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I KELT-4Ab Orbital Characteristics

  1. Apr 2, 2016 #1
    I am trying to determine the orbital characteristics of KELT-4Ab, given that KELT4Ab orbits its star every 2.9895936 ± 0.0000048 days, and assuming it has an extremely small orbital eccentricity (< 0.01).

    I come up with a semi-major axis of 6,460,182 km (0.04318 AU), which gives it an orbital velocity of 157.144 km/s. It also means that the planet would be tidally locked. Furthermore, given that the effective surface temperature of the star KELT-4A is 6,206°K ± 75°K, with a radius of 1.61 R, at a distance of 0.04318 AU, that would give KELT-4Ab an effective surface temperature of ≈1,553.84°C (not factoring in Albedo).

    This would imply that KELT-4Ab could not have formed in its current orbit, but had to have migrated to its current position from a distance of at least 6.4 AU, where the temperature would be 150°K (a.k.a. "frost line") for KELT-4A.

    First, given the information (see Source below), does the orbital characteristics above make sense? Second, does anyone have an idea how a Jupiter-like planet could have migrated 6.363 AU closer to its star and still maintain a stable orbit?


    KELT-4Ab: An inflated Hot Jupiter transiting the bright (V~10) component of a hierarchical triple - Astronomical Journal, Volume 151, Number 2, DOI: 10.3847/0004-6256/151/2/45, Published February 4, 2016 (arXiv free reprint)
  2. jcsd
  3. Apr 7, 2016 #2
    Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
  4. Apr 7, 2016 #3
    I don't know anything specific about this one, but 'Hot Jupiters' seem now to be unexceptional.
    Not commonplace, but not rare either.
    Perhaps our expectations of other star systems and their planets was a bit solar system-centric before we recently gained ability to look at them.
    Last edited: Apr 7, 2016
  5. Apr 7, 2016 #4
    I agree, hot gas giants are not that unusual. Which implies that there must be some mechanism that causes these planets to migrate closer to their parent star. Whether or not it is the same mechanism for each of these migrating gas giants or a completely different mechanism, I couldn't even begin to speculate. Although it is unlikely to be a rapid change in the planet's orbit, or the orbit would be much more eccentric. Which would seem to rule out a collision as being the cause.

    Furthermore, when the gas giant formed at, or beyond, its frost line it would have had a much slower orbital velocity than it does in its current orbit. In order for the gas giant to migrate closer to its star, it would have to somehow increase its orbital velocity since its orbital velocity determines its orbital distance.

    Just for kicks I calculated KELT-4Ab orbital velocity if it were at 6.4 AU (the frost line for KELT-4A) and came up with 11.8 km/s with an orbit of 16.2 years (5,911 days).
    Last edited: Apr 7, 2016
  6. Apr 7, 2016 #5
    One possible mechanism for orbital decay might be that the planet once formed loses angular momentum because there still is a lot of stuff from the original nebula flying around the star in random directions, acting as a sort of friction.
    Just guessing though, I'm sure there are other ways it could be explained.
  7. Apr 7, 2016 #6
    It would certainly have to lose the overwhelming vast majority of its original angular momentum to be in its current orbit, but if it was caused by "stuff" would not the planet also lose angular velocity? As the radius of the orbit decreases, the angular velocity of the planet would have to increase (assuming the mass does not change). If the mass increased as the radius of the orbit decreased there would be much less loss of angular momentum, but angular velocity would still have to increase.
  8. Apr 7, 2016 #7
    I wonder about our own Jupiter.
    Do we know if it's destiny is to become closer to the the Sun? .. or more distant?
    Is there any theory strongly suggesting one way or the other?
  9. Apr 7, 2016 #8
    I am not certain of Jupiter's destiny, but I do know that it probably has not always been in the same orbit that it is in now. One paper suggests that Jupiter was formed at 3.5 AU and moved as close as 1.5 AU before settling down at 5.2 AU.

    In this particular case they suggest that when Jupiter and Saturn reached a 1:2 orbital resonance Jupiter migrated inward. When a 2:3 orbital resonance between Jupiter and Saturn was achieved, Jupiter began moving outward again. This took place over the course of 10,000 years according to their simulation. So the orbits of two neighboring gas giants appear to be the mechanism in our case. Since KELT-4Ab is the only known gas giant in its system, there would have to be another mechanism involved.


    http://www.nature.com/nature/journal/v475/n7355/full/nature10201.html - Nature 475, pages 206-209, July 14, 2011, DOI: 10.1038/nature10201 (arXiv free reprint)
    Last edited by a moderator: May 7, 2017
  10. Apr 8, 2016 #9


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    The increase in angular velocity comes from orbital mechanics. All you need is some sort of friction force against the direction of motion, and the planet will get closer and faster (sounds weird, but that is orbital mechanics).
  11. Apr 8, 2016 #10
    Then expanding upon what rootone suggested, and the model Walsh et al. proposes in the above paper, a large debris field with enough gas, "embryos, and planetesimals" in the early formation of the solar system could be have caused a ~10 Jupiter mass gas giant (that originally formed ~6.4 AU from its parent star) to lose enough angular momentum, gradually over time, to form a stable orbit ~0.04 AU from its parent star. Since there is no known gas giant neighbor in the KELT-4A system, KELT-4Ab would not have been able to form a 2:3 orbital resonance with a neighboring gas giant to get pulled back out again, like Saturn did for our Jupiter. KELT-4Ab's inner orbit would then have been determined by the inner edge of the debris field. KELT-4Ab could have also increased slightly in mass as it migrated inward, so that the loss of its angular momentum might not have been so dramatic.

    It is certainly the most plausible, not to mention the only, mechanism for gas giant migration I have heard yet. :cool:
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