Exoplanet news Part 2: Exosaturn!

By Phil Plait | January 12, 2012 10:03 am

[I'm trying to catch up with all the news that's been released this week while I was off lecturing in Texas. This is Part 2 of a few articles just about exoplanets. Part 1 was posted earlier.]

Astronomers have found one of the most interesting exoplanets yet: one with a very extended ring system!

[That's an artist's impression of the system; click to encronosate.]

The planet was discovered with the SuperWASP (Wide Angle Search for Planets) telescopes — a UK project that employs low-magnification but very sensitive cameras which can observe large areas of the sky at the same time. It orbits a young star called 1SWASP J140747.93-394542.6, which is 420 light years away. The star’s youth — 16 million years — indicates that the rings are probably the leftover remnants from when the planet formed.

The planet and its rings were discovered using the transit method: looking for small dips in starlight as a planet passes directly between us and the star. This is how the vast majority of exoplanets are found. Usually, when you graph the brightness of the star over time, the dip in the plot as the planet transits the star starts suddenly, drops to some minimum, then jumps back up (see here for example). The whole thing is usually over in a matter of hours at most.

But this planet took nearly two months to transit the star! And the dip was weird: there were multiple times the star dimmed then got brighter again, at one point having 95% of its light blocked. Even though the planet wasn’t seen directly, the most obvious explanation is a ring system similar to Saturn’s (though much larger), blocking the light. It must have gaps in the rings, like Saturn’s do, to explain the starlight jumping up again over time. Overall, four rings were detected, and they stretch tens of millions kilometers in diameter!

Saturn’s ring are only about 300,000 km across, so clearly this planet must be much more massive than Saturn, and the rings denser. It may be a little unfair to compare it to Saturn at all; it’s more like a super-Jupiter still surrounded by primordial debris. Unfortunately, we don’t know how massive the planet itself is; you need Doppler data for that and none has been taken yet. The astronomers who discovered this system, of course, are looking into obtaining Doppler data. It’s even possible the object is so large it’s actually a brown dwarf and not a planet.

Perhaps most intriguing about all this are those gaps in the rings. The easiest way to explain them is that there are objects there, moons, sweeping out the material in the rings. Saturn’s rings have gaps for this reason. In fact, there are hundreds of gaps in Saturn’s rings! These are caused by resonances: if a ring particle orbits twice for every one time a moon orbits, for example, the moon’s gravity tugs on it every time it swings by, pulling it into a different orbit. Over time, all the particles in that orbit are gone, leaving behind a gap.

If the planet itself is big, how big are those moons? Could one be Earth-sized? It’s an idea that’s been around awhile, but none has ever been seen… yet. All these super-Jupiters being found have a lot of gravity, and it’s possible they have big moons. We’re also getting better at detecting smaller objects, so it wouldn’t surprise me if that announcement is made sometime relatively soon, too!

I’ll note that the idea of looking for rings and moons is more than an idea: the Hubble observations of the star HD 209458 I mentioned the other day were taken to look for moons and rings around that planet! None were seen, but astronomers will keep trying. There are a lot of planets out there, and one thing we’ve learned is that variety is the spice of nature.

Image credit: Michael Osadciw/University of Rochester

CATEGORIZED UNDER: Uncategorized

Comments (33)

Links to this Post

  1. [LINK] More than an exo-Saturn « A Bit More Detail | January 12, 2012
  1. mozetti

    Phil, considering the age of the star and the size of the planet and rings, is it possible that the rings are in the process of coalescing into moons of the planet?

  2. Bob

    Doing a little jig! Also, I love the word ExoSaturn.

  3. Derek

    Maybe “Exosaturn” implies “planet”, but since you even link to the UR press release “authorized” by Eric: he makes no claim that this actually is a planet, but it certainly is very close in size to a giant planet; still it could also be a brown dwarf or dwarf star ;-)

  4. Okay, had not heard about this one. This is extremely cool! Did the report give any idea how far from the star this planet is?

  5. Kuba the Pirate Navigator

    A claim of “discovery” in this case is a blatant overstatement. The “planet” is just one of thousands of candidates that SuperWASP consortium has stumbled upon, while scouring the sky for possible exoplanetary targets. Until a good quality RV data set is collected – there is no way of telling whether this really is a planet or just some systematic effect.

    Also – a transit duration of approx. one month??? That in itself is a bold claim.

    I call it a bull.

    On the other hand – the original writer did put quite a lot of work into his paper. First publication containing an actual exosaturn discovery will have to reference this attempt. History in the making, ladies and gentleman.

  6. Kendall

    Did I read that right? 95% of the star’s light was blocked at some point? Do the planet’s rings have nearly the same diameter of the star?

  7. amphiox

    Kendall, the sun’s diameter is about 1 400 000 km, so Saturn’s rings, at 300 000km, already cover almost a quarter of the sun’s diameter. So this ring system having a similar if not even greater diameter than the host star is certainly within the realm of feasible.

  8. amphiox

    Indeed, based on my understanding of the report, that 95% blockage at one point would have been caused by just one band of the ring system. The whole ring system would have a diameter much greater than the star….

  9. Phil, could you post actual data (or link) with dimming caused by this planet? Something like this: http://blogs.discovermagazine.com/badastronomy/files/2011/05/55cancrie_dip.jpg

  10. RobT

    Texas? I didn’t know science was allowed in that state. Did you have to smuggle in your notes or did you email them to yourself?

  11. vel

    considering that there is still questions on how “sunspots” affect observations, how can we be sure of any of this? I want to see exoplanets as much as anyone but it surely seems that the things are proliferating like tribbles with some questions remaining.

  12. @vel (#11), they look for repeatability. Sunspots would cause changes that are not strictly periodic like a planetary orbit. That’s part of the confirmation process.

  13. andy

    Yeah calling this one a planet is premature at this point. Saying it isn’t a discovery on the other hand is ridiculous – the identification of this object as a disc system is pretty secure even if the nature of the central object is still unclear.

    Transits of discs in eclipsing binary systems have been seen before: probably the most famous example is Epsilon Aurigae, where an F-type post-asymptotic giant branch star is eclipsed by a disc surrounding a B-type main-sequence star. The difference here appears to be the extreme flatness of the disc which implies that it cannot be a dust disc. As for the analogy with Saturn’s rings, Saturn’s main ring system lies within the Roche limit and therefore will not accrete to form large satellites. The rings of the companion of 1SWASP J140747.93-394542.6 are substantially larger and may well go on to form satellites: the gaps suggest this process may have already begun.

  14. Hannah

    Wait, you were in Texas and didn’t go to the AAS Meeting in Austin? No wonder you’re behind on the big astronomy news!

  15. kevbo

    “1SWASP J140747.93-394542.6″?

    I just logged on to the International Star Registry. You can all call that star “Kevbo” from now on.

    (“click to enkevbofy”)

  16. Gary Ansorge

    Oh goody, a Firefly solar system,,,I wonder if they speak Mandarin/English?

    Gary 7

  17. tim Rowledge

    Planet? Don’t be silly. That was a convoy of ET starships that happened to pass across a star. *Big* starships. Almost lined up with the star, hence the long occultation. Which means they’re heading pretty much this way….

  18. David Vanderschel

    I suspect that Phil’s explanation of the resonance effect is incomplete (also in the earlier article). What I think should be said is more along the following lines: Regarding an orbit with half the period of a moon, when an object in that orbit is closest to the moon, it is always in the same position relative to its own orbit. Thus the effects of the tugs accumulate and eventually make a large change in the orbit. If it were not for this in-phase consideration, the effects of the tugs would not accumulate. Indeed, they do not for objects in non-resonant orbits, which are also tugged by the moon but not when in the same position.

  19. Holy cow it’s raining exoplanets!
    Forecasts call for scattered showers of awe with fascination-storms increasing into the afternoon! :D

  20. Doug Little

    Phil,

    Not sure if the next installment on Exoplanets is to do with this research but you need to include it. It uses gravity microlensing to find planets in the habitable zone, I have never heard of this method of detection as well which is an extra bonus.

  21. Wzrd1

    With rings “tens of millions of kilometers in diameter”, that object could only be a brown dwarf.
    Sounds more like a failed binary star system.
    A really cool one, since the plane of the ring system is at a significant angle away from the plane of orbit. Indeed, one wonders how the rings aren’t bent out of position by the primary’s gravity!
    Can’t wait for DETAILED observations, with doppler and other measurements.

  22. Greetings – I’m first author on the paper reporting this object. I just want to address a couple comments.

    First off – we worded the original press release quite carefully. This is not necessarily “planet transit” as some have called it — but we are very confident that it is a transit by a system of multiple dust rings orbiting *something*. But right now we don’t have enough data to definitely say whether it is a young (16 million year-old) exoplanet, brown dwarf, or very low-mass star (it must be <A claim of “discovery” in this case is a blatant overstatement. The “planet” is just one of >thousands of candidates that SuperWASP consortium has stumbled upon, while scouring the sky >for possible exoplanetary targets. Until a good quality RV data set is collected – there is no way of >telling whether this really is a planet or just some systematic effect.

    The “systematic effect” that we saw was a series of multiple eclipses at the tenths of magnitude to ~3.3 magnitude level over ~54 days in 2007. We spent considerable time trying to find alternative explanations and we discuss them at length in the paper (no stellar eclipse can mimic this signal!). Also, I am not affiliated with SuperWASP or their planet program — indeed we discovered this star when searching for new members of a nearby young stellar association first, then queried the SuperWASP public archive and found this light curve. We also have light curves for hundreds of similarly young stars that show no such eclipses (just normal variations due to starspots). As for the radial velocity follow-up… patience is key. We do not have instant information on all things right away — research takes time.

    >Also – a transit duration of approx. one month??? That in itself is a bold claim.
    >I call it a bull.

    Interested to know how many seconds it took you to conclude that, and what data that you took into account to draw that informed conclusion. Look at the data yourself in the paper or the SuperWASP archive.

    http://adsabs.harvard.edu/abs/2011arXiv1108.4070M (PDF of paper)
    http://www.superwasp.org/ (SuperWASP public archive)

    While there are some systematic hiccups in the photometry at the ~0.1 mag level (seen in other SuperWASP light curves) — I would love to hear your explanation for the variations seen over April and May 2007.

    Also — there is independent confirmation of the variations by a totally different experiment (the ASAS survey at Las Campanas in Chile). Two telescopes on two totally different mountains in two different hemispheres.

    >On the other hand – the original writer did put quite a lot of work into his paper. First >publication containing an actual exosaturn discovery will have to reference this attempt. History >in the making, ladies and gentleman.

    We’ve been studying this thing on and off for a year – and we spent a lot of time trying to make it go away or account for it through some other astrophysically plausible mechanism. No one wants to publish something that could be found to be easily wrong!

    If it is a very low-mass star: we’re dealing with an incredibly compact system of an inner disk and discrete rings within 0.1-0.4 AU of a tiny star. And there are gaps – what is causing them? A Kepler-affiliated group reported a miniature trio of terrestrial planets orbiting with periods of <2 days around a red dwarf at the AAS meeting. Tiny stars appear to have tiny planetary systems. Perhaps 1SWASP J1407 is hosting a tiny planetary system in the making?

    If it is a brown dwarf, it is similarly remarkable. A brown dwarf with rings? Again, what is creating the gaps. How big do "planets" or "moons" get around brown dwarfs?

    And if it is an exoplanet, then indeed we may be talking about a protoexosatellite or circumplanetary disk — while we call them rings because they are thin annuli of dust, these are clearly larger and more massive than Saturn's rings, and these extend *way* beyond the Roche radius of the planet. If it is an exoplanet, then this could be the first hints of *moon* formation outside of the solar system. This is a weird type of system – somewhat intermediate in scale between thin, compact "rings" like we study around giant planets in our solar system and "dusty debris disks" orbiting other stars (which have scales of many AU, Earth masses of dust, and are generally puffier).

    The discovery paper was more or less a progress report of what we know and don't know so far, and some calculations on how common these sorts of eclipse events might be and how we might try to find more of them in the future. It is always to tough to figure out when to stop writing and publish your findings – there is always a tendency to want to wait for more observations.

    I'd like to add that there is a cool opportunity for astronomers with telescopes that monitor variables stars: We *don't* know the period of the system. We looked through several years of data and only saw one long, complex eclipse back in 2007. It will take a lot of follow-up photometry/monitoring to find the next eclipse. When we get the period, then we can really plan some detailed follow-up observations of the eclipse and learn more about this crazy ring system. So help us find the next eclipse and solve for the period of the system! (And I hope it isn't as long as e.g. Neptune's orbit! Please I hope I see another eclipse while I'm alive!).

    And thanks again Phil!

    Eric M

  23. Aidan

    Okay, read a few of the comments… I noticed some of you seem to be confused. Some of you are insisting that because at one point the suns coverage was 95%, that the ring covering it must have been 95% of the suns diameter. That would be wrong. Because stars are round, light goes off in many directions, not just forward, as they go off in these directions, the mover further and further away from the suns central regions, [think of a diagonal arrow].

    Now, because of this, if a planet is sufficiently distant from it’s star, it will only be graced with a fraction of the stars light. Hence: distance was probably more of a factor than size. Besides that, it most likely is a gas giant, which would mean it’s made from light weight elements, which would mean it’s probably quite distant from it’s star, just like our gas giants.

  24. Are there other reasonable explanations besides a ring system? This is an awesome discovery regardless of the explanation, but calling rings the reason for the longer, deeper dip tickled my skepticism gland a bit. I certainly hope that is the answer though, it is one of the coolest bits of exoplanet stuff I’ve seen.

  25. Messier Tidy Upper

    WHOAH! :-o

    That is just superluminous news. To be able to discover planetary rings – far larger than Saturn’s to be sure but rings nonetheless – around an extrasolar planet and a relatively distant one at that.

    That is.. just .. Wow. Marvellously WOW. :-D

    I am offically thunder-struck. :-)

  26. Reminds me somewhat of the mystery companion to Epslion Aurigae also known as Almaaz – click on my name for link to Kaler’s Stars page on that one – although obviously on a much smaller scale.

    Again we have a strange object transiting across its star and creating a wonderfully intriguing puzzle with some exotic proposed solutions. Perhaps we’re jumping too quickly into the Saturn-like rings conclusion but still .. wow. :-)

    @23. Eric M – January 12th, 2012 at 7:33 pm :

    Congratulations! Splendid discovery and work there – I can’t wait to hear & read more about this. Please relay my congrats and thanks to the rest of your team too. :-)

  27. Kuba the Pirate Navigator

    Eric – no hard feelings there. I did have a look at your paper and read through the section concerning other plausible explanations. What bothers me is that SuperWASP light-curves are plagued with all sorts of systematics. Myself, I was very close to announcing “discovery” of a WASP planet once, which eventually was found to be just a systematic effect related to the the way in which SWASP reduction pipeline adds timestamps to data collected by its telescopes. I see that you collaborated with Andrew Collier Cameron on this paper. Well, Andrew, being involved in SWASP project as much as he is, knows all too well just how misleading SWASP data can sometimes be.

    In the whole business of exoplanet hunting, confirmation of a discovery is the key. I would be really interested to see outcomes of the RV analysis when sufficient spectroscopic data is eventually collected. For now it is a very interesting idea that awaits confirmation.

    Oh – and sorry for calling it a bull. That was far too strong. I am just immensely annoyed by how media respond to such announcements. I work on one of telescopes following up SWASP candidates and my primary job is concerned with rejecting planetary candidates that are put forward basing on raw SWASP data (exactly the sort that you were dealing with). In 95% of all situations followup “kills” these “planets” before anyone even starts thinking about publishing any information about them. Coming from that sort of background, your announcement sounds a little preliminary, and the way in which it got picked up by media is just purely outrageous.

  28. VinceRN

    Wow, author happens by and chimes in. That is awesome. Thanks Eric M, for your work and for stopping by.

  29. DigitalAxis

    @1: Mozetti

    Phil, considering the age of the star and the size of the planet and rings, is it possible that the rings are in the process of coalescing into moons of the planet?

    Actually, the presence of rings suggests there’s ALREADY moons. If there’s a gap in a disk, something has to be clearing that gap. For instance, with Saturn’s rings, the enormous Cassini Division is cleared by Saturn’s moon Mimas (which lies outside the rings but exerts gravitational influence on them).

    @30 VinceRN:

    MANY professional astronomers read Bad Astronomy.

  30. Anchor

    @ Kuba: “…the way in which it got picked up by media is just purely outrageous.”

    Yes, it is. At least as outrageous as implying that researchers are responsible for how media may portray their work. But no hard feelings there, to be sure.

  31. Drunk Vegan

    That’s not a planet, it’s a ringworld built around the star. The gaps are the spots where the various sections are loosely linked together to lessen the tidal stress. :)

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