Ustrax, I think you are confusing disappointment with the questions following the briefing for disappointment with the briefing itself. One can be excited about the briefing and then disappointed in the questions following it. In fact, the more exciting the news, the more disappointing the lack of good questions will be.

(I didn't see the briefing myself.)

As was noted in the reply above, you're confused by my definition of Q&A.

I was disappointed by the questions asked during the Q&A session at the end of the press conference. It seemed like the people asking the questions didn't bother to learn anything about Kepler before the briefing.

"How far away is Kepler?" You have got to be kidding me.

Sorry for the misunderstanding Syrinx...and thank you centsworth_II...I was taking it too serious...I have to deal with this another way...hey! I can always anagram it!

Gah! I missed the press conference because I was in the middle of negotiating a fair valuation for my house regarding property taxes!

Anyway, most of it is now on YouTube, if you missed it too -- here's the link to part 1, with two more parts posted by the same user.


http://www.youtube.com/watch?v=iWvpQ5Jwyfg

The space-multimedia archive site will probably have the full video at some point in the next day or two.

Anyway, this is fantastic news! I guess after Hubble's trials, there's always that period of worry and second guessing before you get the first block of science data and the instrument finally checks out okay.

Kepler's HAT-P-7 lightcurve is the subject of a paper in Science this week http://www.sciencemag.org/cgi/content/abstract/325/5941/709

The whole thing is on the official NASA TV youtube channel http://www.youtube.com/watch?v=qRN7fNkZ-IQ

I'm extremely excited. But there's no way to detect the composition of the atmosphere, right?

Depends on the planet. If the star is sufficiently bright then you might get a high enough SNR to say something about the atmosphere (i.e. like HD 209458 b and HD 189733 b, but these planets are exceptional cases). It won't be done with Kepler though, all it has is a photometer.

That came up in the press conference, and the answer is that Kepler can't do that, but some of the future missions being proposed would be able to.

By the way, in case anyone missed it, this is what a Jovian planet looks like when you have four transits to work with. Had this been Earth and the Sun, the transit (the big drop) would have been about 2/3 the depth of the occultation (the little drop). So, yes, Earth-like planets should be detectable, but it's going to be close.

Something else interesting from the conference and some of the links was that the light curves of the variable stars are often unlike anything in the literature. Apparently the atmospheric noise has been hiding significant behavior. This complicates finding planets, of course, since their models for spotting variable stars have to be reworked.

--Greg

ustrax's couple writeups were extremely interesting.

I thought it was cool that the warm Spitzer was mentioned as an observatory likely to follow up on new discoveries. I love it when old missions come in handy.

I have some questions, if anyone can answer them.

1. Kepler's mission is only 3.5 years, barely enough time to confirm it's own initial discoveries. It sounds likely that the mission can be extended, so what is the expected longevity of the mission (assuming funding is not the problem)?

2. From the mission website: "Expected Results:

From transits of terrestrial planets in one year orbits:

About 50 planets if most are the same size as Earth (R~1.0 Re) and none larger,
About 185 planets if most have a size of R~1.3 Re,
About 640 planets if most have a size of R~2.2 Re,
About 12% with two or more planets per system. "

-- would these expected numbers scale linearly with mission extensions (i.e. would another 4 years of observing double these numbers)? Or is Kepler's field of view fixed to one region, so the sample set is difficult to change, and if this is the case, would most new planets found be longer orbital periods?

3. "Stellar evolution models are used to estimate the mass, radius and metalicity of the parent star"
-- how reliable are these models? Is there any way to directly determine these values, or does it require an instrument like the Terrestrial Planet Finder?

4. For gas giants found within a habital zone of the star, would it be possible to search for large moons with either Kepler or astrometry of the gas giant using ground-based instruments?

Four years or so I think. Fuel is the problem. Kepler's gyroscopes must be de-saturated every so often. Eventually the fuel will run out, the gyroscopes will become saturated, and the entire spacecraft will lose its ability to point.

Yes, it's fixed. As such, the useful data return will decline with age.

I would imagine very reliable.

We had that discussion earlier in the thread I think. If I remember, yes it's possible but very difficult.

That is not a free article. Anyone with access care to summarize (not plagiarize) the new and interesting parts, if any?

It's mostly the same things reported in the press conference yesterday. There is a good summary in Sky & Telescope's website
http://www.skyandtelescope.com/community/s...g/52657352.html

Kepler is using a defocused star image, right? Is it a defocused mirror? What if instead of a defocused mirror, they put in a lens with severe chromatic abberation. They'll still get defocused images, since the colors would be spread around the whole star, but a circular integral around the star could yield some useful information. Dumb idea, or should I call a patent lawyer?

IIRC, Kepler has a spherical mirror, which I think is much easier to make than parabolic mirrors needed to fix the spherical aberration. I guess there is not much point is going through the extra work of making it parabolic, just to defocus it with a secondary lens.

Metalicity is measured directly spectroscopically. Mass can be measured directly if the star is binary (or tertiaty...). Of course, for exoplanets the main focus of interest is on single stars (as least partly due to the old discussion about possibility of habitable planets orbiting binary stars) and this is one of the reasons to use stellar evolution models.

There was some discussion of exoplanet moons with help from the transit timing method in this post (#158):

http://www.unmannedspaceflight.com/index.p...mp;#entry139639

And I think I read of the possibility of "amateur" transit timings even contributing to the search. Was this in Sky and Telescope or somewhere? There are some websites mentioning this as well.

We can get masses for members of binary stars. The standard relations between mass and luminosity come from members of widely separated binaries, where the stars are too small compared to the orbits to have affected one another's evolution (yet). Here is a typical set of mass-luminosity and mass-radius relations for main-sequence stars. Eyeballing that scatter, it looks like 20% in luminosity if mass is known or 10% in mass if luminosity is known (since it's a steep function). Radius looks a bit worse; that often has to come from blackbody laws and the effective temperature and luminosity; which come from spectroscopy and from photometry plus parallax. That can be improved for stars not too distant; I saw a result from the CHARA interferometer in which they resolved the disk of one of the stars with a transiting planet, reducing its uncertainty in radius. (They are a long way from getting an interferometric signal from the dark planetary disk, alas).

As has been posted already, we get metallicity from spectroscopy, calibrated to the Sun. (Kind of odd that the best-fitting spectroscopic oxygen abundance there is not the best-fitting one for helioseismology. A lot of other things may shift a bit when that gets sorted out).

I'm quite curious to learn more about the light curves of some of the other variable stars, not necessarily curves which look like they may be planet transits. It sounds like they may have unique and exciting data. I wonder where and when these other data will be reported.
Wouldn't it be something if it turns out that Kepler's Earth sized planets are a minor part of what its data yields?
steve

I have no doubt that that the majority of stars with photometric variability will be intrinsically variable.
But I would sure like to be wrong!

The dataset all comes down together, so the timing of the non-exoplanet science results depends entirely upon the priority the science team gives it, and how many people they have working on it. It's probably not the highest priority, but if something highly unexpected falls out of the data, I suspect they will need to characterize fairly quickly it so they can rule out the possibility that it's caused by some type of exoplanet.

A non-exoplanet discovery would have to be really something to trump the discovery of habitable-zone Earth-twin planets. Unless it was something to do with solving current cosmological head-scratchers, like dark matter or dark energy, then I don't really know what would qualify as a show-stopper.

Discovering a Dyson's Sphere, perhaps....

Either that or a full sized Culture habitat, or a huge Romulan mining ship firing "Red Matter" bombs..?

How do you discover a Dyson's sphere using photometry method? I thought you need infrared imaging for that

Periodic flashes of starlight through the windows...

...or through holes punched in the outer shell by meteoroid impacts..?

They'd have to be some fairly large holes to be detected by Kepler.
But if you build a Dyson sphere, and you still have asteroids in your solar system, you're just begging for it to get hit

A couple of questions. Given how nicely the first results from Kepler have turned out, should it be possible for Kepler to:

a] detect almost-but-not-quite transiting hot Jupiters from the rising and falling phase-induced light curves (i.e. like HAT-P-7b only without the transits).

b] detect the presence of other, non-transiting planets from variations in the timing of the transits of planets they can see?

Yes. The inclination will still be unknown, but constrained as a function of the light curve variation, and models of the planet's reflectivity. I would expect such things to not really be noticed though. I don't know how the Kepler software works. If it's anything like some others, transit-like dips in brightness get flagged as planet candidates, and then examined more closely. You might imagine that a planet detection like one you describe would easily be assumed to be intrinsic stellar variability. (unless it holds up for a very long time, which would indeed be the case if it's a planet. In that event, it all depends on if it's noticed or not. Then the next obstacle is whether or not to devote resources to follow-up on it).

Yes. What really helps is that the planets that Kepler detects will have many, many transits measured, and in high-detail for the bright stars. A long baseline is crucial in detecting transit timing variations. Kepler is uniquely situated to do this, more so than CoRoT due to a longer observation time in a fixed point in the sky.

Yes, Kepler will detect non-transiting, close-in Giant planets.

"The Kepler Mission readily records the modulation of the light reflected by about 870 close-in giant planets as their phases change between superior and inferior conjunction."

http://kepler.nasa.gov/sci/basis/giants.html

In this case, "close-in" means a period of a week or less.

--Greg

But only if they're close in. For planets in Earth-sized orbits, they'd only have 3 or 4 transits during the course of the primary mission. (Or would 3 - 4 transits be enough to detect variations in timing due to other planets?)

Four transits is probably enough to determine that there ARE other planets, but I'd be very surprised if you could usefully extrapolate much of anything about them. Too many variables and not enough data points.

Thanks for the link back to moons... saved me the trouble of searching. There's something that's I've been thinking about regarding moon detection with Kepler data.

Most investigators seem to be focusing on the transit-timing method for detecting moons: the slight advance or delay of transit ingress and egress due to motion of the planet around the planet-moon barycenter. This timing variance will be slight, but within the edge of detectability. I also saw one reference to the possibility of slight additional dimming before or after the main transit due to the physical body of a large moon. For this to be possible, the moon will have to be very large indeed, approaching Earth diameter: much larger than Ganymede, but within the realm of conceivability.

For a moon this size, a possible third method of detection occurs to me. The Hill spheres of hot Jupiters are very small, so that even a moon in a high-inclination orbit is likely to transit and be occulted by its planet as seen from Earth (or Kepler). Also, it is likely to have a very short period even relative to the abbreviated "year" of the hot Jupiter, probably on the scale of hours. Won't we therefore see a signal as the moon passes in front of and behind the planet during transits? This signal will be much fainter than the main transit signal, of course, but shouldn't it be possible to tease it out? Over the course of time, as the number of observed planet-star transits increases, the observed moon-planet transits will increase at some multiple, enabling characterization of both the moon and the planet. Especially coupled with the barycenter/timing method mentioned above, this could tightly constrain masses and hence densities of exoplanets. Assuming, of course, that such moons exist.

I'm sure someone must be working on this; has anyone more familiar with the literature or the community seen or heard a mention?

Won't the transiting moon always be in eclipse? Do you mean you might see signal from a moon when it is not transiting its planet? (And therefore is neither in front or behind the planet and is transiting the star?)

Yes, Steve, that's what I mean. I suppose it depends on what you mean by "signal," but the indication of the presence of a moon would be the magnitude difference between the star transited by both planet and moon and the the star transited only by the planet, because the moon is transiting the planet or being occulted by it. In my quick-and-dirty illustration (not to scale), on the left the star is transited by both star and moon, but on the right, the moon has moved in front of the planet as it tranits, resulting in slightly less occultation of the star. Is that clearer?

Click to view attachment

Yup. Makes sense. And I of course won't pretend to know whether it's detectable, but part of me thinks that if they see the temporal shifts between planet transits, they'll be on the lookout for amplitude wobbles as well.

Also, though, as the planet/moon gets farther from the star, you might just get two or three separate transits.... what would Jupiter+Callisto or Saturn+Titan look like 200 light years away? Do they resolve as separate transits?

I'm not sure what you mean by "resolvable," Steve. It's not like we're able to resolve a star disk image with a transit visible as a blotted-out circle within it. But I would imagine that the "resolution" in this regard would be the same as Kepler's overall ability to detect a planetary transit. A moon would have to be the minimum size necessary for Kepler to detect it, all by itself, as a transit event. So if Kepler can't detect the dimming of a star's light caused by the transit of a Callisto-sized planet, it ought not be able to detect the additional dimming that would occur with a Callisto-sized moon as it would appear in the first frame of 'squid's excellent illustration. And also, therefore, ought not be able to tell the difference between the first and second frames.

As I understand it, Kepler can detect down to about an Earth-sized planet, correct? Then I would have to think that the smallest gas giant moon it might detect would have to be at least as large as the Earth.

Another very interesting thing, though -- any planetary body with a ring system will block more or less of a star's light depending on the angle the ring plane presents to the viewer. I can well imagine that some percentage of the planets Kepler will discover may indeed have ring systems, and that these ring systems may not always present the same angle to us here on Earth during every single transit. It will be very, very interesting to see how fast the investigators suspect they're seeing ring systems in some of their results...

-the other Doug

I think "resolultion" in this case refers to time. According to the mission website, Kepler samples a star's brightness every fifteen minutes. I don't know how long a typical Kepler transit will be but in the preliminary list of COROT candidates the transits ranged from an hour up to sixteen hours, with perhaps three hours being the norm. So if we assume a transit takes three hours, we have only about a dozen samples per transit. That is a pretty small sample size from which to try and weed out moon data, and it is a reasonable question whether it will be possible. It seems to me that, if we have a particularly long, slow transit (say 8 hours), an unusually large moon (approaching Earth sized or at least larger than Mars), and a bit of luck, it will be possible, but won't be obvious in the raw data. However, over time Kepler will obviously accumulate observations of multiple transits for each planet detected by Kepler, so that with rigorous analysis it might be possible even with less extreme examples. This is more believable after seeing how clean and noise-free the data were at the Aug. 6 news conference.

As for a ring: Doug, I hadn't thought of it, but it will obviously wreak havoc with density assumptions at first, as first contact of the rings will be difficult to differentiate from the planet itself, giving a grossly inflated diameter estimate. I imagine the difference will become apparent over time, as more samples are added to the data set, but in the meantime someone will publish a paper they'll have to retract.

Hmmh - it seems to me then that a number of the extremely low density exoplanets already discovered might actually have ring systems, it's just that we have (as yet) no mechanism (other than a very low apparent density) for making that case. I'm specifically thinking about such oddities as TrES-4 (1.67 Jupiter radii but 1/6th the density) although from the discussions I've seen so far the smart money on all of these appears to be on tidal heating.

Although this does also get me thinking that surely it would be nearly impossible for any large moon ( certainly anything earth sized ) or significant ring system to survive around any of the hot Jupiter class of planets? Tidal forces would be immense and since the numbers indicate these are probably enough to inflate the Hot Jupiters by 10-20% in any case then surely they would be more than enough to rip any significant moon apart and lead to environments far to chaotic for rings to form? Or is there a possibility for a class of "Trojan\Greek" style co-orbital dust clouds rather than rings that might be more stable in such an aggressive environment?

Yes, they would be able to detect the transit of a sufficiently large moon separate from the planet, regardless of if the two are transiting simultaneously, just so long as the planet doesn't eclipse or occult the moon at the time of transit (in which case, surely, it won't in at least some other transits).



Normally, yes, but with the transit light curve of a planet being scrutinsed, that photometric data gets much more attention. Photometric data containing evidence for a planet (one of the later OGLE planets) went un-noticed for quite some time. Kepler and others like it gather a lot of data, which isn't too easy to sift through easily and detect very minute transits. I hope they intensely scrutinize Kepler photometry around transits of medium and long-period planets in search of moons. Though I don't know what you could really say about them (other than their radius, with a significant error) without extensive transit-timing observations (Hey, Kepler might give those too).

Did you see the depth of the HAT-P-7b secondary transit in the raw data during the press release? It's apparently the same depth as an Earth-radius planet in transit. It shouldn't be much of a stretch to imagine a transit depth half of that (not half the radius of the planet, of course, but a planet whose disk has half the 'area').



This paper,

Transit Detectability of Ring Systems Around Extrasolar Giant Planets
http://arxiv.org/abs/astro-ph/0409506

discusses the transits of ringed planets, shows example light curves, and describes how scattering may allow for one to determine the size of particles in the rings.

Earths in both size and orbital period, meaning only three transits over the Kepler mission. With more transits, you can pull smaller planets out of the noise. The Kepler site has some information http://kepler.nasa.gov/sci/basis/sizes.html

So that I understand, are you asking about detecting various Trojan objects sharing the orbit of the 'Hot Jupiter' around the star ? Does anyone recall the maximum mass a Trojan object can have (seems like I recall mass ratios affecting Trojan stability) and does that overlap with what Kepler might be able to detect ?


Fascinating contemplating what might (and might not) turn up in this missions results.

In the usual formulation, the medium body must be at least 25x smaller than the large one, and the smaller body must have "negligible" mass. I'd be surprised if even a moon-sized object were stable even with a 10x Jupiter, but someone would probably have to simulate it numerically to know for sure.

--Greg

Well, I've read that for a distant observer, a Jupiter transit would have a duration of 30 hours, a Saturn transit about 40 hours...
Exo-moons could be detected with more accurate detectors, , a Planet-Moon system would have a characteristic transit timing variation, for instance a Jupiter-Europa system would have a variation in the order of 10 seconds, a Saturn-Titan system in the order of 30 seconds...
Ring system might be easier to detect

By "transit timing variation," are you referring to delays or advances in the transit time due to the planet's motion around a planet-moon barycenter? This is the method described in the discussion earlier in the thread. It would seem to me that, in order for such a system to be detectable with Kepler, the moon would need to be more massive, relative to the planet. We've found larger planets than Jupiter; it's only reasonable to assume that larger moons than Ganymede also exist. And such larger moons--if they indeed exist--could be detectable by their own transits across the star as well, I should think, especially if their presence was already suspected from the timing data.

Helvick's concern about the stability of moon orbits for hot Jupiters due to tides is notable, but again, if we tweak our hypothetical we might avoid it. Wouldn't there be more available stable orbits for a large moon of a planet in a 24-day orbit than of one in a 3-day orbit? As a bonus, the 24-day planet will have lower orbital speed and thus a longer transit, giving a longer sample of planet-moon interaction.

I haven't heard anyone talk about work on direct detection of moons transiting their planets. Any graduate students looking for a thesis idea?

One other possible source of noise occurs to me; it's also an opportunity for stellar studies, I suppose. If a planet crosses a large starspot, the variation in total light will be similar to its eclipsing a large moon. There are of course differences; the region surrounding a starspot is usually brighter than the average star surface, no? Has such an event been modeled? What would the resulting light curve look like?

Another possibility is stellar flares during a transit, which in fact have been detected:

A Stellar Flare during the Transit of the Extrasolar Planet OGLE-TR-10b



Bill

From reading the Kepler site, I think several of these questions have easy answers. The short summary is that it'll probably find Earth-sized moons in Europa-sized (or bigger) orbits but only if the primary planet has a one-year period AND if the moon orbits in the same plane as the planet (so that they both transit the star).

If the satellite is Earth-sized, then when it transits the star, it should show the same light-curve effect as any Earth-sized planet. Depending on how large the orbit is, that transit may or may not overlap the transit of its primary. If the big planet has a 1-year period, then Kepler should detect the satellite in much the same way as it detects any Earth-sized planet. If it has a longer period, then Kepler can't do it. If the satellite is only Ganymede-sized, then the primary would need to be much closer to the star, raising questions about whether satellites have stable orbits -- other than orbits SO close that Kepler can't see them.

Another point no one has mentioned is that this only works if the orbital plane of the satellite is close to the plane of the primary's orbit. Otherwise we'll miss the transit in all probability. That makes Jupiter a better bet than Saturn (other than the need for a 36-year mission, of course.)

Observing moons transiting planets (but not transiting stars) is clearly beyond Kepler's capabilities, since Kepler is barely able to measure the effect of reflected light from a giant planet in a one-week orbit. The light-change from full to crescent is a LOT bigger than the smaller change when a moon transits.

--Greg

Just pondering other possible 'yon wee beasties' Kepler may or may not detect, has anyone considered a light curve for a binary 'hot Jupiter' orbiting a star?

I'm not sure how stable a double planet might be in this regard, but if the twins orbited their barycenter in a multiple of their period around the host star maybe we might have a stable situation ?

No one can say for 100% certain that its impossible, but none have been identified yet. It's quite possible that some of the non-transiting Jovian planets known today are actually double planets, we wouldn't know with just radial velocity alone.

If a double planet transited, it would be pretty much the same dynamics as a planet + moon transit, but the transits would be more similar to each other.

This is the three-body problem again, of course. Someone would probably need to do some heavy-duty numerical simulations to find out whether any such combination is "stable" in the sense of "neither body gets thrown into a different orbit nor do they collide." My guess is that they'll eventually collide unless they are very far from their star.

If they orbit far enough apart, Kepler certainly out to see a two-step phase curve -- even if the pair really were far from the star and Kepler only got to witness a single transit. Of course, a single transit wouldn't eliminate the possibility that two planets in different orbits just happened to transit at the same time. Still, in a few years, we'll have a much better idea whether such things do exist in any numbers.

--Greg