Tuesday: 22 December 2009
It’s that time of the year to make lists: lists of the best and worst, lists of movies, books, sexiest examples of the sex of your choice, hot travel spots, hot events, and hottest days.
Here is my end of the year list: A few of the now 415 extrasolar planets known (as of December 18!).
Even just a decade or two ago there were no known planets outside of the solar system - a few unconfirmed suspicions, but that was it. Suddenly improved optical design and several methodologies allowed the means to directly or indirectly tease out the tiny influences of a planet from its overwhelmingly larger star. And now we have a veritable zoo of exoplanets - 415 of them revolving around 350 stars, which means some of those stars, 44 of them, have multiple planets. 87 of these were discovered in 2009 alone - a new planet added just about every four days.
To guide us on our tour, here is a plot I generated at the Extrasolar Planets Encyclopedia, which keeps track of new discoveries.
The plot is of the length of the year in days along the x-axis versus the planet’s mass in Jupiter units. It’s a log-log plot, but read the same way as any. I’ve added the planets of our own solar system to orient us.
I’ve also added the names of exoplanets that represent extremes in one way or another, and it’s these that we’ll be exploring.
(You might have noticed that there’s quite a large hole in the lower left quadrant. That’s of some interest since it’s in this region that we would expect to find earthlike planets. Does that mean that there are very few earthlike planets? The answer lies in the methodologies and their limitations, and I’ll have more to say about that in another post.)
You might well wonder how we could detect such a tiny planet across a distance of 980 light years, not to mention know all these sorts of things about mass and period!
A good number of the stars with discovered planets have the tag “Gl,” or “GJ.” This denotes inclusion in the Gliese Catalogue of Nearby Stars, nearby meaning within about 80 light years. This includes many tiny red dwarfs, which generally are ignored as “vermin of the galaxy.” But it also turns out that quite a few exoplanets circle these ancient dwarf stars. Here are two such systems.
Red dwarf stars are extremely long lived; they will still be here trillions of years from now. It would be strange and perhaps difficult to live on a planet whose parent was a red dwarf.
Finally, and although not labelled on the plot at the top, here is epsilon Eridani b, discovered in 2000. It’s the closest known planetary system, just 10.5 light years away. Eps Eridani is a K2 star, redder and about 4/5 the size of our sun. The only planet we know of is half again as large as Jupiter, and circles eps Eridani in just under 7 years. As the map below indicates, an inner asteroid belt has also been detected.
And so that’s a few, just a few, of the 415 exoplanets known. In just a few days there will be another one or two added to the list - they are being discovered at a rate of one every four days. And to imagine that twenty years ago we knew nothing of any of these, and wondered even if there were other planets circling other stars!
Here is my end of the year list: A few of the now 415 extrasolar planets known (as of December 18!).
Even just a decade or two ago there were no known planets outside of the solar system - a few unconfirmed suspicions, but that was it. Suddenly improved optical design and several methodologies allowed the means to directly or indirectly tease out the tiny influences of a planet from its overwhelmingly larger star. And now we have a veritable zoo of exoplanets - 415 of them revolving around 350 stars, which means some of those stars, 44 of them, have multiple planets. 87 of these were discovered in 2009 alone - a new planet added just about every four days.
To guide us on our tour, here is a plot I generated at the Extrasolar Planets Encyclopedia, which keeps track of new discoveries.
The plot is of the length of the year in days along the x-axis versus the planet’s mass in Jupiter units. It’s a log-log plot, but read the same way as any. I’ve added the planets of our own solar system to orient us.
I’ve also added the names of exoplanets that represent extremes in one way or another, and it’s these that we’ll be exploring.
Image: Extrasolar Planets Encyclopedia
(You might have noticed that there’s quite a large hole in the lower left quadrant. That’s of some interest since it’s in this region that we would expect to find earthlike planets. Does that mean that there are very few earthlike planets? The answer lies in the methodologies and their limitations, and I’ll have more to say about that in another post.)
 Image: R. Hurt, NASA/JPL-Caltech | Starting at the very small planet at the bottom and working our way clockwise we begin with PSR 1257+12 b. The romantic “PSR” means pulsar, a rapidly rotating neutron star, and the “b” at the end means the first planet discovered in this system. The unusual appearance of the pulsar star is due to vast amounts of radiation being flung out of the two ends of the spin axis, and so we would not be comfortable there. This teeny tiny planet, 1/45 the size of Mercury, was discovered in 1992. The pulsar is 980 light years from us, and this planet circles the remains of the star in 25.3 days. There are two other planets in this system, both larger than Jupiter. Both are expected to be rocky, not gassy, and that’s what you’d think might be left after a parent star becomes a supernova. |
You might well wonder how we could detect such a tiny planet across a distance of 980 light years, not to mention know all these sorts of things about mass and period!
| Next up is a “superearth,” CoRoT-7 b. The lovely name comes from the European space-based satellite that made the discovery early this year, COROT. This superearth circles a previously unnamed star 489 light years away. CoRoT-7 b is a little under 4 earth masses, which means you’d feel a little heavy walking on it. You’d also fry - this planet circles its sun, a K0 just slightly smaller and redder than ours, in less than a day. That means it’s really really close, and speeds around at 130 miles per second. |  Image: European Southern Observatory |
 Image: ESA/C.Carreau | WASP-18 b, again named according to the project name, SuperWASP. It lies 330 light years away, and like the above CoRoT circles its star in just under a day. The star is an F9, hotter and somewhat larger than our sun, and the planet is a superjupiter, many times larger than our own Jupiter. It’s expected to gradually spiral into its sun in less than a million years. Wouldn’t that be a sight to behold! |
| This image is actually *not* of HD 43848 b, but it is similar to the “brown dwarf” that is HD 43848 b. A brown dwarf is a failed star. It’s enormous - 25 times the size of Jupiter, but just a bit too small to ignite the fusion process that would turn it into a real star. It does generate a lot of heat, though, and here we see an accretion disk surrounding the dwarf. This one circles a sun very much like our own, 121 light years away, and in this case the star was previously known. The HD tag tells us that it was included in the Henry Draper Catalogue. Unlike the extremes above, HD 43848 b takes 650 years to circle its sun, and so lies much farther out than even our own Pluto. |  Image: NASA |
 Image: NASA, ESA and P. Kalas (UC, Berkeley) | Fomalhaut b may not be extremely large, perhaps as much as three Jupiters, but its massive hot white parent star Fomalhaut is only 25 light years away, practically next door. Fomalhaut b, which circles its sun once in 872 years, is one of the few to have been detected directly by visible light. The Hubble Space Telescope took these over a period of several years and the discovery announced in 2008. |
| OGLE-05-390L b is another superearth, a little more than five times the size of our earth, discovered in 2005. It’s cold, cold, because it revolves around its tiny red dwarf sun, taking 9.6 years to complete the trip. What’s remarkable about OGLE-05-390L b is that it’s 21,500 light years away! That places it nearly in the center of the galaxy. Now how could we know about something so distant? It’s all in the name: the Optical Gravitational Lensing Experiment (OGLE). |  Image: NASA, ESA and G. Bacon (STScI) |
A good number of the stars with discovered planets have the tag “Gl,” or “GJ.” This denotes inclusion in the Gliese Catalogue of Nearby Stars, nearby meaning within about 80 light years. This includes many tiny red dwarfs, which generally are ignored as “vermin of the galaxy.” But it also turns out that quite a few exoplanets circle these ancient dwarf stars. Here are two such systems.
 Image: unknown | Gl 581 e is the smallest known exoplanet circling a “normal” star. In this case, “normal” means a M3V red dwarf, a third the size of our sun, 20.3 light years away. There are four planets in the Gl 581 system: b, c, d, and e. They all circle very close to their cool parent; the widest orbit doesn’t stretch out to the size of even Venus’s orbit in our solar system. It was originally thought that Gl 581 c would be in the narrow habitable zone of this system, but recalculations suggest that it’s actually Gl 581 d, the fourth planet out, that might be comfortable to us. Temperature-wise, anyway, but as a superearth seven times larger than our earth, it would be crushing. |
| Recently there has been considerable interest in another Gliese, this time GJ 1214 b. Again, the star is a tiny (1/6 the size of our sun) red dwarf located 40 light years away. What’s interesting is that the planet, which revolves around its primary in 1.6 days, appears to be a waterworld. A hot hot waterworld, nearly 7 times the size of our earth, on which the planetwide ocean is thousands of miles deep. At those pressures and temperatures, water doesn’t exist in its usual form, but possiby something more like Ice VII. You’ll enjoy this article by DarkSyde on “Water World,” which also features a very nicely imagined painting. NPR had a good piece on this discovery a week or so ago. |  Image: unknown |
Red dwarf stars are extremely long lived; they will still be here trillions of years from now. It would be strange and perhaps difficult to live on a planet whose parent was a red dwarf.
Finally, and although not labelled on the plot at the top, here is epsilon Eridani b, discovered in 2000. It’s the closest known planetary system, just 10.5 light years away. Eps Eridani is a K2 star, redder and about 4/5 the size of our sun. The only planet we know of is half again as large as Jupiter, and circles eps Eridani in just under 7 years. As the map below indicates, an inner asteroid belt has also been detected.
Image: NASA/JPL-Caltech
And so that’s a few, just a few, of the 415 exoplanets known. In just a few days there will be another one or two added to the list - they are being discovered at a rate of one every four days. And to imagine that twenty years ago we knew nothing of any of these, and wondered even if there were other planets circling other stars!
