How solar system arrangements and common planet types don’t match our Solar System.
If you’re a Star Trek fan, then chances are you know about the planetary classification scheme that’s used to identify a planet so you know what to expect of the world before you get there. The solar systems devised were no doubt ahead of their time, however they were modeled largely after the only planetary model the creators had at the time – ours. Hence the reason why most of the rocky planets are nearer their star, while the jovians (large gas planets) were located further out.
Since then, astronomers, using such sci-fi classics as Star Trek as inspiration, have discovered over 1000 exoplanets orbiting dwarf stars of all sorts, and only within a small sector of our interstellar neighbourhood. It has even warranted a planetary classification system; fixed, of course, to represent the true distribution of exoplanets in the galaxy.
Comparing solar systems:
To the right we see our Solar System, click it to enlarge. There are eight planets in our Solar System, which is still more than what any other known solar system has to this date. And notice in the diagram at the top right where the small, rocky bodies are located, compared to the large, jovians.
Keep that in mind…
Now look at this schematic of all the exoplanets discovered not using the Kepler Space Telescope, and compare this to that of our Solar System. Notice where most of the exoplanets fall – Hot Superterrans, Hot Neptunians, Hot Jovians, Warm Jovians, and Cold Jovians. Except for the Cold Jovians (we have two of our own), these groups contain planets that are, in some cases, completely alien to the planets we’re familiar with.
This planetary distribution found by the Kepler telescope shows again a spread – this time, Hot Subterrans, Hot terrans, Hot Superterrans, Hot Neptunians, and Hot Jovians. Except for the Hot Terrans (Venus goes into this category), these planetary groups are also quite different to what we’re used to seeing.
Below you see the most common solar system layouts we’ve come across so far (generally-speaking). Notice how none of the above looks like our own. So why are there so many hot worlds? Well, we’re working on it…These exoplanetary systems really are alien in most cases, and it’s even forced astronomers to reconsider the model of solar system formation that seemed so accurate before we starting finding exoplanets left, right, and center.
It’s even beginning to look as if our Solar System is the mutant of the galaxy, since it, unlike the vast majority of exoplanetary systems, doesn’t seem to show much signs of the inward migration so prevalent in other star systems. It kind-of begins to make Star Trek look unimaginative, doesn’t it?
6 Comments
KinglyCitrus
December 20, 2013, 07:07This ignores the bias inherent in the Kepler data entirely. Planets that are large and close produce much greater observable effects on their host stars. It’s telling that the vast majority of exoplanets we’ve discovered are “hot Jovians”. Most of them, in fact, are significantly larger than Jupiter. Massive gas giants orbiting very close to small stars are far easier to see than planets of Earth-like size and orbital position, so of course the former are the kind we see the most. I think our statistics right now have far more to do with the relative primitiveness of our instruments than anything else.
REPLYJeffrey Daniels@KinglyCitrus
December 29, 2013, 20:28I thank you for your comment, and apologies for the delay..
REPLYFor both
the radial velocity and transit (which Kepler used) methods, it is much
easier to find huge Jovians orbiting 20x closer to their star than
Mercury than it is to locate an Earth-like planet in an Earth-like
orbit.
However, Kepler was designed to passively observe the entire
field of observation simultaneously — to watch every single star in
that field at once, and detect every single planet in a transiting
orbit.
Thus, I think the Kepler periodic table, as it stands now, is
perhaps the best model of relative exoplanet frequency. Of course,
Kepler only scanned a tiny portion of the sky, and so it is entirely
possible that a different layout will come from scanning a different,
equally tiny portion of the Milky Way.
That being said, the
exoplanets found not using Kepler, which are located all over the place,
largely agrees with the Kepler data, and so I think it’s reasonable to
say that the frequencies shown in the periodic tables is, at this point
in time, accurate.
But I will agree that we are limited in our
telescope technology, and that future centuries of exoplanet discovery
will hopefully confirm or deny the current periodic table distributions.
Since
we’re currently looking to find another exoplanet just like Earth
(since Earth’s the only reference we have in terms of a successful
biosphere), yellow-dwarfs and orange-dwarfs I think are our best
choices. Anything above a yellow dwarf is out for good Earth-like
planets anyways, since their lifespans are too short. Though red-dwarfs
are the most abundant type of star, the fact that Earth-like planets,
as you say, need to orbit so close, and that red-dwarfs are commonly
flare stars, finding life on worlds such as Gliese 581 d or Gliese 667 C
c may be hit-and-miss.
AvangionQ
December 20, 2013, 08:08According to these numbers coming from Kepler, Jupiter size planets typically migrate into the inner solar system, swallowing up any rocky planets therein. That we haven’t found any earth sized planets in the habitable zone yet is no surprise, but that there have been ten super-sized rocky bodies thus found still leaves room for discovery ~ that only a tiny fraction of the sky has been thus mapped leaves room for hope.
REPLYJeffrey Daniels@AvangionQ
December 29, 2013, 20:37Agreed. The beauty of science is that a model for a certain aspect of reality (in this case exoplanets) is seen as accurate so long as all the evidence supports it. Once something comes along that challenges the current model (say, scanning a different portion of night sky), then the model is forced to change in order to keep it accurate with the new aspect of reality.
REPLYMandoZink
December 20, 2013, 18:02It is necessary to note that the Mercurians, Subterrans and Terrans are more difficult to detect, in that order, than the larger-class planets. Many systems we scanned may contain smaller bodies that we failed to discern for various reasons. Our own solar system still is surprising us with decent sized objects we have failed to notice, and they are proportionally massive when considering the distances Kepler is operating at.
My guess is that these “exoplanetary periodic tables” will eventually be revised in the direction of our own system. Our arrangement still may be off of the statistical norm.
REPLYJeffrey Daniels@MandoZink
December 29, 2013, 20:56The five star systems I (so terribly) sketched in this article are some of the most common star systems thus found, drawing from thousands of such systems. The most common systems appear to be those with the least amount of planets in orbit about them.
REPLYOf course we may end up finding more systems more like our own in the future, although I still think it’ll be more common for those systems to contain, say, a couple rocky planets with only a couple more jovians orbiting further out. We already see vague instances of this in systems like 55 Cancri.
Cheers!
JD