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I would argue the exo-planets we have found so far are a very biased sample. We don't have great technologies for detecting solar systems like ours. And at le

I would argue the exo-planets we have found so far are a very biased sample. We don't have great technologies for detecting solar systems like ours. And at least the first few years of planet hunting we'll tend to find a lot of large giants orbiting close to their stars since they're the easiest to find.

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3JPLW 13y ago
I was very surprised that the article made no mention of such a bias. It was my first thought, too. I'm not terribly up-to-date on how Kepler has been detecting planets, but isn't it due to the 'wobble' of the stars? Surely they'll find planets that orbit faster (and wobble their stars faster) first. It'll take many many years of very consistent observations (or one very lucky day) to see a wobble (or occlusion) from a planet like our Jupiter.
- schiffern 13y ago
  Kepler uses occlusion, not wobbling.
  - Udo 13y ago
    Still, a very rare event requiring a precise alignment of the star system that gets more improbable and more rare the further out a planet is. And I imagine we need 2-3 data points of transitions at least to exclude false positives - that simply doesn't happen within a reasonable time (e.g. Neptune has an orbital period of 165 years).
  - teamonkey 13y ago
    Yes, and it's designed to detect Earth-sized planets as well as larger gas giants.
    - Tuna-Fish 13y ago
      But detection probability is directly correlated to planet size and shortness of the local year. Eventually, Kepler and Kepler-like probes will be able to locate all earth-like planets with orbits that cross the path between the star and us. However, this technique will find the larger planets with shorter orbits first.
      - teamonkey 13y ago
        Yes indeed, but the point is that according to current theories Kepler should already be finding more Earth-sized planets than it has and few inner gas giants. The observable evidence does not match the predicted model.
    - Retric 13y ago
      Can detect is one thing, can detect at the same distance is another. Assume you can detect an earth like planet at say 100 light years and a Jupiter sized planet at 1,000 light years and you find 1000x as many Jupiter sized planets that does not mean there are 1,000x as many just that you can detect them in 1,000x as much space. Granted I don't know there search pattern but we are talking about less than 1,000 systems at this point. And I think we have found 2 reasonably earth like planets already which suggests there are at a minimum millions of them in this galaxy.
  - lutorm 13y ago
    True, but occultations are also very biased towards planets with small semimajor axis, because the solid angle that produces an occultation goes roughly as 1/a.
    - schiffern 13y ago
      You use "but" as though you're disputing some statement I made, when I intentionally avoided making any statement outside my knowledge. I said Kepler does use occlusion, and does not use wobbling. I'm not trying to single you out here. I'm just curious what one has to say on HN so as not to be corrected. Is there some "don't assume I've implicitly made a bunch of unsupported remarks" initialism that I can tack on to the end of all my posts? /DAIIMABOUR
      - dandelany 13y ago
        No need to get defensive, lutorm was just pointing out that the assumption made by the OP is still supported. His comment was simply a productive, interesting addition to yours, unlike this complaint.
        fusiongyro 13y ago
        I'm glad he brought it up. I have noticed it is getting awfully hard to say anything precisely enough to get it past the cadre of pedants.
        cmccabe 13y ago
        Actually, the correct expression is "cohort of pedants."
        fusiongyro 13y ago
        Are these tears of laughter or rage?
- sbierwagen 13y ago
  Most exoplanets are discovered by the radial velocity method, (tracking the doppler shifts in the spectrographic lines in the light emitted by the star) but the best data is produced by Kepler, which uses occlusion and thus can tell you how big the planet is, by how much the star dims. Both methods are biased towards finding big planets (which produce clear, easy to detect spikes) that are close to their star. (so you get enough data points) Kepler tends to be a little more biased, simply because it hasn't been running long enough: http://en.wikipedia.org/wiki/File:Exoplanet_Period-Mass_Scatter_Discovery_Method_TR.png http://en.wikipedia.org/wiki/File:Exoplanet_Period-Mass_Scat... Kepler was launched in 2009. Realistically, you'd have to run it for another three or four centuries if you wanted to detect exoplanets that orbit as far out as Neptune. (orbital period: 165 years) No wonder we're seeing a lot of planets orbiting close in!
  - S4M 13y ago
    That's a very interesting point. I knew about this method but didn't connect it to the article. it would be interesting to have some stats on the number of stars that were probed and that were found to have exoplanets. I have no idea of what the actual numbers are, but if we need to probe 100000 stars to find one that has an exo planet, it means that the radial velocity method might not be that efficient.
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joshuahedlund 13y ago
Indeed. Couldn't you just as easily say we used to think our solar system was abnormal because we hadn't discovered any planets around other stars?
madaxe 13y ago
Yeah, that was my first thought too. Sample bias all the way. 1) Things which orbit rapidly are easy to spot as we get lots of observations from occlusion and wobble. You can identify a planet over the course of a few weeks if it has a period measured in days. 2) Things which are massive are easy to detect using wobble methods. The closer in they are, the more wobble there is. GMm/r^2 and all that. 3) Massive close-in things with short periods are the easiest planets to detect by a country mile, as you get a nice big occlusion along with a nice big wobble, all happening on nice short timescales. This entire article is oriented around sample bias due to current instrumentation and techniques. We have little hope of spotting the equivalent of, say, Uranus, with the same orbital period and distance, as we'd need, um, a good few centuries of observations using wobble and occlusion to be certain - and the net effects measured would be tiny, so you'd need a huge pile of data to get a statistically significant measure. Finally, earth-mass planets at an earth-like distance - again, tricky. Less tricky than the above, arguably, particularly if they have atmospheres (spectral changes are a dead giveaway), but still tricky. JWST could serve up the goods.
- saalweachter 13y ago
  I think sampling bias is definitely the most likely reason we haven't seen any solar systems like ours, but... Even with sampling bias, we could still be seeing something legitimately weird. We expect to detect some certain number of exo-planets with a certain range of characteristics with our current technology. We don't think this set of exo-planets is necessarily representative of the average exo-planet in the galaxy, because of the limits of our technology, but we expect to look at X stars, and see Y planets, which look about like Z (where Z is probably Jupiter-sized planets orbiting at Jupiter-like distances). We might also expect to see a few weird planets, where the solar systems are aberrations or we just got lucky and detected something better than we'd expect with our technology. But what this article seems to be saying (maybe a little poorly) is that the set of planets observed is not the subset of planets we expected to observe. We expected (say) to look at 100,000 stars, see 100 normal Jupiters, fail to see 9,900 other normal Jupiters that were there but we didn't detect, and see 1 aberration, like an Earth-like planet we accidentally detected or a hot Jupiter representing a weirdly captured wandering planet. Instead, we saw 1000 aberrations, hot Jupiters. More than we expected to see based on how we thought solar systems formed. Just because the unexpected thing is actually extremely easy to detect once you're looking at the universe in the right way doesn't make it any less unexpected. Even if we eventually find millions more solar systems that look like ours than ones with hot Jupiters, we might still have to figure out why there are so many more hot Jupiters than we expected.
mturmon 13y ago
Yes, but they quoted both Steve Vogt and Mike Brown, who have devoted most of their careers to planet-finding. And both of them said that our solar system seems unusual. Since they are well aware of the biased sample issue, I would bet there is more to it than this. For instance, maybe they are able to model the effect of the bias, at least for some range of planetary masses, and the bias is not enough to explain the frequency-vs-mass curve they are getting. Or maybe their use of multiple techniques allows them to characterize the bias. This excerpt from their recent paper on an Earth-like planet in the habitable zone of a nearby M-class star gives an indication of where they're going with this work: "Using the relations given by Charbonneau et al. (2007), the reported candidates have non-negligible probabilities of transiting in front of the star (∼2.7%, 1.1%, and 0.6% for planets b, c, and d, respectively). [...] With the new generation of optical and infrared spectrographs, many nearby M dwarfs will be eﬃciently surveyed for low mass planets. If the detection rate holds, very soon now we may have a real chance of searching for spectroscopic signatures of water and life on one of these worlds." In other words, detect a bunch of candidates, watch for transits, and use spectroscopic information to detect water.
- lake99 13y ago
  tocomment is right. Watch http://www.youtube.com/watch?v=8JTneTI82nE http://www.youtube.com/watch?v=8JTneTI82nE from 5:00 to 7:00. Yes, scientists may have been quoted in this article, but I have seen how most journalists mangle, ignore, or sensationalize things that were said.
  - scott_s 13y ago
    She also agrees with the main point of the original article, which is that our solar system is probably unusual. (From 6:47 to 7:00.)
    - lake99 13y ago
      Unusual is a subjective term. She says 10%-20% of the planetary systems are like ours. Are you saying that that was the point made in the original article too?
namuol 13y ago
Precisely my thoughts. Has this notion been refuted? I would think this is common thought among the community of exoplanet hunters.
Alex3917 13y ago
"I would argue the exo-planets we have found so far are a very biased sample." All of science is based on sampling bias, by definition. That's one of the main reasons why science isn't a perfect lens for discovering truth. But just because something is probably wrong doesn't mean it isn't a 'scientific fact.'

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3JPLW 13y ago

I was very surprised that the article made no mention of such a bias. It was my first thought, too. I'm not terribly up-to-date on how Kepler has been detecting planets, but isn't it due to the 'wobble' of the stars? Surely they'll find planets that orbit faster (and wobble their stars faster) first. It'll take many many years of very consistent observations (or one very lucky day) to see a wobble (or occlusion) from a planet like our Jupiter.

schiffern 13y ago
Kepler uses occlusion, not wobbling.
- Udo 13y ago
  Still, a very rare event requiring a precise alignment of the star system that gets more improbable and more rare the further out a planet is. And I imagine we need 2-3 data points of transitions at least to exclude false positives - that simply doesn't happen within a reasonable time (e.g. Neptune has an orbital period of 165 years).
- teamonkey 13y ago
  Yes, and it's designed to detect Earth-sized planets as well as larger gas giants.
  - Tuna-Fish 13y ago
    But detection probability is directly correlated to planet size and shortness of the local year. Eventually, Kepler and Kepler-like probes will be able to locate all earth-like planets with orbits that cross the path between the star and us. However, this technique will find the larger planets with shorter orbits first.
    - teamonkey 13y ago
      Yes indeed, but the point is that according to current theories Kepler should already be finding more Earth-sized planets than it has and few inner gas giants. The observable evidence does not match the predicted model.
  - Retric 13y ago
    Can detect is one thing, can detect at the same distance is another. Assume you can detect an earth like planet at say 100 light years and a Jupiter sized planet at 1,000 light years and you find 1000x as many Jupiter sized planets that does not mean there are 1,000x as many just that you can detect them in 1,000x as much space. Granted I don't know there search pattern but we are talking about less than 1,000 systems at this point. And I think we have found 2 reasonably earth like planets already which suggests there are at a minimum millions of them in this galaxy.
- lutorm 13y ago
  True, but occultations are also very biased towards planets with small semimajor axis, because the solid angle that produces an occultation goes roughly as 1/a.
  - schiffern 13y ago
    You use "but" as though you're disputing some statement I made, when I intentionally avoided making any statement outside my knowledge. I said Kepler does use occlusion, and does not use wobbling. I'm not trying to single you out here. I'm just curious what one has to say on HN so as not to be corrected. Is there some "don't assume I've implicitly made a bunch of unsupported remarks" initialism that I can tack on to the end of all my posts? /DAIIMABOUR
    - dandelany 13y ago
      No need to get defensive, lutorm was just pointing out that the assumption made by the OP is still supported. His comment was simply a productive, interesting addition to yours, unlike this complaint.
      - fusiongyro 13y ago
        I'm glad he brought it up. I have noticed it is getting awfully hard to say anything precisely enough to get it past the cadre of pedants.
        cmccabe 13y ago
        Actually, the correct expression is "cohort of pedants."
        fusiongyro 13y ago
        Are these tears of laughter or rage?
sbierwagen 13y ago
Most exoplanets are discovered by the radial velocity method, (tracking the doppler shifts in the spectrographic lines in the light emitted by the star) but the best data is produced by Kepler, which uses occlusion and thus can tell you how big the planet is, by how much the star dims. Both methods are biased towards finding big planets (which produce clear, easy to detect spikes) that are close to their star. (so you get enough data points) Kepler tends to be a little more biased, simply because it hasn't been running long enough: http://en.wikipedia.org/wiki/File:Exoplanet_Period-Mass_Scatter_Discovery_Method_TR.png http://en.wikipedia.org/wiki/File:Exoplanet_Period-Mass_Scat... Kepler was launched in 2009. Realistically, you'd have to run it for another three or four centuries if you wanted to detect exoplanets that orbit as far out as Neptune. (orbital period: 165 years) No wonder we're seeing a lot of planets orbiting close in!
- S4M 13y ago
  That's a very interesting point. I knew about this method but didn't connect it to the article. it would be interesting to have some stats on the number of stars that were probed and that were found to have exoplanets. I have no idea of what the actual numbers are, but if we need to probe 100000 stars to find one that has an exo planet, it means that the radial velocity method might not be that efficient.

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joshuahedlund 13y ago

Indeed. Couldn't you just as easily say we used to think our solar system was abnormal because we hadn't discovered any planets around other stars?

madaxe 13y ago

Yeah, that was my first thought too. Sample bias all the way. 1) Things which orbit rapidly are easy to spot as we get lots of observations from occlusion and wobble. You can identify a planet over the course of a few weeks if it has a period measured in days. 2) Things which are massive are easy to detect using wobble methods. The closer in they are, the more wobble there is. GMm/r^2 and all that. 3) Massive close-in things with short periods are the easiest planets to detect by a country mile, as you get a nice big occlusion along with a nice big wobble, all happening on nice short timescales. This entire article is oriented around sample bias due to current instrumentation and techniques. We have little hope of spotting the equivalent of, say, Uranus, with the same orbital period and distance, as we'd need, um, a good few centuries of observations using wobble and occlusion to be certain - and the net effects measured would be tiny, so you'd need a huge pile of data to get a statistically significant measure. Finally, earth-mass planets at an earth-like distance - again, tricky. Less tricky than the above, arguably, particularly if they have atmospheres (spectral changes are a dead giveaway), but still tricky. JWST could serve up the goods.

saalweachter 13y ago
I think sampling bias is definitely the most likely reason we haven't seen any solar systems like ours, but... Even with sampling bias, we could still be seeing something legitimately weird. We expect to detect some certain number of exo-planets with a certain range of characteristics with our current technology. We don't think this set of exo-planets is necessarily representative of the average exo-planet in the galaxy, because of the limits of our technology, but we expect to look at X stars, and see Y planets, which look about like Z (where Z is probably Jupiter-sized planets orbiting at Jupiter-like distances). We might also expect to see a few weird planets, where the solar systems are aberrations or we just got lucky and detected something better than we'd expect with our technology. But what this article seems to be saying (maybe a little poorly) is that the set of planets observed is not the subset of planets we expected to observe. We expected (say) to look at 100,000 stars, see 100 normal Jupiters, fail to see 9,900 other normal Jupiters that were there but we didn't detect, and see 1 aberration, like an Earth-like planet we accidentally detected or a hot Jupiter representing a weirdly captured wandering planet. Instead, we saw 1000 aberrations, hot Jupiters. More than we expected to see based on how we thought solar systems formed. Just because the unexpected thing is actually extremely easy to detect once you're looking at the universe in the right way doesn't make it any less unexpected. Even if we eventually find millions more solar systems that look like ours than ones with hot Jupiters, we might still have to figure out why there are so many more hot Jupiters than we expected.

mturmon 13y ago

Yes, but they quoted both Steve Vogt and Mike Brown, who have devoted most of their careers to planet-finding. And both of them said that our solar system seems unusual. Since they are well aware of the biased sample issue, I would bet there is more to it than this. For instance, maybe they are able to model the effect of the bias, at least for some range of planetary masses, and the bias is not enough to explain the frequency-vs-mass curve they are getting. Or maybe their use of multiple techniques allows them to characterize the bias. This excerpt from their recent paper on an Earth-like planet in the habitable zone of a nearby M-class star gives an indication of where they're going with this work: "Using the relations given by Charbonneau et al. (2007), the reported candidates have non-negligible probabilities of transiting in front of the star (∼2.7%, 1.1%, and 0.6% for planets b, c, and d, respectively). [...] With the new generation of optical and infrared spectrographs, many nearby M dwarfs will be eﬃciently surveyed for low mass planets. If the detection rate holds, very soon now we may have a real chance of searching for spectroscopic signatures of water and life on one of these worlds." In other words, detect a bunch of candidates, watch for transits, and use spectroscopic information to detect water.

lake99 13y ago
tocomment is right. Watch http://www.youtube.com/watch?v=8JTneTI82nE http://www.youtube.com/watch?v=8JTneTI82nE from 5:00 to 7:00. Yes, scientists may have been quoted in this article, but I have seen how most journalists mangle, ignore, or sensationalize things that were said.
- scott_s 13y ago
  She also agrees with the main point of the original article, which is that our solar system is probably unusual. (From 6:47 to 7:00.)
  - lake99 13y ago
    Unusual is a subjective term. She says 10%-20% of the planetary systems are like ours. Are you saying that that was the point made in the original article too?

namuol 13y ago

Precisely my thoughts. Has this notion been refuted? I would think this is common thought among the community of exoplanet hunters.

Alex3917 13y ago

"I would argue the exo-planets we have found so far are a very biased sample." All of science is based on sampling bias, by definition. That's one of the main reasons why science isn't a perfect lens for discovering truth. But just because something is probably wrong doesn't mean it isn't a 'scientific fact.'