This was one of THOSE weeks, by which I mostly mean I had a Junior Lab presentation + paper due. So I spent last weekend wracked with spasms of data analysis, trying to find decent initial conditions for fitting 12 lorentzians plus a gaussian to one set of data and cursing matlab and athena and my computer. But the reward was a damn gorgeous hyperfine spectrum of magnetite, if I do say so myself, and several other lovely plots as well. Oh, you wanna see? Okay look:

And if that does not make your heart beat a little faster, well, either 1) you are far saner than I, or 2) you’ve never taken Junior Lab.

And if it does, well, there’s more where that came from. Aww so pretty.

I think, though, that I need to limit my hours spent in Junior Lab. I’m officially scheduled for lab six hours per week, on Monday and Wednesday mornings — but then there’s Open Lab on Fridays, and from noon until 2 daily, and nobody really minds me going in during another section, and so on… Lab is disturbingly addictive, actually. If Anna and I work efficiently, we can usually finish the prescribed experiments in three of our five allotted days, which leaves us two sections (and a week of realtime) to play. By which I mean my favorite part of Jlab: digging through the cupboards and through arxiv and through wikipedia for other cool things to do with a given apparatus. And then appropriating and occasionally misappropriating whatever I can find to scrabble together something-or-other. And then HAVING IT WORK.

Actually, maybe that’s the best thing about experiments. MIT’s physics courses do grind you down some, and it’s been a while since I’ve thought it obvious that I Can Do Physics, and a longer while since I’ve placed complete trust in my brains to get me through things. But experiments are as much about figuring out how to do things, and as such they reward the goofball sneaky thinking I’m actually decent at.

Oh, my other new best friend? The Real World. By which I mean, the one I used to snort at and brush off. The one with friction and air resistance and things which are not, in fact, spheres, with all the mess and all the crap. But I have come to realize something truly glorious, which is: The Real World always works. Unlike me, it is never off by a factor of two. It doesn’t make sign errors. It never fails to realize that something is impossible, and it’s never swayed by the prevailing theory of the moment.

Also, it is just pretty.

I’ve been on an observing run in Arizona, working with the very telescopes I use from Cambridge. The observatory is at the top of Mt. Hopkins, which is about halfway between Tucson and the AZ-Mexico border. It’s home to several scopes, including two siblings of 48″ and 60″ (i.e., they have primary mirrors of 48″ and 60″). The two scopes are housed in the same building, on the mountain’s shoulder. In a chamber next door is a 51″ telescope which does infrared imaging and is run entirely remotely — its observer can open and close its dome, uncover its mirrors, etc. from afar. Behind the 51″ is a big gamma-ray dish, which looks a bit like a satellite dish, except the dish is made of hundreds of hexagonal mirrors. A few steps up the road is a cluster of five little robotic scopes called the HATs (Hungarian Automated Telescope, I think), which are really just adorable. They’re refractors, instead of reflectors — they have lenses instead of mirrors — and have tiny 4-inch apertures. They look so earnest when they’re observing, little noses pointed skyward…

I realize I’m talking of telescopes the way other people talk of kittens. But really…aww, HATs!

Up on the summit of the mountain is the MMT, which stands for Multiple Mirror Telescope. The name is a little puzzling, because the MMT has, as far as I can tell, just the usual number of mirrors. I take it that there once were seven, arranged in a flower pattern to comprise the primary mirror, but they’ve recently swapped those for a single primary 6.5 meters wide. Which is pretty big.

The smaller telescopes live inside chambers with a domed roof; the dome opens a window and rotates around so that the scope can see out. At the MMT, the entire BUILDING rotates. I heard that someone once parked his car too close to the edge of the building, and when he went to rotate the scope…well, it was not pretty. Something like

Dear Insurance Company,
A telescope ran into my car. Please advise.
-Astronomer

When working from Cambridge, I observe with the 48″ and do direct imaging, a.k.a. pretty pictures. The 60″, on the other hand, is used for spectroscopy, which, oh man, spectroscopy. Taking a spectrum involves capturing light from an object, separating it into its component colors — think rainbow — and measuring the amount of each color. I love spectroscopy. I really do. I love it with an unbridled and possibly inappropriate passion. Spectra are just SO COOL. You can look at a spectrum and know immediately whether it’s a star or a planet or a supernovaor a galaxy; how hot it is; how fast it’s moving with respect to you; what it’s made out of; how intense its surface gravity is; and whether, somewhere far, far away, something has gone terribly wrong…

Anyway, I was working with the 60″ this time, which meant I got trained in the physical operation of the telescopes: the dewar-filling, drive-enabling, dome-opening, real day-to-day care and feeding of the beasts. And oh man it was awesome. But now I’m back, and there are p-sets to be completed and posts to be written and data to analyze. To work, world, to work!

To illustrate, here’s a fun exercise: find the brown dwarfs.

See? So very easy.

Okay, that was mean. The image above is an infrared view of the galactic center — each one of those tiny pixels of light is an entire star and might possibly be worth looking at. The fundamental question in searching for brown dwarfs is HOW DO YOU FIND ANYTHING IN SUCH A HUGE UNIVERSE? How do you separate the sheep from the goats on your brown dwarf day of judgement? Where do you even start?

You start by figuring out what, exactly, to look for. You want a set of brown dwarf characteristics that, in conjunction, will let you pick your dwarfs out of a sky of stars. Here’s Adam’s list:

1. Look in the near-infrared.
2. Look for “funny” colors.
3. Look for high proper motion.
4. Look in young star-forming regions.
5. Look around nearby stars

Let’s start with the first, shall we? Okay. In theory, an object that absorbs all radiation incident upon it — a “blackbody” — should re-emit that energy as a characteristic spectrum dependent only on the object’s temperature. Hotter objects emit more light at shorter wavelengths, and colder objects at longer wavelengths. While not perfect blackbodies, most stars are a reasonable approximation thereof. So really hot stars look bluish; medium-hot stars look yellow; cool stars look reddish; and cold little brown dwarfs look redder-than-red, a.k.a. infrared.

Brown dwarf surface temperatures run from around 3000K for young dwarfs to 700K or so for the oldest, coldest objects. 700 K is about 800 F. Your oven bakes bread at 400 F…700 K is pretty damn cold for a star.

Astronomers often plot a star’s magnitude against its temperature, as measured by its color. Here’s a diagram from Hipparcos, a survey program that took pictures of, um, MANY stars:

Brown dwarfs are very faint, and very red. See the tail of stars in the lower right, where the plot looks like a tornado? Follow that down off the chart for a few inches. Brown dwarfs are there.

The best way to get data on LOTS of stars at near-IR wavelengths is to use any of several recent all-sky surveys. There are three really useful ones:

1. DENIS, which took images in I, J, and K bands (centered at 0.8, 1.26, and 2.16 micron wavelengths, respectively). It covered the Southern sky, and has a lovely 355 million objects.
2. 2MASS, which took J, H, and K-band images (H is at 1.6 microns). 2MASS covered the entire sky and has something like 471 million sources
3. SDSS, which took u, g, r, i, and z-band images.

Once you have LOTS of stars, you can plot various sets of data. Color-color diagrams plot the difference between two sets of filters, so you might have J-K one one axis, and J-H on the other. Color-magnitude diagrams plot a difference on one axis and a straight-up single-filter magnitude on the other. The point of all this is that populations of similar stars will tend to cluster in a particular region on the plot, and stars with more unusual colors will make themselves known. Then you go AHA! and draw a big circle around them:

I love this population-analysis thing. It appeals to the same bit of brain that thinks stat mech is awesome.

Note that these are just some images I found on the 2MASS site and stuck some circles on. The location of brown dwarfiness is about right, I think, but I’m going by the sketches I have in my notes. I was considering asking Adam for his pictures, but… “Hi, can I blog your lecture slides?”…would that be weird? Maybe they’re classified, or something. If you were/are a professor, what would you think?

For a little more insight into this spectrum thing, consider this diagram. It’s showing (approximately) the normalized spectra of Vega, a bright star of spectral class A, and of a brown dwarf.

Vega emits much of its light in the optical region of the spectrum, which is why we see it as shining so brightly. The brown dwarf does a lot of emitting in the near-IR, but is fraught with absorption lines of this and that, so the spectrum ends up a little raggedy.

Now let’s superimpose a few filter bands on those two spectra. The V band is at the center of the optical range (V for visible?); J and K are infrared. Vega is very bright in V but much less so in J. The dwarf, on the other hand, is bright in J but dim in V. So on a plot of J-V, the dwarf would have a high value and Vega a low one.

Searching for brown dwarfs all over the sky has its advantages: you’ll end up with the dwarfs that are closest to the Sun, which means they’re more easily studied, and can be detected at fainter magnitudes and colder temperatures. Plus, you can use existing sky surveys. The disadvantages? Well, for one, brown dwarfs are RARE. For his dissertation, Adam looked at half of the entire sky, and found all of 10 T-dwarfs. Also, the sky surveys don’t let you constrain the parameters very well — you don’t know the distance of your object, or its mass, or its age…

One solution to the parameter problem is to look for dwarfs in star clusters. If the dwarf is, indeed, part of the cluster, you can place constraints on its age and distance. The biggest problem here is to make sure that what you’re looking at really is a brown dwarf. And for this, you look once again at the spectrum — specifically, you look for the signature absorption pattern of lithium. Stars with a mass higher than about 0.065 solar masses will quickly consume their lithium, and it will be entirely absent from their spectra. Brown dwarfs, on the other hand, won’t. So finding lithium absorption indicates that your object is below that mass limit, which makes it nice and small and probably brown (purple!) and therefore of interest. And there is much rejoicing.

Lecture ended at this point. I won’t be at seminar this week, but I’ll ask Anna or someone to give me a report.

Adam’s research pages have some nice brown-dwarf information, plus links to his papers.

This paper by Oppenheimer, Kulkarni, and Stauffer gives a really nice overview of some features of brown dwarfs, and explains just what makes a star a star, and a planet a planet, and why a brown dwarf isn’t either. I particularly like Figure 2, which shows the evolution of luminosity for objects on the borderlines, and explains each bump and wiggle.

P.S. The image of the galactic center is from 2MASS, about which I am supposed to say, “Atlas Image obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.”

The Hipparcos HR diagram is from http://orca.phys.uvic.ca/astrocourses/a120/A120/lab5.html.

The color-color and color-magnitude diagrams, minus the red circles, are from http://www.ipac.caltech.edu/2mass/releases/allsky/doc/sec1_2.html. They, too, are from 2MASS, which means you should go read that “joint project..funded by” blurb again.

Plus, in a stunning display of totally awesome, WordPress added support! Which means I can now present seminar synopses WITH EXTRA EQUATIONS.

Here goes.

When I was a little kid I used to think that if I squinted really hard at a star, I might be able to see planets around it. I never really succeeded. Armed as I am with physics wisdom, I now appreciate that this is completely obvious, and that my seeing any difference at all had mostly to do with my needing glasses. BUT! The usual conception of astronomy does rather assume that, you know, what you do is LOOK at things and take pictures. And then you look even harder with the help of a computer, and you FIGURE STUFF OUT. And sometimes that is the case. Malcolm, the guy I observe for, does pretty much exactly that: he looks at pictures of a supernova and plots its brightness as a function of time. But with exoplanets, the LOOKING method is all kinds of difficult, for two main reasons:

1. CONTRAST. Unlike stars, which are inherently luminous, planets “shine” by reflecting the light of their sun. How much they reflect depends on
   1. The luminosity of the star;
   2. The effective surface area of the planet;
   3. The distance of the planet from the star, ; and
   4. How reflective the planet’s surface is, which is termed its albedo, .

   The ratio of the flux from the planet, , to the flux from the star, , is
   
   For a planet of the size and distance of Jupiter, . Which means the planet, on your pretty photograph, is TWO BILLION times fainter than the star. Which means good luck seeing it.

3. RESOLUTION. The other problem is that in terms of interstellar distances, the distance from a planet to its sun is pretty tiny. If you were looking at the Earth-Sun system from a distance of 10 parsecs (32.6 light years), the Earth-Sun radius would be about 0.1 arcseconds. For comparison, the telescope I usually observe on has a 1.2-meter mirror, and on a good day can resolve objects about 2.0 arcseconds apart. The 6.5-meter Magellan telescopes in Chile (the only other scopes I’ve observed with) can get down to 0.3 arcseconds or so. So even with some fancy scope work, the star and planet tend to blur into one blob of light.

Happily for exoplanets, astronomers are inured to such hardships, and have devised alternative methods of detection. There are three big ones:

1. Astrometry!

which means measuring the position of objects on the sky.

2. Spectrometry!

or tracking the amount of specific colors of light emitted by objects.

3. Photometry!

i.e. measuring the brightness of objects.

Both spectroscopy and astrometry rely on the dynamical truth that PLANETS DO NOT JUST ORBIT THEIR SUN. Heresy! you cry. No, no. In any system of orbiting whatnot, both bodies orbit around the system’s center of mass. Like this:
Which means that the star, too, moves.

The astrometric method is the more straightforward: just watch for a star with an otherwise-inexplicable wobble, take some pictures of it, and figure out its orbit. Then backsolve to find what kind of object is perturbing it. Here, again, you need very fine resolution; you also need the patience to sit around and wait out a good fraction of an orbit. The more massive and farther out the planet is, the larger the star’s wobble — but the longer its orbit.

Josh says that astrometry alone has never succeeded in finding an exoplanet, though it has been used to confirm the exoplanitude of objects otherwise discovered.

Spectrometry is a bit more complex. It combines the star’s orbital motion with the characteristic pattern of different colors of light the star emits: its spectrum. A stellar spectrum generally contains a combination of continuous, rainbow-looking regions and places where a few colors are just missing. Maybe there’s no turquoise-green-but-not-too-green, or none of two kinds of yellow: these absences are called absorption lines, and they’re the result of that color being absorbed by some other atoms in the star. Anyway, these spectroscopic features are useful landmarks within a given star’s spectrum.

When a luminous object moves away from an observer, the observer sees the light as redder than what was originally emitted. Conversely, if a star moves towards the observer, the light is shifted towards the blue (this shifting business is called the Doppler effect, and it’s the same kind of thing that makes the siren on the ambulance passing you go from a high-pitched EEeeEEee to a lower pitched OOooOOoo). So. If we’re looking at a star-planet system edge-on (or at any angle other than face-on) the star moves towards us part of the time, and away from us part of the time. But the pattern of light actually given off by the star hasn’t changed — and that means we can track just how much things are shifted by watching the movement of spectral features over time.

The star’s velocity as a function of time is given by
,
where is the distance from the star to the center of mass, and is the distance from the star to the planet.

The velocity along the observer’s line of sight (that’s the eyeball in the picture) is

where is the inclination angle, i.e. how much the orbital plane is tilted with respect to us.

What you look for, when you’re selecting candidates to check for spectroscopic Doppler effect, are stars with lots of narrow absorption lines. The more lines there are, and the better defined they are, the more accurately you can pin down their movement. You also want “quiet” stars — ones without surface explosions or pulsations or starspots that might mess things up. But the most significant limitation of the spectroscopic method is the ” sin i” term in the equations. The planetary mass you derive from all this work is actually the minimum mass — which means that it could be a planet in that system, or it could be something much larger. Like, I don’t know, a brown dwarf. Or another star. Or a giant alien spacecraft. Furthermore, you have no way of knowing the planet’s radius.

Even so, spectroscopy has been by far the most successful method of exoplanet discovery: of the 212 known planets, something like 185 were found spectroscopically.

Hokay. The final major method is the one Josh is currently working on: photometric detection of exoplanets via transits. A transit, here, is basically an eclipse: a planet passes in front of its star, blocking some of the star’s light. It’s not a lot of the star’s light — it’s on the order of 1% — but it’s enough to be visible. The problem with most planets is that their orbits are just too long and the transits just too short. If we’re talking Sun-Jupiter, the transit happens for 14 hours ever 12 years. And if you don’t know when or where a transit might happen, you have to look EVERYWHERE, ALL THE TIME, and then maybe someday you’ll see something. The happy resolution to this problem is a type of planet called a “hot Jupiter”, which has a period of about three days. Fourteen hours out of three days is not nearly so unmaneageable, and, in fact, such planets have been found. The data for a transit looks like this:

The little dip in the middle? That’s a transit.

The transiting-planet technique is plagued by false positives. A potential planetary transit could be an eclipsing binary star system, or a blending of light from multiple stars, or a complicated artifact of the star field, or who knows what. BUT! If you do find a nice transiting planet, you can learn all manner of things: its mass, its radius, its orbital period, its orbital inclination…all the things that make planet-hunters happy.

Here are the planet-finding-related readings assigned for today’s seminar. I’ve linked to the astro-ph versions.

A new transiting extrasolar giant planet There’s a nicely-explained process of eliminating false positives here.

Evidence for a co-moving sub-stellar companion of GQ Lup. A possible planet — and, contrary to all my objections above, a direct image of it!

Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing. Another method for planet-discovery.

Also, you can read The Transit Light-Curve Project III. Tres transits of TrES-1 which was co-authored by my lab partner, Anna. Which is pretty cool. The paper has some nice plots of transit data fit to a model.

I guess I should get with this publishing-papers business myself, or I will never get into grad school…

I could totally make a HOT XXX DATASETS calendar. So, world, make some suggestions.

(My current calendar is EXTRAORDINARY CHICKENS. And damn, world, I never knew chickens could be so EXTRAORDINARY.)

….

Anna is taking a Mössbauer exposure. It’s absolutely gorgeous, with six lovely Zeeman-splitting peaks in all the right places. We flaunt our data before Sasha and Mark, who are performing the same experiment on a separate apparatus. I think Sasha is hilarious — he’s the enthusiastic puppy-dog of everything physics. He has wide brown eyes and wild brown hair and is so incredibly earnest about everything that it’s hard to believe he’s been here three years. Every so often he’ll produce a tiny digital camera, look around for the professors, and whisper, “Take a picture of us!” Then he’ll stand by the apparatus, grinning goofily in genuine delight.

….

Nancy from the Physics Department office has made us cupcakes as a token of her love. It’s nice to be loved, especially if there’s food involved.

….

Sasha has involved Prof. Yamamoto in a discussion of…I don’t know what. I think it’s from Scott’s GR class. He mentions Sean Carroll’s textbook, which makes me feel worldly, because I read the blog of someone important. Who writes textbooks.

Suddenly Sasha grimaces in pain and cries, “Yes, but in the sense of the deeper truth –”. Mark gives him a wry glance. Anna and I can’t stop laughing. “You just have to accept it,” Yamamoto says. “Physics is a religion. First you believe, and then you’ll come to understand it.” I write that in my lab book, because it seems profound. It’s just below the quote from Peter that says “Don’t touch the source and then pick your nose.”

Anna says Sasha reminds her of Dostoevsky. I write that down, too.

….

High-density Mössbauer samples take FOREVER to run.

….

I spend the two hours before seminar in the common room, reading over the assigned papers. Some of them say funny things. For instance:

Astronomers (not only those at Lick) sustain a cultural tradition of fashionable foolishness. The mortuary of make-believe planets contains numerous tombstones, and many astronomers lugged these planets to their private graves. (Marcy & Butler 1999)

Or:

The survey implies a star-brown dwarf binary frequency of less than 1%, although more than one specimen is needed to make this statement significantly meaningful.(Oppenheimer, Kulkarni, & Stauffer)

HA.
….

A girl — I don’t know her name — is working on a nuclear&particle problem set with another guy. He says something about cross-sections and orders of magnitude. I look up: I like cross-sections! But the girl snorts. “I HATE scattering!” she says. The guy wonders what she’s doing in particle physics, if that’s the case. “I really hate scattering. I just want to play with quarks!”

I go to seminar.

….

Max is running down the hallway. Why is Max always running down the hallways? I shout a greeting at him, at which point he stops running.
“Hey Max,” I say, “when you’re beginning a Nobel speech, and you say ‘Your Majesties, your Royal Highnesses, Ladies and Gentlemen,’ who are the Majesties and who are the Highnesses?”

Max scratches his head and says he was just at a Nobel ceremony, so he should know…he tells me that it’s important to bow to the correct people. I assure him I’ll keep in practice. Just in case.

Will profs I know win Nobels? I mean, the ones I know well enough to chat with in the hallways. I’ve had the odd recitation with Wilczek or Ketterle, but I wouldn’t know what to say to them. Although I guess I could ask about the Highnesses.

Will students I know win Nobels? Will Sasha?

Oh man, that would be the greatest speech ever.

….

Note to you people waiting for seminar notes: there are a lot of figures this week. Give me a day to Inkscape those up.

Figure 1: An IR weather picture of the southwestern US. Note the lack of threatening green blobs over southern Arizona.

Figure 2: An optical all-sky photograph on Mt. Hopkins, in southern Arizona. Note the horrible soul-consuming clouds.

WTF, sky? You were perfectly photometric an hour ago. Perfectly.

If you are going to pull such stunts, I would appreciate your doing so BEFORE the T stops running, so that I could go home.

I don’t have any particular interest in math tournaments, and I wouldn’t otherwise have had anything to do with HMMT, but three years ago I got suckered into painting the HMMT logo on the bottles. Come to think of it, mollishka did the suckering. Anyway, this is what I did with my weekend. Because getting to play with Klein bottles is just cool.

Ready, world? Here’s what Josh said:

Exoplanet research has its roots in two fundamentally philosophical questions: Where do we come from? and Are we alone? Answering the former leads you into planet formation, and answering the latter gets you into exobiology. Either of them gets you into some fascinating physics.

The idea of “other worlds” goes back to Old Man Epicurus in 300 BC. In opposition to Aristotle, Epicurus believed that there were an infinite number of atoms, and that, consequently, “there [would] be nothing to hinder an infinity of worlds”. (I’ve never understood this argument. You can quite easily have an infinity with a hole in it, can’t you? Somebody explain this to me.) Some centuries later, Giordano Bruno favored a universe that was infinite, homogeneous, and isotropic, and contained many stars and planets. He was burned at the stake. Later, people put up a statue of him to make up for it. Sorry, Giordano Bruno! Here’s you in bronze. And we’ll quit killing planetary scientists, maybe.

Back in the day a.k.a. before 1995, scientists worked under the assumption that extrasolar planetary systems would be characteristically very similar to our solar system. They’d noted some salient features of ours:

• The orbits of planets are coplanar to within 10 degrees;
• The orbits of planets are nearly circular;
• The type of planet are correlated to both the distance from the sun and the planet’s mass. Rocky planets are small and close; gaseous giant are huge and at a medium distance; icy giant planets are big and far away. And Pluto…well, you know what happened to Pluto.

For reference, here are some ratios:

• Mass of Earth: Mass of Neptune : Mass of Jupiter : Mass of Sun :: 1 : 15 : 300 : 300,000
• Radius of Earth: Radius of Neptune: Radius of Jupiter: Radius of Sun :: 1 : 4 : 10 : 100

The traditional hypothesis about the formation of the solar system was that it had formed when a spinning disk of matter congealed. The sun was at the center, of course, and stayed there; since they’d all been in a disk to begin with, the planets remained roughly coplanar; since the disk was circular, the resulting orbits were roughly circular.

The modern theory, called core-nucleated growth, goes something like this:

1. Some stars have disks around them made of gas and dust.
2. The dust settles into the midplane of the disk.
3. Somehow (handwave, handwave) the dust clumps together.
4. Eventually, the clumps grow large enough that gravitational attraction can cause further collisions and clumping.
5. When a clump has approximately the mass of the earth, it begins to accrete the surrounding gas into an atmosphere.
6. At approximately 10 times the mass of the earth, the gas accretion goes out of control and the planet puffs up. POOF.

Rocky planets are cores that were too small to poof out. Maybe they weren’t sticky enough; maybe the dynamics were unfavorable. Gas giants made it all the way through and puffed up properly. Ice giants grew too slowly and were too far away. Reasonable? Reasonable.

With a decent theory in place, it remained only to find some planets. Like the brown dwarfs, exoplanets had a series of false starts. The most famous involved Barnard’s Star, which is the second-closest stellar system to us. As a consequence of its nearness, it appears to be whipping by on the sky. (Adam broke in here to point out that by “whipping by” Josh meant “at the speed of a turtle walking across the moon.” Astronomers develop a strange sense of scale.) In 1963, somebody called Peter van de Kamp found that the star had just a little extra wobble, which could be accounted for by a Jupiter-mass planet with a 25-year period, or by two planets with 12- and 26-year periods respectively. In the first scenario, the orbit was highly eccentric (i.e. more elliptical than circular); in the second, the orbits could be “nice” and circular — just like our solar system. Unfortunately, the “wobble” was actually a flaw in the archival plates van de Kamp was using. Ouch.

A really good start came, at last, in 1992, when a planetary system was found around a millisecond pulsar. With masses comparable to Earth’s, these planets are still the lowest-mass exoplanets known. But the pulsar business is just weird. A pulsar is pretty much a dead star, the leftovers from a supernova, and nobody knows whether these planets formed before or after all the explosions. Anyway, WEIRD. But finally, in 1995, a really nice system was found. It had a Jupiter-mass (or possibly larger) planet orbiting a fairly ordinary non-dead star, and exoplanetologists rejoiced.

As of yesterday afternoon, 210 exoplanets had been found. They don’t all conform to the patterns of our solar system, either: there are huge gas giants close in; there are highly eccentric orbits that whip the planet around its star on one end and send it out to freeze, slowly, on the other. But the important thing is THERE ARE PLANETS OUT THERE. And if planets are not uncommon, and Sun-like stars are no rarity, then it’s very possible that Earth-like planets exist. Somewhere.

Sweet.

Next week: Ways of finding brown dwarfs & exoplanets.

There are, Josh tells us, a couple of objectives to this seminar. One, obviously, is to give us a fairly detailed understanding of a subfield of astrophysics. We should someday be able to scan astro-ph for brown dwarf or exoplanet papers and read them and know what’s going on. But the other objective has more to do with training us in the Ways of Physicists. To that end, we’re expected to discuss things and review literature and finally produce a nice little term paper on some relevant topic. And Josh and Adam will hammer us into shape.

Now, world, I can’t help you with the how-to-be-a-physicist thing, because I don’t have any extra academia cred. But if you’re interested, I’ll give you a writeup of the lectures and discussions in seminar. Today’s program: fiddling with the projector.. followed by an overview of the historical and physical features of each kind of not-really-a-star. Good stuff! Okay, world, here we go:

First up: Adam & the [500] Dwarfs.

Stars are classified by temperature, on a scale that runs O-B-A-F-G-K-M-L-T, highest to lowest. The Sun, for instance, is a type G star. The L’s and T’s are the brown dwarfs; they’re the very coldest variety of starry thing. Brown dwarfs are formed like stars, but differ in that they can’t fuse hydrogen in their cores. Like stars, they’re self-luminous, and can be found in isolation. They exhibit other characteristically “stellar” behaviors, too, such as circumstellar disks and binary companions. On the other hand, they have low-temperature atmospheres, like planets; they have clouds, like planets; their masses are on the order of 10 Jupiters, and their radius only around 1 Jupiter. So they’re sneaky, double-crossing little beasts. They’re like teenagers, or mermaids, or the goop you make out of cornstarch and water: not really one thing and not really the other.

So the big question is: What separates a planet from a little tiny star? And that is what we’ll be wondering allll semester.

Anyway, here’s another question: Why are these things cool? Well, for starters, there seem to be a damn lot of them, and they have been there all this time, and NOBODY KNEW. Says Adam, what if we suddenly found a hidden species of people, 6 billion strong, living right alongside us? When you put it like that, yeah, it’s a little creepy. On a more practical note, though, brown dwarfs are useful sources of data on low-temperature atmospheres, cloud behavior, EXXXTREME climatology…all of those planet-type features. In the aggregate, they tell us about galactic processes: What kind of conditions did they need to form? How are they distributed throughout the galaxy? What do they do to galactic dynamics? Again, answers may or may not arrive in the coming weeks. But first, here’s a short historical overview.

Back in the day a.k.a. 1963, some people started batting around a conjecture. They noticed that there are some fairly strict lower bounds on the temperature and pressure you’d need to fuse hydrogen nuclei in the center of a star. A star like the Sun is massive enough that gravity pulls it strongly together and maintains that pressure. It doesn’t keep collapsing forever, though, because at some point the inward force of gravity is balanced by the outward push of all that fusion – it’s a bit like a well-behaved explosion. But there’s nothing stopping the formation of an arbitrarily smaller star. If a star doesn’t start life with much mass at all, it never gets to the point where fusion can take over. It still doesn’t collapse forever, because eventually the electrons just refuse to get any closer to each other. So we’re left with this ball of STUFF that’s not very good at glowing. And as time goes on, they just become colder, and fainter…and colder…and fainter…

At this point in history, everyone started saying, “ZOMG these things are dark and massive and that means they are DARK MATTER.” And because they couldn’t go around calling them “things”, a bunch of names were proposed – planetar, black dwarf, failed star, substar – but none of them really caught on. Finally, at some point, Dr. Jill Tarter, who is now famous for being an important SETI person, said, “We don’t know what color they are, so let’s call them brown dwarfs.” I don’t understand the logic there, but the name stuck. Later, when people actually found the things and gave them a spectroscopic once-over, they turned out to be PURPLE. (Wait, if hot stars are white and HOTT stars are blue, why are cold stars purple? It’s just an artifact of absorption features, that’s why.)

In 1985, people started to actually observe the things…or so they thought. The first one found was called VB 8B, and caused a big ruckus and an entire conference, but turned out not to exist. In 1988, another funny object was found.It was a companion to a white dwarf, but it had a bizarre spectrum, and nobody really knew what to do with it. And so on, and so forth. The big year was 1995, when people found Gliese 229B. Its spectrum showed that it was clearly not a planet, and other people confirmed that it was actually there, and a proud article was sent off to Nature. Somehow, though, that issue of Nature has a goofball octopus on the cover. Come on, Nature, a brown dwarf can totally take an octopus!

After that, people started finding brown dwarfs with relative ease. The problem wasn’t really with the observers, but with the technology necessary to detect in the near-IR and IR band. Today, there are about 500 known brown dwarfs. Here are some other relevant numbers:

• Maximum Mass: 0.07 Sun-masses, or 75 Jupiter-masses;
• Radius: 0.1 Sun-radii;
• Minimum Luminosity: 10^-6 Sun-luminosities;
• Lowest Temperature: 700 K;
• Central Pressure: 10^11 bars;
• Spectral Classes: 3;
• Ratio of # BDs to # Stars: 1;
• Percent of Dark Matter they make up: less than 2.

This is a looong post. I’m going to split it here and put the exoplanet stuff on its own.