More than hot Jupiters

One of the nice things about the Extrasolar Planets Encyclopedia, which I came across whilst writing about Kepler, is that you can produce plots which summarise the properties of all the extra-solar planets discovered so far. I was particularly interested in the plot below, of orbital period (in days) against mass (scaled relative to the mass of Jupiter), to which I’ve added Earth, Venus and Jupiter for reference.

mass and orbital period of known extra-solar planets

In terms of mass, it’s clear that most of the planets discovered so far are big gas giants, on the scale of Jupiter or bigger, with a scattering of smaller gas giants and Super Earths. If, as I’ve argued before, this reflects the limits of our current detection technology, in the next few years Kepler and CoRoT should increase the number of points in this range, and below it (for those of you wondering about the ultra-small outlier, it’s orbiting a pulsar).
More interesting is the distribution of orbital periods, which is actually bimodal: there’s a cluster in the 1-10 day range, representing the ‘hot Jupiters’, and another, larger, cluster of gas giants with orbital periods of about 1-10 years, which Jupiter is just about on the edge of. It’s probably my fault for not paying enough attention, but I was genuinely unaware that so many of this latter type had been discovered: I was under the impression that almost all of the extra-solar planets discovered thus far were sun-skimming hot Jupiters, if only because they are much easier to detect. The fact that we’re detecting gas giants in a much more familiar place (when compared to our own galactic neck of the woods) is actually quite encouraging, and actually increases my confidence that we don’t need to chuck out the Copernican Principle just yet.
Update: for those in the comments who astutely pointed out that many of these planets have a much higher eccentricity than planets in our solar system (~0.05 on average), it turns out this may indeed be an issue:

“If Jupiter’s orbit around the sun was just a bit more eccentric (oblong), it would have scattered a lot of the material that delivered water to the Earth, kicking it out of the solar system instead,” [Sean Raymond, who has been modelling extra-solar planet formation] says. “The result would have been an Earth that had only 10 percent of the water it does now.”

Interestingly, the passage of “hot Jupiters” from the outer solar system where they form to their sun-skimming orbits, whilst disruptive to the formation of the rocky inner planets, does not preclude their survival.

Categories: geology, planets

Comments (9)

  1. Markk says:

    As you get older ever so often you have these mind blowing moments where you realize the future is really a new world. Look at this chart of PLANETS IN OTHER SOLAR SYSTEMS! Whoah. In my life, and zillions more to come! The times are tough and all that, but it is nice to be alive now!

  2. Navy Davey the Quartermaster says:

    Soon we may have hundreds of Earthlike planets. Yet SETI has nothing. NADA! Reasons abound, theories mostly of course, but the fact remains that we have zero contacts via radio waves. There are messages there but we haven’t realized it yet.
    If any extra-terrestrials have visited us in the past, there is absolutely nothing dug up nor discovered anywhere on earth or under it nor in the oceans nor lakes. Odd, no?
    I hope there are some advancements in CONTACT in our lifetimes.

  3. andy says:

    However the majority of the planets in the long-period clump have orbital eccentricities significantly higher than those in our solar system.
    Only a handful of long-period, low-eccentricity systems… not vanishingly rare, but apparently not an especially common architecture either.

  4. geekosaur says:

    Actually, I’d say that is unclear; quite possibly it’s the irregular ones that make enough of a “splash” to be noticed, and with better tech we’ll find lots of regular long-period gas giants.
    As for me, I’m still stunned at hot gas giants. I’d give a pretty penny to see one of those systems close up.

  5. andy says:

    You mentioned the pulsar planet at the bottom of the diagram. Every so often someone claims some recently-detected super-Earth with a few times Earth’s mass is the “smallest exoplanet ever detected”. However that particular pulsar planet was discovered way back in 1994 (i.e. before the famous discovery of the hot Jupiter planet orbiting 51 Pegasi), and has a mass approximately twice that of our moon. Seems that the people writing the press releases don’t know the history of the field…

  6. Lab Lemming says:

    “The fact that we’re detecting gas giants in a much more familiar place (when compared to our own galactic neck of the woods) is actually quite encouraging, and actually increases my confidence that we don’t need to chuck out the Copernican Principle just yet.”
    Um, have you looked at the eccentricity of the orbits of those “jupiter-like” planets?
    The mean is higher than the most eccentric planet we have here…

  7. Chris Rowan says:

    That’s true: but ‘our’ planets still seem to fall within the main distribution, even if we’re towards one end of it.

  8. Lab Lemming says:

    You need to add detection limit lines to that first plot for it to be meaningful.

  9. Chris Rowan says:

    Modelling indicates that large eccentricities can indeed be quite disruptive. See the update at the end of the post.