almost all galaxies that we see are in other clusters, which are much, much farther off.
    - Retric 8y ago
      I think the Von Neumann machine bit is a vastly larger issue than people assume. The Milky Way is 105,700 light years across. At voyager probe speeds that’s ~1.8 billion years. Stopping requires 4x as much energy, and cutting that time in half requires 4x as much energy. Worse slowing down to build a probe adds more time as does however long it takes to select targets, gather resources, and build several more probes. On top of a huge number of issues could slow things down such as lost probes, or needing to avoid the center of the galaxy resulting in an even longer trip. Not to mention collisions, at a minimum you need to survive grain of sand sized collisions, but interstellar space may contain significant numbers of much larger objects. Finally these probes would both come here, and stick around for us to see. A true space based civilization might have little interest in planets like ours, or have sent multiple probes to our solar system that failed. It seems possible for hundreds of civilizations to all be expanding across the galaxy even if we have seen none of them.
- mbell 8y ago
  Our understanding of exoplanets is analogous to a 3 month old toddler exploring the world. It's _way_ too early to be making definitive statements about it.

by mbell 8y ago

btilly 8y ago

Thank you for that correction. I am having trouble finding the reference that I found that claim in. But note that Kepler-90 does not have rocky planets that look like the Earth. It has super-earths, and gas giants. A gas giant will do a worse job of protecting a super-earth than it will of protecting small rocky planets. So Jupiter still seems rare. Incidentally while looking for the article that I saw before, I stumbled across https://www.reddit.com/r/askscience/comments/4ohkkn/how_unique_is_our_solar_system/ https://www.reddit.com/r/askscience/comments/4ohkkn/how_uniq... which pointed out something I had not thought of. Our Sun is an unusual combination of old and metallic. It is likely that a neutron star collision happened near the stellar nursery where the Sun is born. This is important for a couple of reasons. The first is that metallic stars give rise to more planets. The second is that metallic stars can give rise to planets with richer chemistry. This is another way in which we are weird, which could be potentially important for the development of intelligent life.

cgriswald 8y ago
Jupiter is hard to find. It’s orbit is ~11 years making it harder to measure and more time consuming. It’s pull on the sun is a lot harder to measure than a hot Jupiter, making the radial velocity method more difficult. We aren’t finding average systems. We are finding systems that are easy (and quick) to find using the methods we have available.
- btilly 8y ago
  Harder but not beyond us. Per https://physics.stackexchange.com/questions/230973/how-can-we-see-planets-thousands-of-light-years-away-but-dont-know-if-there-are https://physics.stackexchange.com/questions/230973/how-can-w... at this point we would be able to pick up Jupiter and Saturn around another star.
  - cgriswald 8y ago
    Yes, but I don’t see how that is relevant. Your claim is that we aren’t seeing Jupiter/Earth type systems and so they are rare. The counterargument is that we aren’t looking for them, so we don’t see them. We don’t know whether they are rare or not.
  - sliken 8y ago
    Keep in mind that there's many more orbits that don't occlude a star than those that do. So there's some luck required, and likely at least 2 orbital occlusions (22 years) to be sure.
joshvm 8y ago
Statistics suggest that there is at least 1 planet per star. Multi-planetary systems are the norm, not a rarity. There is a huge amount of observational bias in exoplanet research and given that we've "only" found a few thousand planets, it's naive to think our Solar System is unique. It's practically certain that with better instruments we'll find new planets in currently known systems. For example HD_10180 (7 planets currently) also has two potential gas giants. See also HD 219134. Also: stats of 100 billion to one, those are reasonable odds. That would make two species in the Milky Way, let alone the other billions-trillions of galaxies in the Universe.
- btilly 8y ago
  I suspect that you almost exactly reversed the point. The point of the Fermi paradox is that an advanced technological system should be able to colonize the whole galaxy in a geologically short time scale using Von Neumann machines. So if it had happened anywhere else in the galaxy, we'd expect to know about it because they would be already here. But they are not here. So we're probably the first in this galaxy. Given that there are around 250 billion stars in the galaxy, if the odds were above 1/100 billion per star, the observed outcome would be unlikely. So it is probable that the real odds are below 1/100 billion. I was offering potential requirements under which real odds on the order of 1/trillion could be justified. These odds are low enough that the observation "we're the first technological civilization in the Milky Way" is not a surprise and the Fermi paradox stops being a paradox. Neither the Fermi paradox nor my argument suggests that there aren't lots of other galaxies which contain advanced technological civilizations. The time for an advanced galactic civilization to cross intergalactic space is NOT a geological eye-blink. And almost all galaxies that we see are in other clusters, which are much, much farther off.
  - Retric 8y ago
    I think the Von Neumann machine bit is a vastly larger issue than people assume. The Milky Way is 105,700 light years across. At voyager probe speeds that’s ~1.8 billion years. Stopping requires 4x as much energy, and cutting that time in half requires 4x as much energy. Worse slowing down to build a probe adds more time as does however long it takes to select targets, gather resources, and build several more probes. On top of a huge number of issues could slow things down such as lost probes, or needing to avoid the center of the galaxy resulting in an even longer trip. Not to mention collisions, at a minimum you need to survive grain of sand sized collisions, but interstellar space may contain significant numbers of much larger objects. Finally these probes would both come here, and stick around for us to see. A true space based civilization might have little interest in planets like ours, or have sent multiple probes to our solar system that failed. It seems possible for hundreds of civilizations to all be expanding across the galaxy even if we have seen none of them.
mbell 8y ago
Our understanding of exoplanets is analogous to a 3 month old toddler exploring the world. It's _way_ too early to be making definitive statements about it.