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On Kepler’s 1284 New Planets

If you look into the software that made possible yesterday’s exoplanet results, you’ll find that VESPA (Validation of Exoplanet Signals using a Probabilistic Algorithm) is freely available online. The work of Princeton’s Timothy Morton, who spoke at the announcement news conference, VESPA is all about calculating the probabilities of false positives for signals that look like transiting planets. Transits, of course, are what the Kepler space telescope has been about, catching the slight stellar dimming as a planet crosses across the face of a star.

The numbers quickly get mind-boggling because while Natalie Batalha (NASA Ames), joined by Morton, NASA’s Paul Hertz and Kepler/K2 mission manager Charlie Sobeck (a colleague of Batalha at Ames) could point to 1284 newly confirmed exoplanets, they represent only a fraction of what must be in the Kepler field of view. Out of its over 150,000 stars, Kepler can only see the planets that transit their host stars, making this a problem of orientation. We now have 2300 confirmed exoplanets in the Kepler catalog, but it’s clear that countless stellar systems are simply unviewable because they’re not lined up so as to make the identifying transit.

But back to VESPA and the technique that made yesterday’s announcement possible. The problems of false positives are legion when you’re looking at a light curve suggestive of a planet. For one thing, a brown dwarf or extremely low mass star may pass between Kepler and the star. For that matter, a larger star in a binary system may just ‘graze’ the limb of the host star, sending a planet-like signal that has to be untangled from the true planetary count. With these and other possibilities, we’ve relied upon verification through follow-ups, usually performed through radial velocity checks or even direct imaging of exoplanets.

All of this takes time and is resource-intensive, serious issues given the number of candidates (4700) found since launch. Morton calls the false positive signals ‘imposters,’ describing the VESPA method, which allows researchers to quantify the probability that any candidate signal is in fact a planet without requiring the lengthy follow-up investigations cited above. Two different kinds of simulation come into play, one involving transit signals and their causes, the other simulating how common the ‘imposter’ signals are likely to be in the Milky Way.

Kepler_fig01

Image: Kepler candidate planets (orange) are smaller and orbit fainter stars than transiting planets detected by ground-based observatories (blue). Credit: NASA Ames/W. Stenzel; Princeton University/T. Morton.

As this Princeton news release explains, the duration, depth and shape of a transiting planet signal are thus weighed against simulated planetary and false positive signals even as VESPA factors in the projected distribution and frequency of star types in the galaxy. Says Morton:

“If you have something that passes all those tests, then it’s likely to be a planet. We know small planets are common, so if Kepler sees a small-looking planet candidate and it passes the strict internal vetting, it’s more likely to be a planet than a false positive because it’s hard to mimic that signal with anything else… It’s easier to mimic something the size of Jupiter, and we know Jupiter-sized planets are less common. So the likelihood of a Jupiter-sized candidate actually being a planet that large is typically relatively low.”

VESPA works with information from both kinds of simulation to produce a reliability score between zero and one for each candidate signal. The candidates with a reliability greater than 99 percent are considered validated. The 1284 exoplanets announced yesterday all fit this standard, meeting what we can consider the minimum requirements for validation, while another 1327 candidates are considered likely to be planets although they do not score as high. 707 candidates turn out to be caused by non-planetary phenomena. It’s worth pointing out, too, that 984 candidates were revalidated — these had previously been verified by other methods.

Kepler_fig02

Image: Since Kepler launched in 2009, 21 planets less than twice the size of Earth have been discovered in the habitable zones of their stars. The orange spheres represent the nine newly validated planets announcement on May 10, 2016. The blue disks represent the 12 previous known planets. These planets are plotted relative to the temperature of their star and with respect to the amount of energy received from their star in their orbit in Earth units. The sizes of the exoplanets indicate the sizes relative to one another. The images of Earth, Venus and Mars are placed on this diagram for reference. The light and dark green shaded regions indicate the conservative and optimistic habitable zone. Credit: NASA Ames/N. Batalha and W. Stenzel.

The announcement of 1284 confirmed exoplanets is the largest single announcement of new planets ever made, doubling the number of confirmed planets, with VESPA being used to calculate the reliability values of over 7000 signals from the latest Kepler catalog. This work gains additional weight as we consider the upcoming TESS (Transiting Exoplanet Survey Satellite) mission, which will be performing an all-sky survey of bright, nearby stars that will doubtless reveal tens of thousands of new candidates, all in need of confirmation.

Natalie Batalha recalled early work on transit photometry with a small robotic telescope at Mount Hamilton (CA), where researchers were plagued with false alarms, up to 70 percent of the signals proving to be non-planetary. Clearly, as Kepler became available, our methods increased greatly in accuracy, but as we move toward the final discovery catalog of this mission next year, we’ll be using methods like VESPA to continue sorting through candidate data, looking ahead not just to TESS, due to launch in 2017, but also the European Space Agency’s PLATO (PLAnetary Transits and Oscillations of stars ), designated for launch by 2024.

The paper is Morton et al., “False positive probabilities for all Kepler Objects of Interest: 1284 newly validated planets and 428 likely false positives,” Astrophysical Journal Vol. 822, No. 2 (10 May 2016). Abstract / preprint.

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Comments on this entry are closed.

  • Tulse May 11, 2016, 10:56

    it’s clear that countless stellar systems are simply unviewable because they’re not lined up so as to make the identifying transit

    Is there potentially any bias in the orbital planes of planets around stars, relative to our orbital plane? That is, should we expect planetary systems to be randomly distributed in terms of their angle of orbit relative to us, or would there be greater (or fewer) than would be expected by chance? In other words, can we extrapolate the results of transit studies by simply presuming the number of systems we couldn’t see transiting is the result of a purely random process?

    I suppose this question is ultimately about whether there is any non-local influence that would impact star and planet formation, such as the overall spin of the galaxy. As I understand it, our solar system’s orbital plane (or the earth’s ecliptic) is anywhere from 60-90 degrees off from the galactic plane, so clearly that’s not the case for our system.

    • Eniac May 14, 2016, 12:28

      I think the distribution is random, and that has been tested by looking for anisotropy in known inclinations across the sky. I think enough eclipsing binaries have been known for a long time to reliably answer this good question.

  • Robert May 11, 2016, 12:40

    What do we know about the density of near earth sized worlds? It would be interesting to consider what the gravity is like on their surfaces.

    • DJ Kaplan May 11, 2016, 18:16

      @Robert, these measurements use the transit method, and tells us nothing about the mass of the exoplanets, but only their size. A next step could be to return to these stars to make a radial velocity measurements. That would help answer the question of mass. Combining the two observations can then allow us to derive the density and make some guesses as to the planets’ composition.

      • Robert May 12, 2016, 11:58

        Thanks. I assume people have models and some assumptions based on limited data. In some systems with larger planets also, I assume Kepler’s laws might be useful too.

  • Harry R Ray May 11, 2016, 12:43

    I just read the PDF. The ONLY planet in the habitable zone(KOI2418.01, or Kepler 1229b) is ALMOST EXACTLY IDENTICAL to Kepler 186f in terms of parent star temperature(slightly cooler), radius(just a tad smaller) and temperature(11 Kelvins warmer). The only difference at all is in the orbital period(33 days LESS) and lack of other transiting planets.

  • Harry R Ray May 11, 2016, 12:45

    OOPS: I meant 8 Kelvins warmer, NOT 11.

  • spaceman May 11, 2016, 13:38

    There is evidence from radial velocity surveys that suggest a similarly high percentage of MS stars having compact systems consisting of mini-Neptunes and super-Earths. This is independent evidence that the Kepler results are correct. Microlensing results, which look further out into the galaxy than Kepler or RV surveys, also suggest a large population of exoplanets in the galaxy. Each of these three methods come to more or less the same conclusion that planets are very common around other stars in a variety of galactic environments. I am pretty sure that the planet occurence rates derived from Kepler are a valid extrapolation to the galactic population, as the concordance of results indicates that the Kepler occurence rates are basically correct. Think about it, this did not have to happen: RV surveys could have shown no evidence of close-in mini-Neptunes which would suggest some kind of bias that Tulse seems to be hinting at.

  • andy May 11, 2016, 14:34

    The newly-validated planets listed as being in the optimistic habitable zone don’t look particularly promising: most of them are large enough that they are most likely to be mini-Neptunes. Kepler-1229b (KOI-2418.01) looks reasonably-sized but is rather on the cold side, plus it has the caveat that there is a close companion star that may or may not have a significant effect on the parameters.

    • Andrew LePage May 15, 2016, 9:46

      A detailed assessment of the potential habitability of Kepler 1229b and the eight other HZ exoplanets with radii smaller than twice that of the Earth in this latest group of Kepler discoveries can be found here:

      http://www.drewexmachina.com/2016/05/14/habitable-planet-reality-check-keplers-latest-finds/

      • Michael May 16, 2016, 14:17

        These mini Neptune’s are not automatically anti life as they could also be further out and their thick atmospheres giving them a cosy temperature. They are also more likely to rotate faster which tends to even out temperature extremes and have magnetic fields. They are however much more likely to have oceans of water but without nutrients been circulated they may actually be sterile except on the ocean floors.

        • Andrew LePage May 17, 2016, 11:39

          I is highly improbable that “mini-Neptunes” would be habitable. As currently being defined, these worlds would have atmospheres rich in hydrogen and helium that account for something on the order of 1% to 2% of their total mass. That would result in minimum atmospheric pressures at the “surface” (if there is even a “surface” present in the conventional sense that corresponds to a change in phase and/or composition) on the order of tens of thousands of atmospheres and temperatures of many hundreds of degrees K. These are *NOT* the kind of environments that can support Earth-like habitable conditions.

  • Jim Strom May 11, 2016, 22:35

    Anyone know hiw much of the clustering to the middle half of the brightness range is from observation bias versus planet formation conditions?

  • RobFlores May 12, 2016, 11:15

    I can see two ways to interpret the diagram on small habitable planets.

    1) The apparent GAP in what should be standard distribution of Terrestrials, in the HZ, is a fluke. We can’t detect them with Kepler, but Earth size planets are statistically likely to occur there.

    2) The GAP shows a bias against any terrestrials forming in the HZ of planets overall.

    Kepler has sampled 150,000 stars, a small percentage aligned correctly has resulted in a confirmation of around 2,500 planets.

    The elephant in the room is the number of any terrestrials in the HZ of stars in the Kepler sample is suspiciously low IMO. a paltry 21. You could argue that there are lots of Mars to Earths sized planets we could not detect with Kepler to it’s limitations.

    Note: Obviously, having a bias against terrestrial planet formation in the HZ of stars is one potential answer to the Fermi Paradox.

    Well, I think the J. Webb telescope will have to settle it, and we know where
    to target thanks to Kepler.

    • Eniac May 14, 2016, 12:36

      There is unlikely to be a physical explanation for this putative GAP. One possible explanation is that planets in the habitable zone have been studied more intensely and therefore we have a lower rate of false positives. The false positive rate is highest at the detection threshold, and that one is unfortunately just around Earth-like.

    • Andrew LePage May 15, 2016, 9:43

      @RobFlores

      The “gap” that you find so suspicious is merely the result of known observation biases of the transit method in general and Kepler in particular. These biases can be taken into account in determining the true prevalence of Earth-size planets in the HZ:

      http://www.drewexmachina.com/2015/11/03/the-prevalence-of-earth-size-planets-around-sun-like-stars/

      While there is still an uncomfortable amount of extrapolation from short-period planets to those with longer periods in the HZ of Sun-like star (and this latest batch of Kepler finds should help improve the statistics considerably), I have not read of anything “suspicious” in the distribution of approximately Earth-size planets in the Kepler data.

  • DJ Kaplan May 12, 2016, 12:21

    No doubt there’s a lot of observation bias this early in the science of planet hunting.

  • Holger May 13, 2016, 10:34

    @Rob Flores:
    “The elephant in the room is the number of any terrestrials in the HZ of stars in the Kepler sample is suspiciously low IMO. a paltry 21. You could argue that there are lots of Mars to Earths sized planets we could not detect with Kepler to it’s limitations. ”

    It’s not even 21 HZ terrestrials: As shown in the diagram, 12 of the 21 planets don’t lie in the conservative HZ (dark green); and those measuring 1.5 to 2 Earth radii are likely mini-Neptunes rather than terrestrials.
    Let’s hope that this low number is indeed due to Kepler’s limitations – Kepler probably just wasn’t able to detect an exact Earth analog. But it should have found closer Earth analogs around cooler stars…

  • RobFlores May 13, 2016, 18:11

    Actually for completeness sakes the report on new planets in the habitable zone should include any 2.0RE and bigger giants as separate categories.
    Unless there are none that are new.

    It would also be use full to update number Worlds .6RE-1.20RE sized
    that have been discovered in total not just in the HZ. There are extrapolations that can be made from THAT data.

  • Stevo Darkly May 13, 2016, 20:49

    Regarding the strange gap or dearth of Earthlike worlds around Sunlike stars ….

    Far-out premature speculation: Such worlds are nearly all inhabited/colonized and have taken discreet steps to prevent themselves from being detected — either because they’re afraid, or are simply wanting a little privacy, just like people drawing their curtains across their windows. :)

    Actually, I would kind of expect something like this gap from our current detection methods. Detection bias, certainly.

    It’s just easier to detect bigger planets closer to smaller stars than to detect smaller planets orbiting more widely around bigger stars. The former would block proportionately more of the star’s light, wouldn’t it? (And more often.)

    That’s the Kepler detection method (transit photometry). The same bias would apply to the radial velocity method: Big close planets orbiting smaller stars also jerk their primaries around more than smaller planets distantly orbiting bigger stars.

    (Apologies if I’m simply voicing what everyone knows. I just don’t see the reasons for the bias in our detection methods spelled out so plainly very often. Maybe it’s just too obvious to do so?)

  • RobFlores May 14, 2016, 14:59

    For the sake of clarity

    I know we cannot detect Earth sized and bit larger (to RE 1.3?) in the HZ
    of stars with Kepler sensitivity.

    But finding out that ANY planets regardless of size are less likely to form
    in the HZ of stars is an important bit of information.

    In that case we have 2 possible answers.

    a) the HZ of stars are were Mars to Earth sized (RE1.1?) planets are most
    likely to exist, they are there, we just can’t detect them.

    b) the HZ of stars is antagonistic to the formation of Mars to Earth sized (RE1.1?) planets

    This would be especially true around cooler K type and M type stars, where the Habitable Zone moves much more slowly.

  • Ronald May 15, 2016, 10:29

    Further to RobFlores (and also Eniac, Holger, andy) with regard to the paucity of roughly earth-sized planets in the HZ, I looked up the NASA press release and its briefing materials:
    https://www.nasa.gov/press-release/nasas-kepler-mission-announces-largest-collection-of-planets-ever-discovered

    and in particular Figure 8: ‘Known Transiting Planets by Size’.
    I think this is by far the most important diagram of them all. Paul, could you add this one to this post?
    This bar chart shows all Kepler planets, both the newly discovered and the previously known, all by radius class.
    There are just over 200 Earth-size (0.7 – 1.2 Re) and just over 700 super-Earth-size (1.3 – 1.9 Re), together about 950.

    We know from recent research (Rogers 2014, Chen & Kipping 2016) that the upper size limit for terrestrial planets is probably around 1.6 Re, or even less, planets above this limit most likely being mini-Neptunes/gas dwarfs.

    I have not had time to browse through the Kepler data in detail, so I just want to make a few observations here;

    If we take just the Earth-size planet category plus those from the super-Earth-size category that are < 1.6 Re (roughly half of that number), we are left with about 550 terrestrial planets.

    With regard to observational bias: although the transit method has a clear and well-known observational bias against orbital distance (detection chance simply decreases linearly with distance, i.e. double the distance = half the chance), as far as I know observational bias is not so great with regard to planet size, this contrary to the RV method. Correct me if I am wrong here, maybe there is a lower detection limit (there most likely is), but that limit is then lower than terrestrial size, since these planets have indeed been discovered in great numbers.

    Of the mentioned 550 'true' terrestrial Kepler planets, only about 4 are in the HZ (sensu Kopparapu et al. 2013) and only 1 or maybe 2 of those in the HZ of roughly solar-type stars.

    Even if we, very optimistically, allow all super-Earths in the optimistic HZ, it is only 21 out of about 950 such planets.

    This does not bode well: the drop-off in number per orbital distance bin seems too sharp to be attributable only to observational bias in the transit method.

    Further analysis and extrapolation is required, but I now tend to RobFlores option 2 (in addition, of course, to observational bias): a real, physical bias against terrestrial planet formation in the HZ (in particular of solar-type stars).
    This in turn is probably correlated with the prevalence of compact systems of medium-sized gas planets.

    Though probably not unique, rather rare we are.

    • Andrew LePage May 17, 2016, 11:48

      As I mentioned in a reply above, the “gap” that you are discussing is just an illusion. It is the result of known observation biases of the transit method in general and Kepler in particular. These biases can be taken into account in determining the true prevalence of Earth-size planets in the HZ:

      http://www.drewexmachina.com/2015/11/03/the-prevalence-of-earth-size-planets-around-sun-like-stars/

      Attempting to conclude anything from a simple visual inspection of scatter plots of Kepler finds as a function of effective stellar flux, etc. without a detailed mathematical treatment of the known biases is guaranteed to lead to incorrect results.

      • Ronald May 18, 2016, 9:58

        I agree with (almost) everything you say in this thread, however, perhaps I did not make myself sufficiently clear, but I certainly did not limit myself to a ‘simple visual inspection’;
        – I have read your two (very interesting) above-mentioned posts, in particular the one about the-prevalence-of-earth-size-planets-around-sun-like-stars.
        – I have also read the Petigura paper.
        – I am aware of the observational biases, both in the RV and in the transit method.

        With regard to the last point, o.b. in transit method:
        1- There is a well-known (and simple, almost linear) observational bias in the transit method with regard to orbital distance. Ref. the 2nd histogram in your above-mentioned post.
        2- Apparent from the Petigura paper, there is also an observational bias with regard to planet size. Ref. the 1st and 2nd histograms in your post. This o.b. and hence its required correction is rather small for planets larger than 2Re.
        3- Not apparent from the Petigura paper is whether there is a *combined* effect, i.e. whether smaller planets are even harder to detect at greater distance than larger planets. The correction for orbital distance in the Petigura paper is the same for all planet sizes. And so is the extrapolation.

        Furthermore, from the Petigura paper:
        a) Size distribution appears to reach a plateau at about 2 Re and the population seems to remain flat out (downward) at least to the 1 RE limit of the survey (the 2nd hist. seems to contradict this though).
        b) Occurrence rate of planets appears to be constant per interval of the log of orbital period (this is the crucial issue: is this still true for terrestrial planets or is there a drop-off with increasing orbital period?).
        c) Statistical analysis and extrapolation were based on only four extrasolar planets < 2 Re (i.e. rather unreliable, sample population is much greater now).

        Again, I have not been able to look at the data yet, to see whether there is an abrupt and sharp drop-off with orbital distance. I will try to do this. And I sincerely hope that you are right and that the paucity of terrestrial planets at HZ distance around solar type stars is merely o.b.

        But my question (and increasing doubt) is, with such a large sample size of terrestrial planets, and so very few of those in the HZ (whereas many of larger sizes in the HZ), whether, even with known correction factors, this can still be attributed to observational bias only. Unless there is an even stronger combined o.b. effect that I mentioned in 3 above.

        See also RobFlores latest comment directly below.

        What is needed now is an update of the Petigura paper with present data.

  • RobFlores May 17, 2016, 13:52

    Yes, there is a bias due to the selectivity of the data. also of course the
    HZ is not static. ( But in Red dwarfs the HZ should not move too fast)
    Too bad that there is insufficient data to make rigorous models.

    A model allowing high chance of (1) one Mars to Earth size world in the HZ in the current group of detected suns with planets would yield an estimate of 60-75 total worlds in the survey area (150,000 stars) which is pretty paltry, and place the nearest star with such a planet in the survey area fairly far away. Which would in turn mean that in our local neighborhood of 50 LY, they maybe very sparse overall.