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Kepler Mission
JRehling
post Apr 5 2012, 07:28 PM
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An extension is really great news.

As I get deeper into the weeds of the data analysis, I understand what that means. The Signal to Noise Ratio improves as a function of the square root of the number of observed transits. So, for a small body with an orbital period of less than 1 year, this means that the SNR that might be accomplished for a body of radius r with a 3-year mission, the same SNR will be achieved with an 8-year mission for a body of radius 0.6 r. In circumstances where the threshold might have been a Venus-sized object, we will see a Mars-sized object.

For larger bodies in more distant orbits, this will push the outer threshold of large planet detections by a factor of about 1.5... actual detections will depend on luck.

The outer threshold of earth-sized bodies will also move outwards... again, actual detections will depend upon luck and the actual frequency of such worlds, but this allows the detection of earth-sized bodies orbiting sun-like stars beyond 1.5 AU, which may be an important part of the range where earth-like surface temperatures occur. I suspect the bottleneck on detections here will be the frequency of such worlds and the unfortunate but unavoidable geometric bias against favorable alignments for worlds farther out.

As a larger comment, the issue of noise is a very thorny one. Top reasons include:

1) The time granularity of observations is 29 minutes, so measuring a transit duration is inherently noisy.
2) The reported stellar parameters are not only inaccurate on an individual basis but almost certainly systematically skewed, but it is not clear how to correct these errors. See Plavchan et all: http://arxiv.org/abs/1203.1887

In the work for my SpaceDaily piece, I took a very holistic approach to all sources of noise. In contrast, Catanzarite and Shao broke the noise down by the key factors, but because there is unconstrained uncertainty other than variations in stellar brightness, their approach also leads to some inconsistencies in analyzing the full data set. (They limit their analysis to data from a region of candidate parameter space where noise is inconsequential.)

This is to say that even the best work on the Kepler data will not give us a crystal-clear measure of what types of worlds we're seeing: The error bars remain large. It will be an ongoing effort to see if we can correct for the systematic biases and end up with a good statistical encapsulation of planetary types.
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marsophile
post May 14 2012, 07:28 PM
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http://swri.org/9what/releases/2012/unseen-planet.htm

Gravitational perturbation method that utilizes Kepler transit variations to detect additional planets.
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Fran Ontanaya
post Jun 13 2012, 06:44 PM
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Small Planets Don't Need 'Heavy Metal' Stars to Form

"PASADENA, Calif. - The formation of small worlds like Earth previously was thought to occur mostly around stars rich in heavy elements such as iron and silicon. However, new ground-based observations, combined with data collected by NASA's Kepler space telescope, show small planets form around stars with a wide range of heavy element content and suggest they may be widespread in our galaxy."

http://www.jpl.nasa.gov/news/news.cfm?release=2012-171


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Paolo
post Jul 24 2012, 11:55 AM
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a reaction wheel appears to be malfunctioning Kepler glitch may lower odds of finding Earth's twin


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rlorenz
post Jul 24 2012, 01:02 PM
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QUOTE (Paolo @ Jul 24 2012, 07:55 AM) *
a reaction wheel appears to be malfunctioning Kepler glitch may lower odds of finding Earth's twin


Well, any anomaly is a concern. But I remember when one of Cassini's reaction wheel showed anomalous torque, and we switched to the backup (in principle leaving us with very little redundancy, although not none as the wheel didnt actually fail)
That was back in 2000, so don't panic, Kepler folks!
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Holder of the Tw...
post Jul 24 2012, 02:39 PM
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Here is the Mission Manager's Report on the incident. It sounds like the problem may be other than mechanical. By that I mean it may not be wear and tear on the wheel, but some other hardware or software problem. The mission can proceed just fine with three working wheels.

UPDATE came out this afternoon. LINK
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JRehling
post Jul 24 2012, 06:42 PM
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Hopefully, the nominal operations can continue much longer. I am planning an analysis of the potential of the long-term discoveries as part of another publication.

Note that Kepler has an extraordinary requirement for stable pointing. We've seen useful imaging of the outer solar system from missions with degraded pointing: You get a blurrier image, and that's worse than a sharp image, but often useful. Kepler data could be degraded tremendously with less reliable pointing. In fact, the pointing has varied over the mission duration already, with the mission team figuring out ways to improve this from the first couple of quarters by Q4 or so (IIRC).

The problem is that an individual star's signal is what you want to be tracking, hourly. If the light from the star is known to fall on a certain pixel, then this is straightforward. Inevitably, some stars' light will fall across 2 or more pixels. If that varies over time, then working backwards to get that star's signal could be hard or impossible, and even if you do reconstruct the signal, there's more noise, which could make smaller planets' signal basically disappear.

Hoping for the best on this...
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Mongo
post Aug 16 2012, 01:21 AM
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One Of Our Planets Is Missing

Possible Disintegrating Short-Period Super-Mercury Orbiting KIC 12557548

We report here on the discovery of stellar occultations, observed with Kepler, that recur periodically at 15.685 hour intervals, but which vary in depth from a maximum of 1.3% to a minimum that can be less than 0.2%. The star that is apparently being occulted is KIC 12557548, a K dwarf with T_eff = 4400 K and V = 16. Because the eclipse depths are highly variable, they cannot be due solely to transits of a single planet with a fixed size. We discuss but dismiss a scenario involving a binary giant planet whose mutual orbit plane precesses, bringing one of the planets into and out of a grazing transit. We also briefly consider an eclipsing binary, that either orbits KIC 12557548 in a hierarchical triple configuration or is nearby on the sky, but we find such a scenario inadequate to reproduce the observations. We come down in favor of an explanation that involves macroscopic particles escaping the atmosphere of a slowly disintegrating planet not much larger than Mercury. The particles could take the form of micron-sized pyroxene or aluminum oxide dust grains. The planetary surface is hot enough to sublimate and create a high-Z atmosphere; this atmosphere may be loaded with dust via cloud condensation or explosive volcanism. Atmospheric gas escapes the planet via a Parker-type thermal wind, dragging dust grains with it. We infer a mass loss rate from the observations of order 1 M_E/Gyr, with a dust-to-gas ratio possibly of order unity. For our fiducial 0.1 M_E planet, the evaporation timescale may be ~0.2 Gyr. Smaller mass planets are disfavored because they evaporate still more quickly, as are larger mass planets because they have surface gravities too strong to sustain outflows with the requisite mass-loss rates. The occultation profile evinces an ingress-egress asymmetry that could reflect a comet-like dust tail trailing the planet; we present simulations of such a tail.

Evidence for the disintegration of KIC 12557548 b

Context. The Kepler object KIC 12557548 b is peculiar. It exhibits transit-like features every 15.7 hours that vary in depth between 0.2% and 1.2%. Rappaport et al. (2012) explain the observations in terms of a disintegrating, rocky planet that has a trailing cloud of dust created and constantly replenished by thermal surface erosion. The variability of the transit depth is then a consequence of changes in the cloud optical depth. Aims. We aim to validate the disintegrating-planet scenario by modeling the detailed shape of the observed light curve, and thereby constrain the cloud particle properties to better understand the nature of this intriguing object. Methods. We analysed the six publicly-available quarters of raw Kepler data, phase-folded the light curve and fitted it to a model for the trailing dust cloud. Constraints on the particle properties were investigated with a light-scattering code. Results. The light curve exhibits clear signatures of light scattering and absorption by dust, including a brightening in flux just before ingress correlated with the transit depth and explained by forward scattering, and an asymmetry in the transit light curve shape, which is easily reproduced by an exponentially decaying distribution of optically thin dust, with a typical grain size of 0.1 micron. Conclusions. Our quantitative analysis supports the hypothesis that the transit signal of KIC 12557548 b is due to a variable cloud of dust, most likely originating from a disintegrating object.
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Mongo
post Aug 21 2012, 02:25 AM
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More on the above peculiar object:

Modelling the light-curve of KIC012557548: an extrasolar planet with a comet like tail

An object with a very peculiar light-curve was discovered recently using Kepler data from first two quarters. Authors argue that this object may be a transiting disintegrating planet with a comet like dusty tail. The aim of the present paper is to verify the model suggested by the discoverers by the light-curve modelling and put constraints on the geometry of the dust region and various dust properties. We modify the code Shellspec designed for modelling of the interacting binaries to calculate the light-curves of stars with planets with comet like tails. We take into account the Mie absorption and scattering on spherical dust grains of various sizes assuming realistic dust opacities and phase functions and finite radius of the source of light (star). The light-curve is reanalysed using first six quarters of the Kepler data. We prove that the peculiar light-curve of this objects is in agreement with the idea of a planet with a comet like tail. Light-curve has a prominent pre-transit brightening and a less prominent brightening after the transit. Both are caused by the forward scattering and are a strong function of the particle size. Dust density in the tail is a steep decreasing function of angle (distance) from the planet which indicates significant dust destruction along the tail caused by the star. The particle size of the grains in the tail is about 0.1 micron but can be slightly larger if data with the shorter exposure (short cadence) were available. The orbital period of the planet was slightly improved. This light-curve with pre-transit brightening is analogous to the light-curve of $\epsilon$ Aur with mid-eclipse brightening and forward scattering plays significant role in both of them.
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Ondaweb
post Oct 18 2012, 03:52 PM
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At the daily press conferences put on by the Division of Planetary Sciences over the past week there was a discussion of results from Kepler that confused me as well as many others. The recording can be found here http://www.ustream.tv/recorded/26175189.
It is the first briefing of the day.
In it, Julia Frank describes how using Kepler data she shows that the range of inclinations of detected planets is "small" so that planetary systems look like something between "pancakes" and "crepes". Later, in the Q and A part someone asks how inclination angles are determined, i.e., compared to what. Julia says an arbitrary plane. The questioner persists, pointing out that since Kepler can only see transiting planets, those in orbits whose angle of inclination is greater that the diameter of the star won't be seen in the first place. This is verified here.. Further, I do not see how it's possible for a transit time to translate into an angle of inclination (when you can't see the path of the transit across the disc of the star since Kepler can't resolve the stellar disc).

Answers provided by Julia and others did not aid my understanding (or the questioners and others in the audience). So I'm posting this here in the hopes that someone could direct me to a reference that might explain how the reported results were obtained.

Thanks
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Hungry4info
post Oct 18 2012, 04:00 PM
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The part of the conference you mention confused me a bit too, but I later realised that the speaker may have misunderstood the question. I think the audience member was asking about the inclination of the orbit of a planet, and the speaker thought he was talking about the inclination of the orbits of two planets to each other.

Inclination of an exoplanet's orbit is defined relative to the plane of the sky. 0 degrees (or 180 degrees) is a face-on orbit. 90 degrees is an edge-on, transiting orbit. 89 degrees may still transit the star if the star is large enough or the planet orbit is close enough. But because it's not exactly 89 degrees, the transit will not be a central transit (the impact parameter will be > 0), and thus the planet will spend less time on the stellar disc.

These two planets have the same semi-major axis but different inclinations (and thus different impact parameters)
image

If you know the orbital period, and you have a value for the stellar mass, you can get its semi-major axis. Based on this, you can get the orbital velocity. With an estimate of the stellar radius, you can predict how long a transit should last depending on what impact parameter the planet's orbit has (or where it transits on the stellar disc), where the impact parameter is calculated with b = a cos i.

Edit: Wording.

Edit2: As for the coplanarity results, they just simulated various planetary system architectures and matched it to what they see in Kepler data, taking into account the observational biases.
http://arxiv.org/abs/1207.5250


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Ondaweb
post Oct 18 2012, 05:34 PM
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First, thanks a lot for your detailed response.
I think I get most of what you are saying. I can see how, by observing the star, you can get it's temperature/spectral class. How you can plot that on a H_R diagram and get an estimate of the mass (though I admit I was surprised to discover that such an estimate is considered accurate enough to make the subsequent calculations. I always saw the main sequence as a broad band.) Then you can use the mass and period to get the semi-major axis. Per what you say, I can see know how it's possible to use that same mass estimate to also estimate volume and hence diameter. Then the estimated diameter can be combined with the observed transit time to calculate the angle of inclination of the planet from a line/plane from Earth to the center of of the star.

As you note, however, any such angle is going to be very small; i.e., an orbital plane very close to 90 degrees.

All that said however, since the only planets that can be detected in a system are those in that narrow range of transiting orbital planes and since planets, at least in theory (e.g., Pluto, if it were still a planet) can have orbits highly inclined to other orbital planes, I don't see how Kepler data can lead to the conclusion Frank describes (i.e., that planetary systems are like pancakes [rather than, say, bagels]).
Put as the questioner put it, Kepler can only detect planets that transit (and hence must be in one highly constrained planar orientation) so how can anything be said about undetected planets in possible non-transiting planes? As you may recall, the more senior responder basically described what you illustrated by your diagram below.
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Hungry4info
post Oct 18 2012, 05:57 PM
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If a planetary system is perfectly coplanar and there's a small inclination away from 90 degrees, then each transiting planet out from the star will have an increasingly large impact parameter. If planetary systems are well-aligned, then you would expect that, in general, the impact parameters of outer planets are greater than for inner planets. Otherwise, you need orbit planes to be inclined relative to each other.

If, amongst the transiting multi-planet systems, you observe a typical distribution of the difference in inclination between two planets, then it's reasonable to assume that this distribution applies to non-transiting planets as well.


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JRehling
post Oct 18 2012, 11:00 PM
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Hungry4info's answer is right on the money on the technical details. (Not sure about the various speakers' intentions; I didn't hear that audio.)

I'll add a couple of notes:
1) Kepler's time resolution is actually pretty coarse as far as timing transit durations, so a single measurement has considerable error. Given many transits, however, the accuracy increases.

2) BUT, any Kepler results are confined to inner systems, which includes planets whose orbits are tidally influenced by the star's rotation. This need not apply to planets further out.

3) There are also major discrepancies seen in the estimated stellar properties and observed transit durations. Peter Plavchan and his colleagues have been researching this. All told, the stellar properties estimated contain errors, and likely contain some systematic errors. As a simple demonstration of this: If you estimate that a planet should transit the star for a maximum of 2 hours and its observed to transit for 3 hours, then your estimate of the star's radius is probably too low (or your estimate of the star's mass is too high, or both). And there are such cases.
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Ondaweb
post Oct 19 2012, 05:34 PM
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QUOTE (Hungry4info @ Oct 18 2012, 12:57 PM) *
If, amongst the transiting multi-planet systems, you observe a typical distribution of the difference in inclination between two planets, then it's reasonable to assume that this distribution applies to non-transiting planets as well.


It's that "assume" in your quote above that defines my problem. If you assume that all the planets in a multi-planet system are in the same plane, then the math will prove your assumption true. But the interest in the "pancake" result stems, I believe, from the fact that this is yet to be determined. We only have one system in which we know the planes of all the orbiting planets and they are pretty much co-planar. But to know that this is also a fact for other systems goes a long way towards verifying our planetary formation models. Thus proving it is so is quite important.) Further, just as indicated by the post above, some of our assumptions about stellar mass (as a function of spectral type) are quite clearly incorrect, so too must we be cautious about any assumptions of orbital inclinations.

So in my mind, I'm still unclear how it is possible to say anything about the orbital inclinations of UNSEEN planets based on only those observed transiting stars.
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