How Weird Is Our Solar System?

Posted: August 24, 2014 by oldbrew in Astrophysics, solar system dynamics

Exoplanet: artist's interpretation [credit: NASA]

Exoplanet: artist’s interpretation
[credit: NASA]

Why haven’t exoplanet searches uncovered any solar systems similar to our own? Most appear to have fewer planets – although detection can be difficult – than ours, and often orbit a lot closer to their star than our planets do, plus there’s something else.

astrobites asks: ‘Earth and its Solar System compatriots all have nearly circular orbits, but many exoplanets orbit their stars on wildly eccentric paths. Is our home system strange? Or is our sense of the data skewed?’

One conclusion is that eccentricity declines as the number of planets in the system increases.

Read the rest here: How Weird Is Our Solar System? | astrobites

There are exceptions though. The two known planets orbiting star HD 45364 are in a 3:2 mean motion resonance i.e. 3 orbits of one takes the same time as 2 orbits of the other.

A 2009 paper notes: ‘This is the first time that such a resonant configuration has been observed for extra-solar planets, although there is an analogue in our Solar System formed by Neptune and Pluto.’

HD 45364, a pair of planets in a 3:2 mean motion resonance

  1. crosspatch says:

    I don’t think planets like ours would be noticed at the great distances we are talking about. Only really massive planets orbiting close enough to cause a noticeable change in light from the star or some wobble in its movement will be noticed. Mercury, Venus, Earth, and Mars are too small to create any significant change in transit when viewed from any great distance and the large planets are too far from the sun. Also, we only notice planets where we are looking along the ecliptic of these stars and we can notice a transit change.

  2. Sparks says:

    Hypothetically, if we assume that main sequence stars without a noticeable wobble in it’s movement, have a similar planetary system as our own solar system, then our solar system wouldn’t be that weird or unusual, it should also help narrow down which stars we should be looking at to find solar magnetic polar field reversals.

    We already know of other stars with polar field reversals, these are stars with giant planets orbiting them and their solar field reversal occurs over weeks and months, whereas the sun’s field reverses at rate of about 11 years.. so therefor, maybe a star with a smaller planetary system would have a field reversal that takes much longer to complete, possibly at a rate of over 30, 50 or 100 years and longer depending on the planetary configuration.

    It seems to me that there is a possibility of this type of solar planetary relationship occurring. and it can be proven or falsified, but it shouldn’t be dismissed outright and declared that our sun and solar system is weird and unique when we don’t have enough data for verification either way.

  3. oldbrew says:

    AFAIK the longest orbit of an exoplanet so far detected is 704 days (Kepler-421b). That’s about the same as Mars, which begs the question: is our solar system unusual, or are we just not able to detect distant (from their star) planets in other systems with current methods?

    (for comparison: Neptune’s orbit is 60,189 days)

    A bit more on the detection question:
    ‘In general, planets further away from their host star have a lower probability that their orbital inclination is small enough that they appear to pass in front of their parent star. A planet like Kepler-421b has a minuscule 0.3% chance of transiting, and yet here she is. To make things worse, with only two transits observed by Kepler, the signal is much harder to find than a planet on a shorter orbital period which would hundreds of transits over the same time range. Put together, these effects generally reduce the yield of long-period planets in transit surveys by a scaling of P^(-5/3). This means that a hot-Neptune (~3 day orbital period) is ~9000 times easier to find than a frost-line-Neptune, like Kepler-421b. Although it’s a bit premature to go through an occurrence rate calculation, since we have just a single planet in this regime, it seems likely that these cold-Neptunes are everywhere, but just very hard to find.’

  4. Sparks says:


    My view is most Astronomers are just not looking, here’s an example;

    Tau Boo’s polar magnetic field rotation takes only two years to complete compared to 22 years for the Sun. It is host to a giant exoplanet about six times the mass of Jupiter, which orbits Tau Boo every 3.3 days. The interesting thing about this is, Tau Boo is one of two binary stars that orbit each other, this configuration behaves the same way Uranus, Jupiter and our sun interact. Tau Boo is also slightly hotter and brighter than our sun and more active.
    We can probably measure the resonance between the binary stars and the giant exoplanet with Tau Boo’s polar magnetic field rotation. I can’t find if this has been done, but it would be very interesting if the results are similar to the sun and planetary orbital resonance.

  5. tchannon says:

    Longer time orbits take longer to measure.

    Now what is the curve of likely to be found even though there?

    [reply] hard to say until a few are found? Estimate above says 9000 times harder to find

  6. oldbrew says:

    ‘Tau Boo spins on its axis once every 3.3 days — the same amount of time as it takes the hot Jupiter to complete one orbit.’

    Spin-orbit coupling 🙂

  7. oldbrew says:

    They did find one distant exoplanet but didn’t give an orbit period. It made a headline in the popular press.

    At the quoted 650 AU from its sun (Earth = 1 AU from our Sun) its orbit should be roughly 16570 years [square root of the cube of 650], making the current ‘record’ of 704 days look a bit puny.

  8. Curious George says:

    oldbrew says: Tau Boo spins on its axis once every 3.3 days — the same amount of time as it takes the hot Jupiter to complete one orbit.

    Maybe the “hot Jupiter” is just a “Great Red Spot”. We know so little about magnetohydrodynamics of huge bodies.

  9. Sparks says:


    I don’t see how a 3.3 planetary orbit and the 3.3 day axial orbit of Tau Boo translates to a complete magnetic polar field reversal every 2 years with the “Spin-orbit coupling” alone. There has to be a secondary influence on the star, what I think is going on is that the polarity of both binary stars are changing due to;
    1) Their orbit around the mutual center of gravity, without the planet the stars polarities would always be at rest probably even facing each other negative to positive.
    2) The planet orbiting Tau Boo interrupts this solar polarity attraction/repulsion over the course of the binary stars orbit around the mutual center of gravity setting in motion the field reversal over a period of a year for one solar cycle and again for the full two year cycle.
    3) The distance traveled by a planet in a coupled spin-orbit is irrelevant because both the star and the planets position relative to each other never changes.

  10. oldbrew says:

    No I don’t see a link between 3.3 days and 1 or 2 years either. But the spin-orbit 1:1 is interesting.

  11. w.w.wygart says:

    “The world isn’t only stranger than you suppose, it’s stranger than you CAN suppose.” ~ JBS Haldane

    That said, I think it’s way to early to be theorizing about exoplanets, we are still completely in the discovery and exploration phase. In twenty years we’ll have enough reliable data to have some hope in determining what the range of ‘normal’ actually is – at least in the very near solar neighborhood.

    Still, its always fun to spin yarns.