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Why don’t exoplanets match astronomers’ expectations? A dispatch from the American Astronomical Society meeting

The views expressed are those of the author and are not necessarily those of Scientific American.


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Kepler 10 b, a rocky exoplanetSEATTLE—The most exhilarating science conference I’ve ever been to took place in San Antonio 15 years ago this week, when planet hunters Geoff Marcy and Paul Butler announced they had found two planets orbiting sunlike stars beyond our solar system. Coming a couple of months after another team, led by Michel Mayor and Didier Queloz, made the first such discovery, Marcy and Butler’s presentation was a veritable Woodstock of astronomy, such was the crush of scientists trying to get into the room. I’ll never forget the chill that went up my spine when they said one of the planets orbited within the habitable zone of its star, the range of distances where liquid water could be stable.

The excitement has scarcely dissipated since then. This week Marcy, now at the University of California, Berkeley, spoke again at the American Astronomical Society’s winter meeting here to take stock. He proceeded to demolish the prevailing theoretical models for how planets form. Observers in any field of science take a peculiar pleasure in seeing their theorist colleagues collapse into sobbing heaps, but it happens with unnerving regularity with exoplanets. Modelers have consistently failed to predict the diversity of planetary systems out there. And they are the first to admit it. "These models are crap," says Hal Levison of the Southwest Research Institute in Boulder, Colo. "They may be the best we can do, but they are still crap."

Most of the hundreds of exoplanets that astronomers have found are Jupiter-size, but a growing number look tantalizingly like Earth. At this week’s meeting came a milestone with the announcement of Kepler-10b [see artist's conception above]. Not only is it the smallest world yet discovered around a sunlike star—1.4 Earth-radii and 4.6 Earth-masses—it has a high density that matches an Earthly composition of rock and iron. NASA’s Kepler space observatory discovered it by seeing a periodic dimming of the star’s light, which suggested that a planet was crossing in front, and the Keck Observatory on Mauna Kea in Hawaii confirmed that the star is wobbling slightly in response to a planet’s gravity. (Even smaller planets orbit a pulsar, a very un-sunlike star, but they get less attention because astronomers are seeking closer cousins to our own solar system.)

Kepler has also found a multiple-planet system around the star Kepler-9. What makes it so cool is that the timing of the dimming varies slightly, confirming that the planets are gravitationally perturbing each other—the first time the dance of the planets has been observed in all its glory. The variation provides a reality check on the existence of planets and an independent way to ascertain their masses. The system has, at least, two Saturn-mass worlds and a 1.6-Earth-radius one.

These discoveries further demonstrate that planetary systems come in all shapes and sizes. Theorists have long since gotten over their surprise that most systems look nothing like our solar system. Still, they argue, systems should have some common features. Marcy went down the list. Planets should go around their stars on nearly circular orbits lying in the equatorial plane of their stars. They should move in the same direction (clockwise or counterclockwise) as the star spins. Over eons, planets can leave their birthplace and migrate to other parts of their system—a process that should clear out any smallish planets from the region immediately around the star. Moreover, to migrate inward, a planet must transfer angular momentum to more distant material; thus, any tightly orbiting planets—including the plentiful breed of massive planets known as hot Jupiters—should be accompanied by more distant ones.

Marcy took down these predictions one by one. Lots of planet orbits are highly elliptical and tilted. As many as a third move in the opposite direction as their star spins, a fact deduced from the way the periodic dimming of starlight brings out the Doppler shift of the spinning star. Perhaps an eighth of sunlike stars have close-in smallish planets. And not a single close-in planet has a more distant companion. Marcy called the success rate of theorists "shocking and a little disturbing."

Theorists dispute some of Marcy’s specifics but broadly agree with his critique. "The issue of hot Jupiters without additional companions needs serious attention," says Doug Lin of the University of California, Santa Cruz. "There is no obvious reason for it." The question is whether he and others can fix their basic scenario for the genesis of planets or will have to give it up.

That scenario, known as core accretion, supposes that planets start as small grains of dust that agglomerate to progressively larger sizes. Some sweep up gas and grow into giants. An alternative scenario, gravitational instability, holds that planets start as large clouds of gas that fragment and collapse under their own weight, much as stars are thought to do, but on a smaller scale. It has fallen into disfavor, though, because it would explain gaseous Jupiters but not rocky Earths.

Although Levison bluntly calls core-accretion models "crap", he sees their failure as one of implementation, not of basic principle. Planet formation is complex, and modelers don’t have enough computing power to run full-up simulations for all possible permutations of planetary systems. To make statistical predictions, they rely on simplified computer codes, and this, Levison says, leads to the failings that Marcy identifies. "The only lesson to be learned from the fact that the models do not reproduce the observations is that the modelers need to work harder," he says. "I do not think it endangers our basic concepts of planet formation." Lin agrees: "I still believe core accretion scenario is a good paradigm to develop the theory of planet formation. But it needs to become more sophisticated."

Others are less sanguine. Over the past several months, Sergei Nayakshin of the University of Leicester has mashed up the two standard scenarios to create a radical new one. In it, gravitational instability first creates a family of giant planets at large distances from the star, way out past the present-day orbit of Neptune. Within each, solid material settles to the core. They all migrate inward, and the star strips the innermost planets of their outer gaseous layers, reducing them to their rocky cores [see video below]. Accretion rounds them out. In the core-accretion scenario, gas giants are bulked-up rocky words. In Nayakshin’s, it’s the other way around: rocky worlds are slimmed-down gas giants.

Nayakshin claims his model can account for the discrepant observations. The early stages of the process are fast and messy and could lead to orbits of all sorts of shapes and tilts. Because planets form far from their stars, any material left over after they migrate need not be enough to give rise to new planets. "Multiple planets aren’t needed in hot-Jupiter systems in my model," he says.

That said, Nayaksin’s model has yet to receive the same scrutiny as the other scenarios have. When it comes to the universe of planets, nature has a way of confounding scientists’ best ideas.

 

Artist’s depiction of Kepler-10 b: NASA/Kepler Mission/Dana Berry

 





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  1. 1. tacitus5 5:06 pm 01/13/2011

    "Perhaps an eighth of sunlike stars have close-in smallish planets. And not a single close-in planet has a more distant companion"

    How do we know that? We can indirectly observe some extrasolar planets, but no method can tell us, that the inferred planets are all the planets of any system.

    It may be, that with more sensitive methods we would find additional planets in the hot-Jupiter systems.

    Link to this
  2. 2. Bruce Voigt 5:42 pm 01/13/2011

    quote tacitus5 – It may be, that with more sensitive methods we would find additional planets in the hot-Jupiter systems.
    —————-

    Or we might look a little closer to home.

    Our NINTH Planet third or fourth from the sun comes bar-ling around the Sun appearing to be heading right for us.

    I Bruce Voigt claim that the recently discovered Asteroid 2002 AA 29 is really a Moon of (and yes I have named this Planet) OLD BRUCE.

    OLD BRUCE orbits the Sun in close proximity to Earths orbit every 365 point something day’s and the Earth is slowly catching up or visa versa.

    Link to this
  3. 3. Wayne Williamson 6:56 pm 01/13/2011

    Love these kind of findings….ie the current "main stream" thought may not be the best….I personally always hated that the planets formed after the sun ignited…just doesn’t make sense…

    Link to this
  4. 4. Cosmoknot 11:24 pm 01/13/2011

    Sounds like there are some very basic assumptions astronomers make that are wrong. Accepted assumptions community wide that lead them astray.

    Link to this
  5. 5. rloldershaw 12:23 am 01/14/2011

    The CoRot Team has just submitted a very interesting preprint :

    http://arxiv.org/PS_cache/arxiv/pdf/1101/1101.1899v1.pdf

    describing a remarkable new planetary system. CoRoT-14b is a 7.6 Jupiter-mass planet orbiting a F9V star at less than 5 stellar radii, with a period of 1.5 days!

    It seems like one would have to do quite a bit of analytical hand-waving in order to produce such a system within the ancient Laplacian formation scenario.

    On the other hand if free-floatiing planets, which are regularly observed in star-formation regions ( see: http://arxiv.org/PS_cache/arxiv/pdf/1012/1012.5915v1.pdf
    , if you do not believe it), were captured by stars, then such an "unexpected" system would be actually be fully expected.

    Furthermore the planetary capture model can explain the following.

    (1) Why a sizeable fraction of planets orbit in retrograde orbits.

    (2) Why many planets orbit in planes that are different from the equatorial plane of the host star, and some orbit at large angles to that plane.

    (3) Why non-circular orbits are not uncommon.

    (4) How exotic systems like pulsar/planet systems form.

    Maybe its time to radically rethink our ancient models and assumptions regarding the formation of stars and planetary systems?

    RLO
    http://www.amherst.edu/~rloldershaw

    Link to this
  6. 6. jtdwyer 2:50 am 01/14/2011

    Well put – the conditions you list almost demand that the subject planets were captured from elsewhere.

    I recall a recent article: "Alien Planet May Have Come From Another Galaxy", describing a giant planet orbiting close to an ancient, violent, metal-poor star. The star is a member of the Helmi stream, an orbital ring of ancient stars that cuts through the plane of our galaxy, believed to have been captured from a local galaxy ripped apart by the Milky Way. Please see: http://www.sciencemag.org/content/330/6011/1642

    That the star was ‘swallowed’ from a smaller ‘merged’ galaxy and the gas giant orbits close to a violent star indicates to me that the planet may have been captured (from another Milky Way star) by the alien star as it and its brethren were violently captured by the Milky Way. Violence begets violence, even in this neighborhood.

    That so many planets (detectable with current technology) may have been captured indicates that stellar systems are (over periods of billions of years) often subjected to highly disruptive events.

    My ‘favorite’ subject is that dark matter is thought to be necessary to explain why the rotational velocity of stars in the planar discs of spiral galaxies do not diminish with distance from the galactic center. I think that general characteristic of rotational curves described by the laws of Planetary Motion result from the extremely centralized mass of stars in planetary systems.

    However, I wonder how many newly discovered planetary systems (once smaller planets can be detected) will be found to also violate the established laws of Planetary Motion, especially the diminishing rotational curve?

    Link to this
  7. 7. rloldershaw 12:38 pm 01/14/2011

    The most important implication of the recent exoplanet observations, which are radically different from anything predicted by the standard astrophysical models of the 1980s, is that the Laplacian "collapse of gas-dust clouds" scenario is a wholly inadequate model for star formation, and for star-planet formation.

    The Laplacian collapse scenario always sounded like strained hand-waving to paper over the fact that we did not have a clue about star, planet and star+planet formation.

    A much better paradigm is primordial "free-roaming" stellar-mass and planetary-mass ultracompact objects that can interact through scattering, or becoming bound systems, or becoming unstable by doing too much of the latter.

    This is what atomic scale systems like nuclei and electrons do, and it may also be what stellar scale systems do.

    RLO
    http://www.amherst.edu/~rloldershaw

    Link to this
  8. 8. Bruce Voigt 2:15 pm 01/14/2011

    jtdwyer wrote:
    My ‘favorite’ subject is that dark matter is thought to be necessary to explain why the rotational velocity of stars in the planar discs of spiral galaxies do not diminish with distance from the galactic center. I think that general characteristic of rotational curves described by the laws of Planetary Motion result from the extremely centralized mass of stars in planetary systems.
    ————————————

    Science will tell you that our Universe is expanding out ward into space at an expeditious rate of speed.

    There are little orbits and there are big orbits, no matter the size, what goes round comes round with no beginning and with no end.

    Sending a satellite into orbit we know the thing is orbiting in a circle but sending it away from Earth it’s god only knows where it will end up. My discovery shows this satellite coming right back to where in space it was launched from. It will make many turns in its travels but and is never to, that far away from home!

    Would it not be nice to understand new true science of why and how things are attracted and stick to each other, really simple STUPH! (not complicated at all)

    Explaining why the quark, molecule, cell, atom, magnetism etc attract and repel would soon have eyes glazing over. I will make it real simple for you to understand why bread crumbs don’t stick to the plate yet toast crumbs and egg do, why snow flakes and rain drops are separated.

    OK this is what you do. Sit your-self down in front of the six o’clock news and take note of the weather report. Animation shows the high pressure area (clockwise) and close by will be a low pressure (counter clock wise). If you look carefully its easy to see the forces of the low melding into the forces of the high (attracting). Looking closer, take note that the forces of the high and low merge into a figure 8 and thats how every natural thing having multiple orbits is held together. Look for two highs or two lows and you can see where the forces are running into each other (repelling).

    Take this to the smallness of the quark or the hugeness of the heavens and Bobs your uncle!

    Link to this
  9. 9. rshoff 3:00 pm 01/14/2011

    With so many planets out there, why no contact? It could be that the habitants of exoplanets are evolved but choose to be silent, evolved beyond interest in us, out of phase with our place in evolution, or possibly many other passing-in-the-night scenarios. Most curious to me would if the manifestation of intelligence is a short phase in evolution of life. In other words the silence from all of these (tip of the iceberg) exoplanets indicates to me that we can expect Human intelligence will soon be displaced by evolutionary forces.

    Link to this
  10. 10. Bruce Voigt 7:32 pm 01/14/2011

    tacitus5 wrote: that the inferred planets are all the planets of any system.

    It may be, that with more sensitive methods we would find additional planets in the hot-Jupiter systems.
    —————————–

    Forces that are cork screwing their way to each of Earths poles, exit the planet as an immature air cell. These cells are in evolution and must first evolve or mutate to other gasses (OZONE) etc.

    The Ozone at this point is still an immature (force) and will not be detected by infrared. This will be incorrectly interpreted by people that are not knowledgeable in this new technology as to be a HOLE in the OZONE!

    rshoff wrote: With so many planets out there, why no contact?
    ——————

    A person is put in a sound-proof box and is hooked up to electrocardiogram etc. Oblivious to what is going on outside the box, the box is taken to a packed arena where a very exciting game is in progress. This box will absorb the aura or info cells of the crazed crowd. The person in the box, without knowing why, will feel or experience the excitement of the crowd which will show on the electrocardiogram.

    I find most Chinese people look alike. When I visit Richmond I think I know everyone. It’s no wonder we have not been contacted by aliens as they would probably see all Earth species as looking alike and are out there trying to communicate with a tree, or my ex-wife.

    Link to this
  11. 11. bewertow 10:29 pm 01/17/2011

    jtdwyer, what does this article have to do with dark matter? Why must you bring this up in every article?

    Link to this
  12. 12. jtdwyer 5:40 pm 01/19/2011

    I don’t bring it up in every article. In fact, I respond to many other subjects and never bring it up at all.

    It’s relevant in this case because dark matter was intended to compensate for the fact that galactic rotational curves did not comply with the laws of Planetary Motion. Many exoplanet gravitational systems contain massive stars orbiting ‘too close’ to their stars.

    I suggest that when smaller planets can be detected in these systems, they too will also be found to violate the law of Planetary Motion.

    If that is the case, it must be considered that the relationship between distance and rotational velocity in a centralized mass gravitational system simply describes the most stable rotational velocities for isolated planets, not a law of orbital systems.

    I hope this explanation meets with your approval, if you understand.

    Link to this
  13. 13. Ashjay 1:21 am 01/22/2011

    The summit of human intelligence lies in the discoveries of unidiscovered heavenly civilizations!

    Link to this
  14. 14. rshoff 1:28 pm 01/22/2011

    Ashjay, with the sheer volume of exoplanets and the age of our own universe combined with the expectation that the force of life evolves through a finite variation (nothing is really unique, nothing) , it’s not possible that human intelligence is enduring. If it were, there would be no undiscovered heavenly civilizations. We would be the discovered if not the discoverer. We would be gossiping at the water cooler by now. The fact we are not, means our intelligent window into the universe is a limited view. If not by uniqueness by our nature, which is unlikely, then by the dimension of time.

    Link to this

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