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HK Tauri: Misaligned Protoplanetary Disks

When I was a boy in ninth grade, I asked our science teacher whether the nearest star was likely to have planets. He loved the question because it gave him the chance to explain to the class that Alpha Centauri was a binary star (we left poor Proxima out of the discussion), and that as a binary, it couldn’t possibly have planets because their orbits would be too disrupted by gravitational effects to survive. That sounded reasonable to me, and I began putting my hopes on places like Tau Ceti and Epsilon Eridani, single stars with no disruptive companion.

Since then we’ve begun finding binary stars with planets and are learning about the diversity of exoplanetary systems, putting Alpha Centauri back into the game. A good thing, too, given the fact that binary stars are common, and keeping them in the planet hunt allows that many more chances to find an Earth 2.0, not to mention all the other interesting kinds of planets including ‘super-Earths’ that we’re locating. But the fact that binary systems can have planets doesn’t mean we can ignore the powerful effects two stars in the same system can have on the objects orbiting them.

Take the case of the interesting HK Tauri system, located some 450 light years from Earth in the constellation Taurus. Here we’re dealing with a young system, the two stars being between one and four million years old, the age range in which planet formation is believed to occur. Their separation is about 58 billion kilometers, which works out to 386 AU. Given the youth of the system, it’s not surprising to find protoplanetary disks here, one of them (HK Tauri B) edge-on and observable in visible or near-infrared wavelengths. The orientation of the disk helps to block the light of the central star, making observations at these wavelengths possible.

The other disk, around HK Tauri A, is best observed in millimeter-wavelength light because we do not see it edge-on and visible light observations are overpowered by the star’s light. Now we have new results from the Atacama Large Millimeter/submillimeter Array (ALMA), which draw on observations of the planet-forming disks in this system. A team led by Eric Jensen (Swarthmore College), able to measure the rotation of the HK Tauri A disk for the first time, has discovered that the two disks are mutually misaligned by at least 60 degrees.


Image: This artist’s impression shows a striking pair of wildly misaligned planet-forming gas discs around both the young stars in the binary system HK Tauri. ALMA observations of this system have provided the clearest picture ever of protoplanetary discs in a double star. The new result demonstrates one possible way to explain why so many exoplanets — unlike the planets in the Solar System — came to have strange, eccentric or inclined orbits. Credit: R. Hurt (NASA/JPL-Caltech/IPAC).

What we’re seeing is that when stellar orbits and protoplanetary disks are not in the same plane, the planets under formation are likely to end up in eccentric, tilted orbits, with the gravitational effects of one star perturbing the disk of the other. Is the disk arrangement we find here a unique case or a common process around binary stars? A good deal of work lies ahead before we can answer that question, and not all oddball exoplanet orbits can be explained by this mechanism. But in at least this case, disk misalignment is a powerful indicator. Says Jensen:

“Our results show that the necessary conditions exist to modify planetary orbits and that these conditions are present at the time of planet formation, apparently due to the formation process of a binary star system. We can’t rule other theories out, but we can certainly rule in that a second star will do the job.”


Image: This picture shows the key velocity data taken with ALMA that helped the astronomers determine that the discs in HK Tauri were misaligned. The red areas represent material moving away from Earth and the blue indicates material moving toward us. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).

The paper on this work notes that the team’s findings are consistent with recent simulations of binary formation that predict such misalignments, especially in systems with separation greater than 100 AU, as we find here. Moreover, we may not always be aware of the companion responsible for a perturbed disk. From the paper:

While it remains to be seen how the protoplanetary disks in a statistical sample of young binary systems are oriented, it is suggestive that in the handful of systems where this measurement has been made, the misalignments are large. If this is a common outcome of the binary formation process, and especially if it extends to lower-mass binary companions (which may easily go undetected) as well, then perturbations by distant companions may account for many of the orbital properties that make the current sample of extrasolar planets so unlike our own solar system.

The paper is Jensen and Akeson, “Misaligned Protoplanetary Disks in a Young Binary System,” published online in Nature 31 July 2014 (abstract).


Comments on this entry are closed.

  • ljk July 30, 2014, 16:11

    Quoting Paul Gilster in the main article:

    “When I was a boy in ninth grade, I asked our science teacher whether the nearest star was likely to have planets. He loved the question because it gave him the chance to explain to the class that Alpha Centauri was a binary star (we left poor Proxima out of the discussion), and that as a binary, it couldn’t possibly have planets because their orbits would be too disrupted by gravitational effects to survive. That sounded reasonable to me, and I began putting my hopes on places like Tau Ceti and Epsilon Eridani, single stars with no disruptive companion.”

    And then there was the old trope that our Sol system was probably one of the few in all of existence because of the way that certain astronomers thought planetary systems formed: By having a passing star pull material off the surface of Sol and the remains went into orbit and later cooled into the planets and moons. This is known as Jeans Tidal Theory:


    Since stellar passages were clearly infrequent and not all of them would result in a “birth”, we were a one in a multibillion chance happening – or the special act of a deity who chose only us and maybe a few lucky others to exist in the Milky Way. This process no doubt occurred with every galaxy across the Universe – which themselves were not even officially confirmed as stellar islands until the 1920s!

    These old attitudes are why even today we still wrestle with those who think there is no extraterrestrial life, or that it is at the algae level or lower at best. Oh heck, there was a recent survey that showed quite a few members of the general public still think the Sun and the rest of the planets go around Earth!

    We have made strides in our knowledge of astronomy, but we still have a long way to go. We know of a few thousand exoworlds, but very little beyond their basic statistics. We don’t know who or what is out there yet, or where. And now we are detecting strange new radio signals which the astronomers have not yet a clue about. It took them years just to discover they weren’t some kind of fluke.

  • Mike Lorrey July 30, 2014, 18:18

    I had a similar experience with a teacher teaching science in the 3rd grade, who claimed that seasons on Earth were due to the elliptical orbit of Earth around the Sun, and when I corrected her that it was due to axial inclination, I got detention. Thereafter she was titled by myself as the Science Misinstructor….

  • Paul Gilster July 30, 2014, 19:11

    Although in this case, I can’t fault my teacher. He was just repeating what was commonly said in that era, long before the discovery of any exoplanets. We’ve just learned a lot in the years since.

  • NS July 31, 2014, 2:18

    Re what’s out there, if we detect e.g. a truly artificial radio signal or pollution we can be fairly certain that intelligence/complex life is not a fluke. If all we detect (well, it’s still a lot) are biosphere atmospheric signatures in other stellar systems, it may be centuries or millennia before we can study those worlds in enough detail to know what sort of life is present. My hope is that life exists elsewhere in the solar system so that we can begin to answer our questions sooner than that.

  • Robin Datta July 31, 2014, 7:56

    In all the rush to find microbial flatulence on exoplanets, might they miss an endostar (a star inside a Dyson sphere)?

    One endostar is worth more than all the exoplanets anyone can find.

  • Eniac July 31, 2014, 23:59


    These old attitudes are why even today we still wrestle with those who think there is no extraterrestrial life

    This is incorrect. There are several valid reasons to think so, no “old attitudes” are required.

    One of them (the valid reasons) is the Fermi paradox, the other the unfathomable improbability of abiogenesis.

    Furthermore, your “we still wrestle with” formulation implies that the notion of extraterrestrial life is ascending. That is clearly incorrect. ET is and has been steadily receding the more we look and don’t find. Think Martian canals, for just one example.

  • August 5, 2014, 6:02

    But we haven’t really looked! Though, your other points are valid… if we (or someone, anyone!) would send a fleet of life-detecting probes to places with a possibly substantial amount of liquid water–say, Europa comes to mind immediately, followed by Enceladus, Titan, and Ceres, and end up empty-handed, maybe then we would have the right to doubt the presence of ET (at least within our own solar system).

    Obviously, we should be more concerned about getting our transportation/propulsion methods up to par with ‘c’ or space-time, in general, which would enable us to more easily probe those extrasolar, Goldilocks territories we have peered into. If and when we find nothing, then we would be able to more accurately question the probability of abiogenesis. Just because we haven’t been able to make it happen in a lab, yet, doesn’t mean our solar system (or universe) hasn’t managed it more than once.

    LJK is right: the only things stopping people from throwing more resources at landing on these planetary surfaces to peep a deeper look (which are very much within our reach) are their attitudes and reluctance, which would undoubtedly not be as they are if such people were informed, educated, and up-to-date on the discoveries made in the last 20 years.

    Believe it or not, I had a discussion with a very young friend of mine (13yo), asking her if she could live forever and go anywhere in the universe before our sun reaches red giant, where would she go… Sadly, her answer was ‘There is no where else to go and there aren’t planets outside the solar system; Earth is the only place for humans and you’ll be long gone before that happens, so why are you even asking that question?’ in a very scolding way, but I have patience with that kind of behavior from a kid. So, I reiterated the fact that my scenario enable her to live FOREVER (which would eventually imply the sun’s red giant phase) and showed her that extrasolar planets existed.

    She seemed very shocked, and went to tell her parents… WENT TO TELL HER PARENTS! Now I’m shocked–not by the fact that eighth grade science hasn’t educated her about what I just told her, but by the fact that it was so unknown to her that she had to verify it with her parents, who also did not know!

    And guess what? They DENIED IT, right in front of the evidence smack dab in their faces. They told me that ‘only god has the power to give eternal life’ and if we went to live off the Earth we wouldn’t be able to get ‘raptured’!

    These people are only separated by ONE GENERATION from me. Looks like we’re in store for quite a bit of ‘wrestling’ if you asked me. These people didn’t even want to use their imagination. ; ;

    Maybe this is an extreme case, but I know we can do a better job at looking, compared to what’s out in space at the moment. ‘Old attitudes’ may not be required to think there aren’t ET, but they sure as hell are still prevalent. In the meantime, I will try to direct them to Centauri Dreams, and maybe their attitudes will change after reading Paul’s exceptional posts and the comments made by very ingenuous dreamers.


  • Eniac August 7, 2014, 21:38

    Oh, we have looked. We have excluded a thriving culture of moon people, jungles on Venus and grand water infrastructure on Mars, all in less than a century. Sure, there is plenty more to rule out, but you can’t say we haven’t gotten off to a good start.

    The only way you could lament things getting worse is if you have a preconceived opinion that ET exists. Then, of course, it would seem like the lifeless world is closing in on you, ET banished further and further just beyond the boundaries of our knowledge, and a smug attitude spreading that maybe they don’t exist. Just don’t call them, in your desperation, “old attitudes”: Many are actually modern and ascendant notions backed by rigorous logic and available evidence.

  • Eniac August 7, 2014, 21:50

    I guess my point is that “old attitudes” include belief in gods, angels, fairies, AND aliens. Vanquishing old attitudes in favor of logic and real evidence will diminish belief in ET, simply because real evidence for their existence is lacking. If I read him correctly, this is the opposite of what LJK was saying.

  • August 8, 2014, 12:08

    We haven’t looked. Your observation is only adequate for extraterrestrial ‘intelligence’ (out of comparison to our own primate brain culture), but the general existence of just ‘extraterrestrials’ has not been ruled out in our own system. We haven’t looked past the surface. Moon people, jungles and canals on Mars were all conjectures that were disproved just from being able to touchdown on these places (even though all of these ideas can ~become~ a reality and are only the result of very specific, limited sample size i.e. a dude’s educated guess/imagination absent of scientific methodology). Only a small percentage of life exists on our planet at the surface and in the atmosphere that is capable of actively inhabiting another celestial body [only one in particular]; all of the interesting stuff that wouldn’t necessarily require the evolutionary history of us landlubbers is underground, in volcanoes/gaseous vents and in bodies of water (both liquid and frozen, or even other non-related liquids) and have been as such for enough time not to require any a lot of that life to have to adapt past ‘extremophilic’ states. These environments more closely resemble the planetary bodies of our solar system outside of what we call ‘habitable’, duly to our own experiential bias. Where we have ~kinda looked~ is one place, Titan; and our looking only points us in the direction (this time based on scientific methodology) that there may be strong evidence for life. What stopped us from being more sure?–Cassini-Huygens wasn’t equipped with the tools necessary to ~really see~. We seem only interested in finding oceans of liquid water when it really isn’t even ~absolutely~ a go to-end all–

    Put yourself in the shoes of an extremophile–to them, our mode of living is pretty extreme. Escherichia coli and Paracoccus denitrificans can not only survive, but thrive at 403,627 x g, are facultatively anaerobic and can live inside of other organisms. If we throw them almost anywhere in the universe somewhere and just slightly engineer their radioactive resistance (or throw them in a place with enough shielding), they will assuredly be able to start a new ecosystem. We are an extremely specified, universal fluke (biologically) comparative to these organisms that could potentially withstand supernovae gravitational conditions unaided, while we’d get made into well-done stellar pancakes.


    “Old attitudes” aren’t beliefs, but moreso the blatant lack or disregarding thereof due to fear, miseducation or maybe even just because you want to and can retain your mind-state. Gods, angels, or what have you–maybe they ~can~ exist. Physicists are just now beginning to realize the need for an accurate explanation on the substance of the seemingly immaterial mind. Where and how did we get these immaterial representatives of our thoughts? And why?

    ET is not something we one day had a dream of and started passing around booklets and giving sermons about. It’s an idea based on the observations of our very material environment. Science enables us to objectify these ideas and test them. If I try to invoke a god or angel with a magic spell written down millenia ago and nothing happens (or something), then scientifically there is(n’t) evidence present to support my belief that they exist–If I threw a handful of microbes into Europa’s oceans and they start multiplying over millions of years (or die within a couple of months), I now have evidence present to support my belief that life can(not) exist there.

    If I haven’t sent someone or something there to scan for life as we know it, then I haven’t created a basis to verify or deny. It’s simple.

  • ljk August 12, 2014, 10:34

    Eniac, while you are correct that Jo(umlat) took the opposite view of my original statements, I do like and agree with what she interpreted. :^)

    For the record, I once attended a friend’s child’s elementary school science fair. I found one student had done his project on the Moon and was using a textbook from 1963 to show the theories of how our natural satellite formed, which included the one by Charles Darwin’s grandfather about how Earth spun really fast back in the day and a piece of our moltenish planet broke off to form the Moon. The place it snapped off from became the Pacific Ocean. Yeah.

    Another student, the one I really want to relate, did her project on the whole Sol system. When I casually mentioned to her that now we know about many other planets around other stars, she looked at me with genuine surprise and even concern. When I added that there could also be life on some of those worlds, I got the impression I was telling someone from 1387 that Earth is not the center of the Universe nor does everything circle around it.

    Please note I was not trying to be mean or show off, I just assumed if someone had done their homework on astronomy that alien planets would come into their knowledge view – especially if they are kids. I was honestly flabbergasted (I just wanted to use that word in a real context) how these children not only did not know these things but that no adult teacher or parent helped to correct or educate them on this. Maybe I am just paranoid and grant you it was a very small data sample, but if our children are not being informed properly on such important subjects as astronomy, no wonder we have so many adults who do not know what stars are or think that the Sun orbits Earth or that travelling between suns is a trivial matter alleviated by a pinch of dilithium crystals.

    Eniac, I will address your comments about life in our Sol system next.

  • ljk August 12, 2014, 10:55

    Eniac said on August 7, 2014 at 21:38:

    “Oh, we have looked. We have excluded a thriving culture of moon people, jungles on Venus and grand water infrastructure on Mars, all in less than a century. Sure, there is plenty more to rule out, but you can’t say we haven’t gotten off to a good start.”

    Yes, we got rid of the big bugaboos of alien life as perceived in the previous centuries, but then again we were also pretty clueless about many of their aspects until the Space Age when we could get a good closeup look.

    Despite this, we haven’t done enough either in terms of probe exploration of our Sol system or with SETI. There are a number of bodies that do have a less than zero chance of being habitats for native life forms – Europa, Enceladus, and Titan are the big three and of course there are others. We haven’t done nearly enough to determine the state of these worlds. Only Titan has gotten a brief landing at one spot so far.

    Did you see the news about finding bacteria living in a body of oil?


    This has very interesting implications for Titan, which we will not be able to determine until we get a serious, dedicated series of missions there to find out!

    As for SETI, the best we have done so far is determine that largely in the case of radio waves that either nobody is shouting in our direction or we are really clueless as to how to tap into the galactic party line. SETI has always been underfunded, understaffed, and undersupported. How can we honestly expect it to make such an important determination about technological ETI under such conditions?

    Eniac, all I am saying is – and I think you will agree with me here – is that we have really only begun to explore our cosmic neighborhood. So much remains to be learned and it will likely happen only when we can get space vessels there. While I may be disappointed and surprised if there is no life elsewhere, I won’t be nearly as frustrated and impatient if we continue to only speculate and base our thinking on insisting ETI needs to contact or visit us in order to prove their reality to us. That to me is ancient thinking and hubris. Besides, another intelligent species might be wondering the same thing, so why aren’t we signalling them or sending out probes? We know the answer to that question.

  • Eniac August 16, 2014, 11:14

    LJK: Mostly agreed. While our increasing knowledge has effectively reduced the potential for life in the solar system over time from thriving civilizations to microbes in un-Earthly environments deep underground, evidence for or against life outside the solar system is practically absent.

    Keep in mind, though, that the Fermi paradox is not about looking and not finding. It is about colonization, and the realization that if interstellar colonization happens, not much looking would be required to see the result just a few million years later, anywhere in the galaxy.

  • ljk August 17, 2014, 17:50

    About colonization, we keep expecting huge astroengineering projects. What if the really advanced ones are working on a very tiny scale? As discussed here back in February:


    About ETI visitation rates, recall this article:


    Which links to this piece:


  • ljk August 18, 2014, 15:54

    Scientific Error Delayed the Search for Alien Life

    Mark Strauss

    August 18, 2014

    In the 19th century, astronomers believed that solar systems formed in nebular clouds throughout the universe. But 20th century scientists rejected that idea, arguing that our solar system was an aberration. This mistake derailed the search for exoplanets, and extraterrestrials, for decades.

    Scientists of the early 20th century argued that tidal forces had caused the sun to spit out the planets when a rogue star passed too close. It was a kind of drive-by shooting theory of planetary formation known as the “Planetesimal Hypothesis.”

    The idea held sway as the dominant theory for planetary formation for nearly forty years—during which time it had a profound impact on how we viewed geology, climate, evolution and the possible existence of life on other planets. One geologist even proclaimed it to be “the greatest contribution to theoretical science since the Darwinian renaissance in biology.”

    In retrospect, that level of enthusiasm is a bit embarrassing, since the Planetesimal Hypothesis was eventually proven to be mind-bogglingly wrong. But, at the time, scientists embraced the idea, since it offered a practical alternative to the existing theory, the “Nebular Hypothesis,” which was slowly collapsing beneath the weight of new discoveries.

    Full article here:


    Shades of plate tectonics and continental drift.