@Roland: Huhn. I hadn’t realized that the probability of seeing a close-in planet was as high as 10%. Do you have a reference for that? And your point about Kepler seeing the Sun as planet-less—very interesting.

They are aware of these limitations and that’s why there’s a distinction between confirmed planets and candidates. Also, if they took the time to manually analyze all candidates we’d still be waiting for the results. This way other people (like the Discover fact-checker or other astronomers) can comb the data for this kind of mistakes. And they could’ve singled out this one and explained it in the paper — but as many people know there’s always a trade-off involved between space limits in a publication and the detail of the analysis you publish.

Even if the Kepler team was nowhere near ready to present their results as authoritative and final at all and only spoke of potential candidates, if any individual planet was elevated by the press the way KOI 326.01 was, should this have not caused them to go back on their own accord and recheck the numbers for that one in particular just in case? It’s fine to argue that they don’t have the resources to validate all their candidates at such an early stage, but they did have the ability to do it for one candidate, didn’t they?

All this is just my gut reaction as a complete layman and grassroots space nut, but it’s the first this that sprung to my mine and made me wonder.

Then to build upon @14. Bruce, we are also missing every planet that has an orbital period > 2 months. That will get better with additional study time of course. How many planets in our solar system have a year that is just 2 months or less?

Kepler is just skimming the surface, grabbing the ‘easy’ detections!

What I learned from the astronomy units of my degree was just how incredibly well astronomers use every last scrap of data made available to them – I recall a (what seemed to me) rather blurred photo of iapetus and then was shown just how much good science was derived from that single photo. When each dataset costs so many thousand dollars, you really have to get everything you can from it, and get the best minds on the job!

I’m not actually sure if Kepler’s field is indeed big enough for the kind of calculations you propose. It’s actually only looking at a relatively small patch of the sky.

One would expect more comprehensive sky surveys to be on the table once sufficient results come back from Kepler, of course.

The reason it was only a candidate still, is they didn’t have time to do all the follow up observations and analysis to confirm that it was truly a planet, Earth sized, in the habitable zone.

It is unfortunate that one of the more promising candidates turned out to be something other than what was initially thought but that’s part of the investigation.

You are correct. Kepler recognizes this and states that the odds of seeing a planet exactly like ours (size and distance from its sun) would be about 0.5% if the planet did exist in the first place.

So, yes, Kepler will see a small percentage of the planets out there. They account for this when taking the survey of 150k+ stars.

The scientists running the Kepler mission realize the detection method is indeed limited for exactly the reason you mentioned. They can indeed only detect planets that cross the visual path between earth and their star. For this reason, it is very easy to use this method to detect a large number of planents orbiting close to their star (a much larger percentage can be detected because the orbital angles can be more varied while still crossing the star’s field of view. However, stars which are much further out (say the equivilant of Neptune) have a very low probability of crossing their star’s visual pathway.

The detection method is also limited by time frame. Only 6 months have passed in the Kepler mission and the planet must pass between us and the star 3 times. Once for the initial dim, once to verify the period of the planet, and once to verify the frequency. This means that only planets with orbital periods of less than 2 months have been detected!

While the Kepler scientists are looking for planets, they’re more interested in the statistics. If 1% of planets can be dected using this method, are they seeing .5% of all stars with planets or 2% or 7%… this will give us an idea of the average number of planets within any given extra-solar system which is the intent of the entire mission.

If we happen to find some potentially habitable planets along the way – awesome!

yes, calculation of the probability: (d* / a). 100 (%)

d = radius of a star
a = Semi-major axis

for the parameters of the Earth ~ 0.5°%

Of course, this makes the assumption that Kepler is surveying a vast amount of sky – but the science could be done, if not by Kepler – and Kepler’s results could be recalculated in light of those results.

@Jon That’s a great question, and thanks to the other commenters for addressing it. Kepler only sees planets whose orbits take them directly between the star and the telescope. Astronomers understand that Kepler is missing out on many planets, and they factor that into their planet frequency calculations.

By the way, this limitation is one of the things that makes the recently announced Kepler 11 system so extraordinary. Consider that the six planets in the Kepler 11 system are so perfectly aligned that they all cross their star in transits visible to Kepler. The eight planets in our solar system orbit along a fairly flat plane, but not flat enough that Kepler would be able to see all of them from afar.

You can calculate how frequent is having the orbital plane aligned with Earth. You then divide the number of solar systems with planet detections by Kepler by the odds of it having the plane aligned with Earth to get the total number of systems with planets detectable by Kepler.

You may want to consider other factors (the farthest the planet is from the star, the lower the chance to observe a transit, not all planets orbit at the same plane, many stars are part of multi-star systems) to get better numbers, but I’ll leave that to the real scientists in the audience.

Not finding a result does not preclude it from having planets orbiting it. But from a purely statistical standpoint, when watching more then 156,000 stars, you can statistically estimate how many WOULD be parallel to the star. Take the size of each star (or the average size as would suffice statistically), the distance of potential planet from said star, and you get a very specific probability of planets being in that plane. Then multiply that 156,000 stars by said percentage and you have the number of stars you SHOULD see any planets at, if this matches (within error) to the number of tracked stars that have candidates then you know all (or at least most) of the stars have planets at all, if the candidates are less then expected then you can statistically estimate how many stars have no planets at all. Then, from ratio of types of planets in candidate stars you can estimate how many planets are around all (even non-parallel) stars.

I’ll bet it’s real fun trying to figure out how random the distribution of orbital planes is, though. How strongly does the galactic plane correlate with individual solar planes? Are there events or objects that perturb them?

Probably some of the questions astronomers are now combing through Kepler’s data to answer.

That said, I believe move star systems have a plane oriented roughly paralell to the galactic plane.

“Kepler finds planets by detecting tiny dips in a star’s brightness during transits when a planet crosses in front of it.”

So I get how that could find planets. But I don’t understand how not finding a result from a star would preclude it from having planets orbiting it. It seems to me that the Kepler method is making a large assumption that our view from Earth is parallel to the plane of each star’s potential system. In other words, what’s to say that we’re not looking top-down on a bunch of star systems… The planets are there, but never pass between us and the star. (I realize this wouldn’t really impact the statistics of what percentage of planets are Earth-like, but wouldn’t it affect the estimates of how many planets there are period?)

Am I way out to lunch?