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.
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.
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_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!
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.
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).
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.
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?
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.
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.
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.
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.
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.
That's a great video, and obviously she is very qualified to speak to this subject. I don't see how this shows that the GP comment is "right" in its assertion that this is just an effect of biased sampling.
I only had time to look at the first few minutes, but around 6:50 she does say that our solar system is "not that common", which she quantifies as "it could only be as much as 10-20% of star systems". Maybe that means <= 10-20%?
So, this does not seem to contradict anything Vogt and Brown were quoted as saying.
Incidentally, I wasn't going by the statement in the NPR article, but also by the press release from UCSC (http://news.ucsc.edu/2012/12/tau-ceti.html), Vogt's home institution. I don't think that quote is subject to journalistic mangling.
I agreed with tocomment that it was a biased sampling not in the sense that planetary systems happen to be this way in the region we are looking at, but in the sense that planetary systems seem to be this way when we look at things from THIS set of instrumentation and methodology.
IOW, I disagree with NPR's claim that "Our Very Normal Solar System Isn't Normal Anymore". Addendum: It may indeed be abnormal. But we don't have data to conclude that yet, or even to suspect it.
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?
"Unusual is a subjective term" -- absolutely. I don't want to go back and forth on this one either.
Listen carefully to the video. She says "It could only be as common as 10-20%". Just before saying that, she pauses and looks upward, to formulate the sentence correctly. The 10-20% number is an upper bound, not a direct estimate.
You have to give her credit for communicating the idea carefully. The difficulty of doing this in real time is extreme. And if you do it wrong, you really get taken to task by your colleagues.
I don't know if it was, but I'm not concerned. What I took away from the original article was: We have a theory for how solar systems form. That theory assumes that our solar system is typical. The data we're finding does not agree with our theory, because the data indicates that our solar system is not typical.
"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.'
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.
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.
I think there's a meta argument encoded in the sampling bias discussion. That being: some people want to expose NPR as just another media center that produces watered down content labeled as science with a complete lack of journalistic integrity. Other people want to believe NPR and perhaps a few other select content producers are publishing articles such as this with the utmost respect to the material. I.e. Things like page views are not considered when it comes to telling a somewhat complete version of the story.
I think this case falls under the lack of journalistic integrity, regardless of whether the overall claim is right or wrong. My view is that when a science article wants introduce the idea that reality may be different than conventionally believed, the goal should be to write an (at least mildly) well rounded, informative piece, not a strictly persuasive piece. When the first response in hundreds of armchair physicists' minds around the world is surprise that selection bias wasn't even mentioned (mine included), I think it's fair to say the article falls more into the persuasive category.
Yeah, but the other possibility is that our tech hasn't lived up to our expectations, and therefore we haven't made the observations and measurements we might have hoped for, which I'd wager is far more likely.
"Hot Jupiter" type planets aren't as much of a revelation as one might expect, however, given the prevalence of binary systems, many of which have very, very short periods (J0106-1000 has one of 39 minutes) - and there's still nothing to say that those same systems don't have rocky worlds tucked away that we can't see due to the overwhelming noise from the massive inliers.
I think everyone here concentrating on sampling bias is missing the point.
The evidence in solar systems so far does NOT fit the theory involving a frost line, and giants like Jupiter are unexpectedly observed tightly orbiting suns. Whether our solar system is normal is conjecture, but our model for how solar systems are(were?) formed is obviously flawed. Pretty interesting find, imho.
I don't see how the fact that gas planets are capable of migrating inward shows that our model for planetary formation is deeply flawed. I didn't think the current model said that it was impossible for planets to migrate inward.
The point about sampling bias is that our current techniques are bad at spotting systems like ours. If this is true (I think it is, but I'm not qualified to say), we wouldn't expect the evidence to fit our theory even if our theory were true because we are nearly incapable of finding evidence that does fit our theory no matter how much of it there is out there.
Perhaps I mangled that a bit, instead of frost line I should have just said the seemingly most popular theory of how the solar system formed. From the article the bit that caught my eye:
Mike Brown, an astronomer at Caltech, wrote me that while everybody is busy hunting for an Earth-like planet, they missed this story. "Before we ever discovered any [planets outside the solar system] we thought we understood the formation of planetary systems pretty deeply." We had our frost line. We knew how solar systems formed. "It was a really beautiful theory," he says. "And, clearly, thoroughly wrong."
I did some reading into this last year while I was stuck in bed sick and what I came across was it was to do more with light pressure than a frost line. The frostline has more to do with where water is likely to form than how a come to exist.
Light has trouble pushing heavy materials outwards, so theres a denser percentage of them in close to a star, the lighter the material the more it gets pushed out, high percentage of hydrocarbons in outer planets. Like a giant centrifuge. So our system should be fairly standard unless the rules of physics are different elsewhere.
That doesn't exclude random formations as you are dealing with super heated eddies, explosions etc... during formation. Or even stars pulling there planets in. Both our planet set up, and hot Jupiters are to be expected because its a giant chaotic exploding mess.
"We have to figure out why our solar system turned out different from all the others."
Not entirely—we have to understand statistics and the anthropic principle, which states "observations of the physical Universe must be compatible with the conscious life that observes it." (Wikipedia: http://en.wikipedia.org/wiki/Anthropic_principle). In other words, our situation appears "perfect" because it was one of the many random combinations available which produced an observer able to look at it. In other words, the fact that our solar system is so stable might just be a random anomaly, but the fact that it was so stable resulted in our existence, and therefore our contemplation of the fact.
As an analogy, winning the lottery is extremely rare—if you go and interview the person who won the lottery, they must think themselves very lucky. But on the grand scale, the probability of someone winning the lottery in the entire pool is exactly one. We know that a winner necessarily exists—just as we know that we, as winners of the cosmic lottery, necessarily exist. But that just tells us that someone won the lottery—something we already knew.
It may be that the lottery is rigged, and that there are some characteristics of our particular solar system which are amenable to life, and those things are definitely valuable to study and find out, so that we might be able to recognize other solar systems capable of supporting life in some way.
But by no means is our solar system special in any light: it's only special in that its particular conditions produced lifeforms able to observe it.
"We are a way for the cosmos to understand itself." - Carl Sagan
Another random question: are the planetary systems we've discovered similar to each other? If ours is significantly deviant from a significant clustering that would be pretty interesting. And of course, that's surely what the original findings were probably about.
You misinterpreted. The point of the article isn't to explore the epistemology of why or how we perceive ourselves to be "special".
The point was that a few years ago we had a solid, broadly-agreed-upon scientific theory of planetary formation. And it was wrong. That's interesting for scientific reasons, not philosophical ones.
In most articles investigating how our solar system or planet is different from the others, they take a pop-sci tone (which I may be reading too much into, for sure) about scientists studying why our special planet got this way.
That is absolutely true to some degree—we want to find out what conditions support life, and why the Earth did end up how it did, and resulted in conditions stable enough and correct enough to allow life to evolve for millions of years essentially uninterrupted.
It's the tone that I read into, and I have a tendency to look out for the earth-centric perspective, mainly because I find the alternative way to look at it basically mind-blowing and fascinating. That doesn't make study of the development of our solar system any less important, any more than the study of evolution makes biology less important. The study of planetary formation, the solar system, geology and every other related field are all extremely important to broadening and deepening our understanding of how the universe works. Understanding statistics and the anthropic principle just gives us a framework in which to view that knowledge that is even more true and complete, in my opinion.
Agreed. For anyone else who routinely reads comments without reading the article, if this is still the top comment, I'd suggest you just go skim the article.
(Aside: I'd guess it's because there are people like me, who often read comments before or without reading the article, that a well-written but not on-point comment like this can get upvoted. I wonder how you'd fix that?)
For pete's sake, can't someone wax poetic on a slightly related subject of interest without it having to be taken as a polarizing direct criticism of the article?
I was surprised at the depth of this thread too. But honestly, yeah. If you're going to wax poetic, it behooves you to let the audience know you're launching into a digression. Instead you started with a quote and a "Not exactly", which sounded like you were making an argument about the substance of the link.
I don't even disagree with your point. But I posted because I did find the linked article interesting (if a little fluffy) and thought your digressive correction was missing some of that substance.
You're surprised that people assume your comments are on-topic? I'm surprised by your surprise. In general, when everybody is talking about a topic and you add something to that discussion, they will try to interpret it in a light that makes it relevant. That's just how our brains process communication.
If you intentionally mean to go off-topic (which you usually shouldn't, but we'll assume you have something really amazing to say), it's best to explain that you're not actually talking about the topic at hand but you thought it was interesting because X. That way people will be able to read your comment in the light in which it was intended rather than straining to find relevance.
(Incidentally, I have no idea why you responded this way to my comment, which is explicitly about ways to ensure people read articles before voting on comments about the articles, and is not about you at all. Maybe you assumed it was on the topic of your comment?)
I never said I was interpreting the article. Can't I just discuss something interesting? Or does it have to be interpreted as an interpretation of the article?
> But by no means is our solar system special in any light: it's only special in that its particular conditions produced lifeforms able to observe it.
sorry, but i don't understand what's the logic here? it's not special, but it's only very special in sustaining/producing life? isn't that more than enough to call it special? :-)
Not in the requisite sense. Suppose that our existence depended on a purely random roll of a six-sided die coming up as 1. If the roll had come up anything else, we would not be here to reflect on it. But if it did come up as 1, that does not mean a mystical force guaranteed that it came up as 1.
Our existence (whether likely or not) does not constitute evidence that our existence was guaranteed or predetermined. Low-probability events happen sometimes, their actually having happened once does not make them high-probability.
it seems like you, and the top comment are interpreting this news story as some attack on atheism? i don't see how that's even related. is that your preemptive strike against non-atheistic interpretation?
Absolutely not. My intention was to broaden the point by introducing the mind-blowing anthropic principle and a new perspective on the conditions leading to life, nothing more.
It's special from our perspective, certainly. It's just not special in a cosmic sense: it just happened, and the universe produced something that could observe it. It could have happened anywhere, but it happened to happen on Earth. To me, that's the special part, and I find it fascinating and mind-blowing to think about.
Unlike a cash lottery for which we intend there to be a winner, winners of a cosmic lottery do not "necessarily" exist--a winner like us is not guaranteed, I mean.
Humor is lost on HN. No, our existence has been verified, and therefore a planet on which we can exist does exist. I hope we don't need to go further with this line of inquiry.
It's true that this could still turn out to be mostly observational bias, but I feel like everyone is missing the monkey wandering across the corridor.
The anthropic principle, that is. Isn't it possible that, while gas planets clustered closely around the star is the normal shape of solar systems, such a configuration is not amenable to complex life?
It would also be a good candidate for the Great Filter - the hidden factor that has stopped the development of all the other intelligent civilisations that would otherwise be out there. That would mean this is good news - we'd rather the Great Filter lies in our past (because that means we've already survived it) than in our future.
I don't think it's a good candidate; certainly it pushes some amount of the Filter into our past, but even if only 1 in 700 solar systems is Sol-like, there are so many we'd still expect to see tons of aliens. For a single-step Filter or for solar systems to even be a majority of the filter, you need a step so unlikely that it can push the presence of interstellar life down from, like, trillions of star systems to ~1.
Thanks for the reply, and yes, that makes sense. What would you predict as the most likely other steps in the filter?
Looking at our own planet, I would guess that it's a combination of a) it being simply infeasible to send large animals to distant stars (we haven't yet colonised the moon or even made a serious attempt to colonise low earth orbit, and we might not do so before we run out of cheap energy) and b) technological singularities.
> What would you predict as the most likely other steps in the filter?
I don't know. Robin Hanson blogs a lot about it, but none of the steps seem terribly plausible although my favorite currently is big brains being feasible - in a number of ways, heads seem to be rare, big brains even rarer, and the costs of big brain almost too expensive to bear because neurons seem to be unable to get more efficient; one paper I liked on the topic was http://www.pnas.org/content/early/2012/06/19/1201895109.full... but you can find a lot of relevant material in http://www.gwern.net/Drug%20heuristics
LEO is a weird place to colonize. There aren't any resources there. The moon is also pretty tough, being no atmosphere and Carbon in ppb quantities. Mars is a good candidate, though.
I imagine that if there were another planet in the solar system capable of supporting native life with just 18th-century technology, we would already have sent people there, even if it were a multi-year trip. Which would provide big economic drivers for interplanetary transport.
Another apparently obvious filter is the development of advanced nanotechnology and AI. However...
Although I'd immediately agree that those are filters hindering the spread of any humanlike civilisation, it doesn't at all stop the spread of other, technological forms - the "vile offspring", to borrow a term.
Honestly, I have no idea what might work as a future filter, at this point. I don't think the expense could be it; that's going to drop, quite abruptly once uploading (or straight-up AI) works well enough.
There's not much suggesting that there is only one Great Filter. After all, we haven't met anyone else yet, so the likelihood of another filter being in our future is just as real as it is if there was one in our past.
Call it the "two neptune filter", to borrow from the article. :-)
While it's a big assumption that this is down to observational bias, at least there is some theoretical support for that statement. The further out a planet is, the less likely it becomes we can observe a transition event (which also becomes extremely rare).
Now that only our system's configuration - as special or as common as it may be - is amenable to complex life is completely unrelated and unfounded.
Kepler has been revolutionary in discovering these 700 odd planetary systems, but we can't even begin to imagine what surprises await for when we point the James Webb Space Telescope at them.
One of JWSTs mission objectives is to study the planetary systems discovered by Kepler, and the origins of life.
Not quite, but JWST will be out at the L2 point, beyond the moon in essentially what amounts to an earth-matching solar orbit.
It's large aperture infrared sensors will have an unobstructed view of the universe (unlike Hubble which is in low earth orbit, and so only gets 90 minute windows of observation).
Basically, using IR spectroscopy we will be able to analyse the atmospheric composition as well as the thermal emissions of exoplanets.
Finally, JWST won't be limited to a small, fixed field of view like Kepler.
"Or maybe it got ejected — astronomers are finding emigrant planets, lonely orbs that wander the universe with no star, just drifting. Maybe one of those used to live here."
There are planets not orbiting stars? I searched Google for "emigrant planets" and the npr article is the number 1 result for that. I think he invented the term. What are these called if I wanted to find more information about them?
Other terms are "orphan planets" and "rogue planets". Naturally, since they don't orbit a star and are usually pretty dark, they're hard to find, though there are candidates:
It probably wouldn't be intelligent life, but maybe there are microbial lifeforms somewhere in the universe which call a rogue planet their home. That's awesome to think about, although it is extremely likely that such a planet will never be discovered. It's just really hard to track down a rogue planet, even a large one.
Being a long time fan of 4x games, well of space games in general, this article makes me smile. I remember reading game forums where one subject that always arose was galaxy and system formations. So many of our concepts are strictly based on the very little observable data we have. Its great to live in a time when many assumptions are proven to be just that.
Now the question that remains is, which configurations are more favorable to life, not just as we know it.
A bit off-topic, but can you recommend any recent space 4x games to check out? The last one I played was Galactic Civilizations II and I enjoyed it immensely.
There's a new-ish one called Endless Space, which I would describe as Gal Civ inspired.
It's a little weird in that all players take turns simultaneously (you still have movement restrictions, etc, but it adds a real-time component because you want to wait to react to your opponent's movements, but also there's a time limit).
The impression I got was that it was a way to speed up gameplay (less sitting around while others make their moves).
But it's a cool little game, lots of race customisation (with a few truly unique race benefits), interesting four-category tech tree (and equivalently, four ways to win), custom ship classes built out of components (like Gal Civ).
The combat is a little weird, they have some card-game-like thingy that can significantly influence battle outcomes.
Neptune's Pride is an always-on real time game played over several weeks in matches of up to 12 players. The mechanics are refreshingly simple, and there's a focus on diplomacy and betrayal in order to come out top.
I think the author has the "frost zone" theory backwards. It's not that it's too cold for rocky planets to form outside the frost zone (after all, there are some planet-sized moons out there) but rather that inside the frost zone it is too hot for planets to hold on to hydrogen, so you don't get gas giants.
I would think that a gassy planet close to a star would have to have a really big solid or liquid core of heavy elements.
The frost line is the point at which ice crystals can form of the major planetary gasses. The theory was that outside the frost line these crystals would add to the density of particulates in the region, therefore larger planets would form outside the frost line and they would have a greater proportion of gasses than the inner planets.
I agree, I'm speculating that by looking at a star wobble or it's brightness decrease that method of detection would bring up all these systems with close orbiting planets that would be bigger than "normal".
Probably not, assuming our current theories about how these systems form are correct. Our best guess is that hot jupiters form about where our jupiter or saturn is and their initial orbit decays inward. If so, that would tend to destroy the orbit of anything vaguely earth-sized on the way in.
All the conventional planet hunting methods are biased towards fast, large, close-in planets. If we could run Kepler for 20 years we might see systems more like ours.
That is AWESOME. The fact that there is so much diversity of planets and solar systems out there flabbergasts me. I don't even care about what kind or whether there's life out there -- just thinking about how there are so many different worlds with prevailing conditions that I never even dreamed of gives me the chills.
Space... space... so much space... gotta see it all...
Does anyone know how far away we are from being able to confirm the presence of planets as we might on a photograph? Or the obstacles we face getting to a high enough resolution?
When I read "nature prefers" and the author admits the data's sample size is less than a drop in an ocean (we found < 1000 solar systems), I just have to stop reading.
Oh Heaven for Betsy. Every time some nitwit looks at new data with results they did not expect, suddenly every thing we have ever known is wrong. Give me a break.
Here's a "lightbulb" for you: A large gas giant near a star is created in the same fashion as a binary star system (a formation that is quite common) with the simple difference that the planet did not get enough material to actually become a star.
