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Mysterious 'disappearing' exoplanet was just a big cloud of asteroid trash, study suggests

In 2014, a planet disappeared from the night sky.

The distant world — known as Fomalhaut b and located a neighborly 25 light-years from Earth — was infamous for being one of the first exoplanets ever discovered in visible light by NASA's Hubble Space Telescope; when astronomers first caught sight of it in 2004 and 2006, the planet appeared as a bright, cool dot moving briskly across the sky. Ten years later, that dot had vanished.

What happened to Fomalhaut b? Did the world have a falling out with its guardian sun (named simply Fomalhaut) and drift away? Did the brilliant planet seek stardom in a bigger, brighter solar system? Or could a nefarious case of planet-on-planet violence be afoot?

Related: 10 interesting places in the solar system we'd like to visit

An illustration of two asteroids colliding in the system Fomalhaut b. (Image credit: © ESA/NASA, M. Kornmesser)

A new study published today (April 20) in the journal Proceedings of the National Academy of Sciences (PNAS) proposes a solution to the "Mystery of the Disappearing Exoplanet" — and, befitting of any good detective story, there's a twist ending.

Perhaps, Fomalhaut b disappeared before the Hubble's eyes, the study authors wrote, because Fomalhaut b was never a planet in the first place; in this scenario, the object astronomers saw in 2004 and 2006 was actually a colossal cloud of icy debris created by a recent, violent collision between two planetary fragments.

This diagram simulates what astronomers, studying Hubble Space Telescope observations, taken over several years, consider evidence for the first-ever detection of the aftermath of a titanic planetary collision in the Fomalhaut star system.

This diagram simulates what astronomers, studying Hubble Space Telescope observations, taken over several years, consider evidence for the first-ever detection of the aftermath of a titanic planetary collision in the Fomalhaut star system. At left is an image of the ring of icy debris around Fomalhaut. At right are the progressions of observations of an object, once thought to be a planet, but now thought to be the dusty remains of an asteroid collision. (Image credit: NASA, ESA, and A. Gáspár and G. Rieke (University of Arizona))

The proposed collision, which likely took place in an icy ring of debris similar to our solar system's Kuiper Belt, must have occurred very shortly before the Hubble first caught sight of the alleged exoplanet, when the expanding cloud of post-collision dust particles was still densely concentrated and apparent in visible light, the researchers wrote. By 2014, that cloud had already grown large and diffuse enough to disappear from view, the idea goes.

In a way, this cosmic case of mistaken identity makes the discovery of Fomalhaut b even more rare and exciting, lead study author Andras Gaspar said in a statement.

"These collisions are exceedingly rare and so this is a big deal that we actually get to see evidence of one," said Gaspar, an assistant astronomer at the University of Arizona's Steward Observatory. "We believe that we were at the right place at the right time to have witnessed such an unlikely event with NASA's Hubble Space Telescope."

Now you see it...

For the new study, Gaspar and his colleagues reviewed nearly two decades of archival Hubble observations, which revealed Fomalhaut b slowly growing dimmer and dimmer before completely vanishing in 2014. Using computer models, the researchers calculated that a collision between two icy bodies roughly 125 miles (200 kilometers) in diameter could have created a dust cloud that matched the Hubble observations.

This dust-cloud-in-disguise hypothesis also explains some unusual behavior of the object. For example, the supposed planet's brightness, which allowed Hubble scientists to see it clearly in visible light, is highly unusual for distant exoplanets, which are often too small to reflect a noticeable amount of light from their home star. On the flip side, Fomalhaut b showed no infrared light signature, meaning it was extremely cold — again, highly unusual for a young planet, which should be warm enough to emit some infrared radiation, the study authors said.

"Clearly, Fomalhaut b was doing things a bona fide planet should not be doing," Gaspar said.

Meanwhile, both of those observations are consistent with the theory that Fomalhaut b is actually the debris of two icy asteroids that met a cataclysmic end. If that's the case, the researchers calculated, then that debris cloud has since expanded significantly, and now it has a diameter greater than Earth's orbit around the sun. Those ever-drifting pieces of leftover ice and dust must each measure smaller than the width of a human hair, far below Hubble's detection threshold, the researcher wrote.

But it's too soon to officially close the case of the disappearing exoplanet — researchers will have to study Fomalhaut's solar system in more detail first. No criminal charges have been brought against either of the asteroids at this time.

Originally published on Live Science.

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Brandon Specktor
Brandon Specktor writes about the science of everyday life for Live Science, and previously for Reader's Digest magazine, where he served as an editor for five years. He grew up in the Sonoran Desert, but believes Sonoran hot dogs are trying way too hard.
  • rod
    "For the new study, Gaspar and his colleagues reviewed nearly two decades of archival Hubble observations, which revealed Fomalhaut b slowly growing dimmer and dimmer before completely vanishing in 2014. Using computer models, the researchers calculated that a collision between two icy bodies roughly 125 miles (200 kilometers) in diameter could have created a dust cloud that matched the Hubble observations."

    Fomalhaut b was considered to be a 3 Mjup exoplanet with orbit near 115 a.u. semimajor axis and eccentricity of 0.11. It had an orbital period of 320,000 days. Now the imaged exoplanet is reduced to debris :) Just think if our solar system was like this today, big objects colliding with Earth or near the Earth or just pushing Jupiter around and towards Earth :)
    Reply
  • dfjchem721
    There might be an alternate explanation for this disappearing act.

    It is possible that the planet has significantly varied albedos over its surface, and if it rotates, the brightness will rise and fall to an observer watching this rotation. For instance, the north polar region of Charon has a very large dark area informally known as "Mordor" (New Horizons ). The dark color is believed to be caused from tholins (as I am sure you know, rod), organic molecules derived from smaller compounds ages ago.

    It cannot be ruled out that the disappearing planet is actually still there, we have just been imaging its brightest side for some time, and now its darker side has come into "full view", so to say. Clearly the disc had not been resolved so this explanation cannot be ruled out based on "brightness" alone. It also could be that most of the planet is dark, and will not "reappear" for many years - it would have to have an extended period of rotation for this to be possible. And this is certainly likely as these things vary substantially. I would guess the decay curve in brightness might rule this notion in or out.

    But perhaps not. It might also depend on the distribution of dark materials vs. light materials, and our line of sight. If the decay curve cannot rule out variable albedos, perhaps extended exposure times will reveal it is still there!

    I noted reading the article that they presume the drop in brightness is due to an expanding cloud of debris, but there is no direct evidence for this. The image showing this is an illustration, not anything from a telescope. All they really see is a point source dimming, and then suggest it is a fading debris cloud. And maybe it is. Considering the probability we are seeing such a thing made me come up with an alternate reason for this disappearing act. There could be others..........


    As usual, aliens cannot be ruled out! :)
    Reply
  • rod
    Yes, aliens can be ruled out. However, the disappearing exoplanet will be disputed. Here is another report, ENSHROUDED EXOPLANET OR PUFF OF DUST? THE NATURE OF FOMALHAUT B, "I certainly would not mind our model being proven wrong,” Gáspár says. For his part, he thinks the object has faded too much to be seen by Hubble — but that doesn’t mean we shouldn’t try, as even an image that doesn’t show the object would still provide useful information. Meanwhile, he will use the James Webb Space Telescope, when it launches in 2021, to look much more deeply at the system’s infrared emission. “Science evolves,” he notes, “as new observations and data are acquired.”

    Follow observations will continue. Mean while, this exoplanet site continues to show the exoplanet as confirmed still, The Extrasolar Planets Encyclopaedia
    The site will likely remove it, like others when confidence that it is a false positive exoplanet identification.
    Reply
  • dfjchem721
    From the Sky and Telescope article:

    "Tracking the object’s movement over a decade, Gáspár and Rieke find that it appears to be moving out of the system, elongating slightly as it does so." - - - Wouldn't this be true if you were watching something like Pluto (as an exoplanet) for a short period? It would appear at times in its orbit "to be moving out of the system". They have only been watching Fomalhaut b for about 10 years, and that object is 115 AU from its star, much further than Pluto. That is a long ways out there. Hard to imagine they can make that determination with only 10 years of data, which I suppose is why it "appears to be moving out of the system". You have a much better understanding of these dynamics then I ever will!

    I considered a shrouded planet, but then wondered about the temperatures required. They never got an IR signal off of it, meaning it is very cold. While a waxing and waning atmosphere could explain the dimming, that would take some energy, and should have provided an IR signal. But maybe not at this distance.

    It sounds to me like they aren't even sure it is a planet anymore, and may or may not even be orbiting the star. You are the resident expert on this. "The site will likely remove it" sounds like the most logical solution. How many other "exoplanets" might fall into this category of "not quite sure"? Yet Fomalhaut b is certainly an interesting object. No doubt its likely position will continue to be watched for some time.

    Thanks for the S & T link.

    And in theory rod, aliens can never be ruled out. We are here, and we are aliens to any other life form(s) in the universe. Even if there are only two planets with life, aliens exist. What they can "do" is another issue! But without direct observations, they are very low on the list.
    Reply
  • dfjchem721
    Tracking down other data on Fomalhaut b , there is one observation that is not even addressed in many of the articles read so far.

    From the discovery team*:

    "In order for Fomalhaut b to be detectable at optical wavelengths, it must have an emitting area much larger than the physical size of a planet. "The data suggests that what is observed is a "circumplanetary ring system large enough to scatter enough starlight to make Fomalhaut b visible only if it has a radius between 20 and 40 times that of Jupiter's radius." This must be based on sound observation calculations, and these people should be on the ball with this. All of the below rides on this being largely correct.

    If we are indeed seeing the light of a circumplanetary ring system, its brightness could change dramatically depending on the reflecting area of debris presented to our line of sight. If we were looking at the broad ring of it face on, or nearly so, it perhaps has now moved into an edge-on view from our perspective, dramatically decreasing light detected on earth. This seems the most likely explanation for the dimming observations. It certainly makes a lot of sense if the above quote is accurate.

    In the original article, Kalas et.al claimed to see variation in the light output, suggesting a rotating ring. While re-evaluation of the same data by various groups did not find "convincing" evidence for this variation, it cannot be ruled out simply by the lack of absolute evidence from such minimal telescopic observations. Such variations may in fact be very difficult to currently detect at 25 ly. Additional higher resolution images cannot be taken currently as it has "disappeared". It is a sure bet that over the coming years, there will be some who are expecting Fomalhaut b to pull a Lazarus on us! I happen to be one of them.

    It will be interesting to see in the future if any of the other exoplanets "disappear". As it seems that "ring plane rotation" is a more likely explanation than a very rare impact event, we should see a few other exoplanets "disappear" as they are periodically monitored. To see even one disappear would decrease the odds of a collision event even more for observations over the next 10-20 years or so. With over 4,000 exoplanets (if I remember this from rod correctly), that should be enough. I will bet again they will see some winking out, because the only reason they are being imaged is because many probably have a large circumplanetary ring system, and the planet is largely not the point source being seen. One would think that a large ring system with bright ices in a plane would be much more reflective than any gas or solid planet, however large it might be. Another reason to look for disappearing acts in the exoplanet story. Hopefully there is a lot of monitoring of known exoplanets to bear out these notions.

    Finally (hopefully), some observations indicate that the size of the source is/was increasing. If this is a ring-plane rotation making the "planet" disappear, one must then consider aspects of brightness of the ring plane as it rotates from our point of view. One would expect a "face-on" view of the rings to provide the greatest brightness, and likely the inner portions of the ring has the highest density of debris. As you work out to the edge of the rings, you encounter decreasing debris density, and therefore decreasing brightness. But consider on rotation, where those outer "dilute" rings are now collectively reflecting much greater light at us as they all begin to emit from a more localized area. Such an effect should increase up to certain point of rotation, before the whole ring plane's brightness thereafter drops to undetectable levels at edge-on. So from this, at some point one would expect the outer-most ring(s) to also light up, giving the appearance of an increase in size, which some have apparently observed. Brightness, and to some extent the object's apparent size, is all in the angle of the ring plane to our line of sight.

    So, it is likely they were never imaging just the point source of a planet (much less a debris cloud!), but the entire planet/circumplanetary ring system. That puts a whole new light, rotation, and spin on things, so to say. :)


    Fomalhaut b's discovery team is this reference:

    * Kalas, Paul; et al. (2008-11-13). "Optical Images of an Exosolar Planet 25 Light-Years from Earth". Science. 322 (5906): 1345–8.


    @ https://science.sciencemag.org/content/322/5906/1345?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=1166609&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT
    Reply
  • Dragrath
    This discussion so far has missed the most crucial element that reveals the likely true nature of this and worse undersells what was observed this is the aftermath of a cataclysmic collision which can be best observed from how the object spreads out and dims. Remember that this is a distant object so in order for the change in size to be so strongly visible the material must be moving fast enough to cover the vast distances involved and calculating the velocity at which the material is expanding outwards.
    Since it was first discerned to be at 115 AU that gives us a scale size for the system which as noted is about the size of Earth's orbit around the Sun i.e. two astronomical units
    From this it can be immediately discerned that the observed object isn't gravitationally bound suggesting it is escaping which is supported by how the object was initially compact and has been expanding outwards as an explosion. This thus is a transient event the destruction of a large icy "asteroid" in a particularly violent collision which is a far rarer and more impressive discovery.

    There is evidence that suggest the aftermath of a similar much more rocky and slightly smaller collision 480 million years ago likely played a major role in setting up the major climatic changes that ultimately contributed to the Ordovician mass extinction while also producing a large family of meteorites which to this day remains the most abundant class of meteorites the L chondrites. We are able to tell this event happened because the rocks preserve evidence of this violent Collison out in the asteroid belt so long ago with the formation of shocked minerals requiring the heat and pressures only a major collision can produce. This along with the observations of transit survey missions like Kepler and TESS have revealed that such cataclysmic events are surprisingly common even in fairly old systems.

    But If there are large asteroids it suggests there was something driving them to cross paths so while Fomalhaut b was a mistaken detection it suggests there might be planets there
    Reply