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How about hot jupiters and super-earths? December 5, 2009

Posted by Jorge Candeias in Extrasolar planets, Giant planets, Terminology, Terrestrial planets.
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A few more pistachios in the belly, and some more ideas coming out of both the posts themselves and the comment boxes. Particularly this comment by Bob Shepard, where he proposes a very detailed classification scheme for the planets, inspired by the spectral classification of stars.

As I told him, with less than 500 known planets (primary and secondary, belt and main, solar and extrasolar) I don’t really see the need for such a detailed scheme for the moment. But I certainly admit that it may be useful in the future, when the number of known planets starts getting astronomical, pun definitely indended. And I may even be wrong right now about this lack of necessity. You see, a more detailed classification scheme is already emerging in exoplanetology. Organically, kind of .

If you browse the literature you’ll find terms such as hot and cold jupiters, cold and hot neptunes, super-earths, etc. These classes of planets are usually not very precisely defined, but that doesn’t stop them from being profusely used, which is a clear indicator that it is felt that they are needed. A “jupiter”, for instance, is defined as a planet whose mass “is close to or exceeds that of Jupiter”, and Jupiter and Saturn are usually indicated as Solar System examples of such planets. Since Saturn’s mass is less than 30% of that of Jupiter, this means that this category might range from some 0.25 MJ to the limit of brown dwarfs (or, as I prefer calling them, planetars as in “intermediate object between planets and stars”), which is about 13 MJ.

However, the “neptune” class of planets gets more definitive limits, ranging from 10 to 30 Earth masses (or ME). In our system, Neptune and Uranus are included in this class and, unless someone comes up with an intermediate class between jupiters and neptunes, this means that jupiters in fact range from 0.0945 MJ to 13 MJ. It’s quite a large interval, including the vast majority of extrasolar planets discovered so far, so it’s possible that intermediate class will indeed appear.

Both the neptunes and the jupiters would fit under my giant planets category, but the next class that has emerged organically in extrasolar studies, the “super-earth” class, would belong to the medium-sized planets. This one, however, is very poorly defined indeed. Although the upper limit is pretty solidly set at 10 ME, some astronomers set the lower limit at 5 ME, wereas for others any planet that is more massive than the Earth is a super-earth. Personally, I think these two perspectives may be a bit too extreme. An interval of 5-10 ME seems too restrictive, while starting super-earths with planets that are basically Earth twins, only slightly more massive, seems to stretch the term a bit too much. I’d call super-earth to planets of no less than 2 or 3 Earth masses, with a slight preference to a range of 3-10 ME.

And that’s it, really. No other size-based classes of planet have been widely used outside theoretical studies of planetary formation, i.e., with real exoplanets, which is, of course, explained by the fact that the first planets to be spotted are always the larger ones and also the closest to their stars. With the exception of pulsar planets, only one planet has been found below 2 ME: Gliese 581 e, a terrestrial planet of 1.94 ME, so close to its star that a year out there lasts little more than 3 days. So there’s no subgroupings below that.

But these three groups are definitely a start in the kind of thing Bob Shepard suggests, only in an ad-hoc, unplanned way. They have the advantage of being born out of necessity and therefore being immediately adapted to the real world, and the disadvantage of not being very orderly.

Hey, nothing is perfect.

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Odd balls August 15, 2009

Posted by Jorge Candeias in Extrasolar planets.
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Here’s something you don’t see every day: last week brought news of not one, but two extrasolar planets that are far more oddballish than anything we have in our Solar System. It started with the announcement that the newly discovered WASP-17b had two very bizarre characteristics. For starters, a planet with its mass, about half that of Jupiter, should be smaller than Jupiter… but that one is much larger. In fact, at twice Jupiter’s diameter (i. e., about 140 000 km), it is the largest planet known so far.

BANG! Strike one.

But what I find really juicy is another fact: WASP-17, the planet’s star, rotates in one direction, and the planet goes round it in the opposite direction.

WHAM! Whoa!

Why is this vastly interesting? Because planets form out of the same rotating clouds of matter that creates stars, which means that the direction of planetary movement around the star, at the time of its formation, has to be the same as the direction the star itself rotates. That’s what happens in the Solar System: not only all the planets go rond the sun in the same direction the sun itself rotates, but the same is true for the vast majority of planet’s moons and smaller objects… all the way down to comets. The only significant exception is Triton, the largest moon of Neptune, whose retrograde orbital motion (it’s the name this phenomenon has) led scientists to believe that at some point it was captured by Neptune from an orbit around the sun. That is: Triton is thought to have been a dwarf planet at some point. Fun fact: it’s about the same size as Eris.

OK, this may be true for almost all Solar System objects but isn’t for WASP-17b.

What this means is that something major happened to it after it was formed. Astronomers don’t seem to be giving much credit to a capture scenario, probably because the planet is so close to its star: it’s a “hot jupiter”. Instead they are speculating that at some point, probably after it was fully formed, it must have had a close encounter (a very close encounter!) with another giant planet that sent it spinning into the traffic, so to speak. The other, unknown, planet, was either sent into a very highly elliptical orbit, or ejected alltogether from the system. This last sentence is me, speculating, so don’t go thinking it’s the truth, OK?

Fascinating stuff!

And it gets better: the very next day, two teams announced the discovery of another retrograde planet, HAT-P-7b. This one wasn’t a new find; the planet was already known. The novelty here lies in the disalignment between the plane of the planet’s orbit and the rotation of its star. In this case, numbers are somewhat conflicting and unclear, so the only thing that is an absolute certainty is that HAT-P-7b does not orbit along the equator of HAT-P-7, as happens with all the major Solar System planets (i. e., all planets excluding most dwarfs): it may be highly inclined, orbiting along the poles or close to them, or, which seems more likely, it’s another retrograde, orbiting along the equator but in the opposite direction, like WASP-17b.

Double Whoa!

Planets, special, orderly, well-behaved objects? Yeah, right…

Addendum: Take a look at Exoplanetology blog, where the method used to make the discovery is very well explained. The only thing I don’t think is correct is where he talks about a violent collision: no collision with an object moving in the same direction could make a planet move in the opposite direction… but a very strong transfer of orbital momentum could.