Re-opening the Pluto Controversy

Image Credit: Alan Stern, Marc Buie, NASA and ESA

Just when it seemed like the issue of whether Pluto is a planet or not was fading away, it was re-opened by the very body that demoted Pluto in the first place. Earlier this week, the International Astronomical Union (or "IAU" for short), the "ruling body" of astronomy world wide, announced that objects like Pluto shall henceforth be known as "plutoids." This decision was announced by the IAU working group tasked with trying to classify objects, and it was quite unexpected. Most of us didn't even know that such a term was under discussion.

The astronomers who still argue that Pluto is, indeed, a planet came out with some fairly strong condemnation of this announcement. Alan Stern, former head of NASA's science directorate and lead scientist of the New Horizons mission (a robot en route to explore Pluto), said, "It's hard to find anyone who thinks this is (i) necessary, (ii) a step forward, or even (iii) useful." From a scientist, that's pretty strong language. An article published on Space.com continues by arguing that a competing body to the IAU is needed.

Frankly, I think those involved need to take a deep breath. I agree that the phrase "plutoid" is stupid; if nothing else, it sounds stupid -- more like a medical condition than a type of heavenly body. (Imagine how you would feel if your doctor came in to a waiting room and said, "I'm sorry, Mrs. Smith, but you have plutoids.") Second, it is actually an over-classification. "Plutoid" is meant to describe icy objects beyond the orbit of Neptune that are large enough that gravity makes them round. But the existing terms "Kuiper Belt Object" already refer to small objects outside the orbit of Neptune, and "dwarf planet" was introduced by the IAU just a year or two ago to describe objects too small to be planets but still large enough that gravity makes them round. I don't know what use a term describing the intersection of these two groups is.

But the other side should calm down, too, I think. The last thing we need is another ruling body to compete with the IAU. The IAU is indeed far from perfect (I think it spends a little too much effort on bureaucracy and not enough on advocacy), but the IAU is not destroying the science of astronomy, and a second major international group would only serve to muddy the waters even more. Let's look at another oft-maligned international body: the United Nations. Most people agree the United Nations is far from perfect, but imagine how much worse the mess would be if we instituted a competing "League of Nations 2". Countries would have to choose between bodies, or maybe belong to both, and then what happens when the two bodies are at cross purposes? Or both trying to work on the same problem but spending more time arguing about who has jurisdiction? That would be worse than a mess; it would be disastrous.

Frankly, the problem is that there is no single good definition for a planet; we humans just have a penchant for trying to classify things that, ultimately, may be unclassifiable. The best classification scheme for planets may be one involving formation scenarios and gravitational interplay and past history of individual objects. But outside of our own Solar System, we have no means of determining most of these things. And even within things we all agree are planets, there are huge differences. Earth is much different from Neptune, which is much different from Jupiter. I'm sure that if Jovians existed, they would be insulted that we would have the gall to compare our puny blue rock to Jupiter's mighty bloatedness.

As humans, we've come to accept that, sometimes, there are not black and white answers, but that most answers come in shades of gray. The same is true even in science. "What is a planet?" Is a question that doesn't have a single answer, and the answer will differ from person to person, and yet all of those answers may be right. Before we go yelling at each other over what Pluto is, we should perhaps ask another question, "Does it matter?" Whether we call Pluto a plutoid, or a Kuiper Belt Object, or a dwarf planet, or a planet, or a "remarkably big chunk of ice and rock really far away" won't change how scientists view and understand the object.

Just don't call Pluto late for dinner.

Earth Day 2008

Image Credit: NASA/JPL-Caltech/University of Arizona

Today is Earth Day! But I promise not to fill this space with discussion of climate change and sustainability and the like, as there is plenty of that discussion out there already.

However, I would like to note how crucial studies of the Earth are to astronomy research, as looking inward gives us a reference point for interpreting what we see when we look outward.

Planetary astronomers get the most direct help from studies of the Earth. Other planets and moons have volcanoes, fault lines, atmospheres, and weather. It is not hard to imagine how understanding lava flows on Earth may help us to understand the history of Martian volcanoes, or how knowledge of cloud formation on Earth can help us understand why Venus is shrouded in clouds, or even the development and structure of clouds on Jupiter.

In the next decade, we will most likely discover Earth-sized planets around other stars. And as we develop more and more sophisticated technology capable of analyzing light from those planets, we will likely begin to detect other atmospheres. How reliably can we interpret those findings from far away? Studies of the Earth from afar (like the above picture of Earth and our Moon taken from Mars late last year) can tell us what we can and cannot detect from far away.

Finally, as we begin to look for life on other planets, we need to know how to look for life. Most importantly, are there any unambiguous signs of life that we'd be able to see from hundreds of light-years away? Our atmosphere has concoctions of molecules, like methane, oxygen and ozone, that we don't think other planets can have without life producing them. But maybe other non-biological processes that we haven't thought of could be at work. Do we need to look for signs of chlorophyll? Would alien plants use chlorophyll? What if we see some strange molecule that we don't have on Earth. Could we identify it and discover if it is a marker of life?

These are all important questions, and while studying our own planet may not provide us with unambiguous answers, maybe we can get some guidelines toward solving these problems. And, in the meantime, we'll continue to learn more about our home planet, our relation with it, and how we should best maintain it.

Why brown dwarfs? No, "Y" brown dwarfs

Image Credit: P. Delorme and the Canada-France-Hawaii Telescope We astronomers are obsessed with classification. Every object we find in the Universe gets pigeonholed into one or more categories. For example, our own sun is a star belonging to several categories: Population I, Spectral Class G2, and Luminosity Class V, among others. Our Milky Way galaxy is morphological class S(B?)b. There are at least three classes of planets in our Solar System: terrestrial (like Earth), gas giant (like Jupiter), and ice giant (like Neptune). So, it should not be surprising that we have already designated classifications for objects that haven't even been found yet.

Brown dwarfs are "failed stars," objects that form like stars, perform a few nuclear reactions early in their lives, but never ignite the basic hydrogen fusion reactions that allow stars to burn for million, billions, and even trillions of years. So, while a brown dwarf starts off as a reasonably hot star, it can't keep itself warm, and gradually cools off.

As a brown dwarf cools, its atmosphere will change. When it is very young, its atmosphere looks a lot like that of red dwarf stars -- glowing a feeble red and filled with some simple molecules, like titanium oxide and carbon monoxide. As the brown dwarf cools below the coolest true stars (about 3100 degrees F), the types of molecules change, and substances like iron hydride (FeH) appear. So, since these objects look different and are too cool to be real stars, astronomers made a new classification for these stars: Spectral Type "L". (Why "L"? Because it was one of three letters in the alphabet left for spectral classes. The others were "T" and "Y", and we'll be using those below.)

Brown dwarfs of spectral class "L" continue to cool, and complex molecules like methane can form in the atmosphere at temperatures of about 2000 degrees Fahrenheit. This changes the appearance of the star quite a bit, so we created yet another spectral class for these stars: "T." Until recently, all brown dwarfs we know were either spectral type "L" or spectral type "T" (with a few having other spectral types typical of the coolest stars). This is only because cooler stars are very hard to detect -- they are faint, and they don't put out any visible, optical light. All of their light comes out as infrared ("heat") light.

Yesterday, astronomers using the Canada-France-Hawaii Telescope on the Big Island of Hawaii announced they had found the coolest known brown dwarf, with a temperature of only about 650 degrees F! At this temperature, a new molecule has formed in the atmosphere, ammonia, and the star's atmosphere looks quite a bit different than the know brown dwarfs of spectral type "T."

Several years ago, Davy Kirkpatrick (who invented the "L" and "T" spectral classes) anticipated that the oldest, coolest brown dwarfs should have atmospheres with lots of ammonia, and he invented the spectral class "Y" for these brown dwarfs cooler than about 900 degrees Fahrenheit. Why "Y"? It was the last useful letter in the alphabet, and these stars will have ammonia in their atmospheres for eons to come. So, the CFHT brown dwarf may be the first member of a spectral class that has existed for 9 years. And we astronomers can all feel pretty clever, because now we can give any star a spectral classification.

In terms of science, though, the new brown dwarf probably represents the most common type of brown dwarf in the Milky Way galaxy, because brown dwarfs will cool to these temperatures in a few billion years, and the Milky Way is three or four times older than this. Now that one has been found, I bet many dozens of these class Y brown dwarfs will be found, and we'll be able to study some of the oldest brown dwarfs in the galaxy.

While this may be the first class Y brown dwarf to be found, it is not the first object of this kind to be found. The planet Jupiter shines partly under its own light (in infrared light; all of the visible light we see comes from sunlight reflecting off the planet's clouds), and its atmosphere is full of ammonia and similar molecules. For that reason, the "Y" class is sometimes thought of as a link between planets and brown dwarfs.

But let's be a little careful here -- planets are different from brown dwarfs. Brown dwarfs are big enough (bigger than about 15 times the mass of Jupiter) that they were able to do some nuclear fusion in the past. Also, brown dwarfs probably formed differently from planets. Planets form in disks around bigger stars, while brown dwarfs probably form more like stars do, from collapsing clouds of gas. Planets are almost always (if not always) found close to their planet stars, while very few brown dwarfs are close to bigger stars. In short, while their atmospheres and the chemistry in those atmospheres may look similar, brown dwarfs and planets are different. One cannot change into another.

And that is the only thing that really bothers me about this announcement. The new brown dwarf is billed by some news outlets as a "missing link" between brown dwarfs and planets. In some senses, this is true. But the term "missing link" carries some baggage; namely, its connection to evolution of one species into another. And, as I said, this brown dwarf is not evolving into a planet. There are links in the atmospheres, and many differences are small. But "missing link" is an unfortunate phrase here.

Hubble sees organic molecules in another solar system

Image credit: NASA, ESA and G. Bacon

This afternoon, NASA announced that the Hubble Space Telescope detected organic molecules in the atmosphere of a planet circling another star.

First, let me emphasize: Hubble did not discover evidence of life on another planet. Organic molecules (generally molecules made mostly or fully of carbon and hydrogen atoms) are quite common in the Universe. Carbon is made in stars and during supernova explosions, and is one of the most abundant atoms after hydrogen and helium. And the organic molecule found, methane, is quite common in the atmospheres of Jupiter, Saturn, Uranus, Neptune, Saturn's moon Titan, and even in some of the coolest brown dwarf stars.

So, why the big deal? It is because we are detecting the signatures of chemistry on a planet outside of our solar system. We can now begin to probe the chemical make-up of this planet, and from that begin to test our models of giant planets around other stars.

How did Hubble detect the methane (and water) in this planet? The planet in question, which has the extraordinarily boring name of "HD 189733b", is about 63 light-years away toward the constellation Vulpecula, a tiny, forgettable constellation near Cygnus, the Swan, visible during the summer months in the northern hemisphere. The planet is about the size of Jupiter but is located very close to its parent star, completing an orbit in only 2 days. Compare that to 88 days for Mercury to go around the sun, a year for the Earth, or 12 years for Jupiter! So it is a hellishly hot place, and certainly nothing can live on the planet (which is a big gas bag, like Jupiter), or on one of its moons (if it has any).

The planet passes in front of its parent star, as seen from the Earth, once each orbit. Every two days or so, the star appears to get a little fainter as the planet's shadow crosses the face of the star. The planet only blocks out some of the star's light (about 1%), so these little variations are hard to see.

Some of the light from the star passes through the planet's atmosphere on its way to Earth, and any chemicals in the planet's atmosphere will put a fingerprint on that light. This is a very tiny signal, and very hard to see. But, with time, patience, and a lot of hard work, it can be picked out. In the past, astronomers had detected sodium in the atmospheres of a few planets around other stars. Now, we can add methane and water to that list.

The next step is to compare the various chemicals and the amount of those chemicals that we see with theoretical models of what these planets atmosphere's might look like. These models are really just educated guesses -- if we assume the planets are like Jupiter, and have a chemical makeup similar to that of their parent star, and we make some guesses as to how hot the planet might be, we can use concepts of meteorology and chemistry to calculate what the planet's atmosphere should look like. And the very early indications are that the models aren't quite right. The models predict that water and methane and sodium should be there, but in different amounts. This disagreement is a good thing -- it means we don't yet know everything, and that we can learn a lot by doing more studies like this one.

So, congratulations to the team who made this discovery. Good going -- now, let's get back to work and see what more we can learn about these planets!

Seeing a re-run

While most people don't like watching TV reruns, most of us have a few shows that we will stop to watch if they come on. My parents had a favorite episode of M*A*S*H, "5 O'Clock Charlie," that they loved to watch and recount. I have a few favorite movies, like Airplane!, that I love to watch over and over, because I often catch something new. And my daughter is just a few years past the phase when she would shout "again!" from the back seat of the car, her request to replay her favorite song from the CD we'd be playing.

In the world of astronomy, information and research is often conveyed by hour-long talks ("colloquia"). But it is extraordinarily rare to hear a "re-run;" with several thousand astronomers to choose from and only one or two colloquia a week, why should the department spend money to bring back a scientist to give the same talk she gave just a year before? But, like with entertainment, we often can get more out of hearing a talk for a second or a third time.

Today, I had the lucky opportunity to get an astronomy colloquium re-run. We had a speaker here at Texas that I saw a couple of years ago at the University of Arizona. Hal Levison is an astronomer at the Southwest Research Institute in Boulder, Colorado. He studies planetary system dynamics (this means he studies how planets, asteroids and comets move in solar systems). And he was talking about the history of our own Solar System.

When we look at our Solar System, we see a nice, ordered structure. Four rocky planets, an asteroid belt, four big gassy planets, and a belt of icy asteroids. If you put the Solar System in a computer and see what will happen a billion years from now, you'll see that the Solar System will look about the same. Maybe a few asteroids have hit some planets, and some comets have come and gone. But Earth will still be in the same orbit; Mars won't suddenly go veering off into deep space. Has our Solar System always been this way?

Detailed studies of asteroids and the icy Kuiper Belt Objects (like Pluto and its family) suggest not -- they suggest that the big planets have moved around. And the moon shows scars from what was a surge of impacts about 3.9 billion years ago. What happened?

Levison and his collaborators took a wild guess, and then tested it with computer models. They guessed that, 4 billion years ago, Saturn, Uranus and Neptune were much closer to the sun than they are now, and fairly close to the planet Jupiter. And they guessed that, outside of Neptune, there were thousands upon thousands of Pluto-sized and smaller icy bodies. And then they let the computers do their thing.

As time passes on, the icy bodies sometimes swing past a big planet and get tossed around. And after about half a billion years, the big planets slowly move a little bit because of the combined effect of a lot of these encounters. (One person may not be able to push a car around, but add hundreds of people together, and you can move the car quite a ways.)

Eventually, Saturn and Jupiter get to a state called resonance, where Saturn goes around the sun once for every two times Jupiter orbits. In physics, resonance is a special thing -- you get feedback. Just like a small whine picked up by a microphone can feed back and give you a piercingly loud noise, the resonance of Saturn with Jupiter started tossing Uranus and Neptune around the outer Solar System. The ensuing chaos of giant planets rumbling through the thick belt of icy Plutos sent rocks and comets and ice everywhere -- deep into space, into the Earth and Moon, into the sun -- everywhere. And then, as quickly as it started, Jupiter and Saturn moved out of resonance, and everything settled down. Uranus and Neptune survived, but were much further from the sun, and 99% of the icy belt that had been the outer Solar System was lost to deep space. In fact, the Solar System looked just like it does today -- nothing has happened since then.

This is a neat idea that explains lots of things we don't understand about the makeup of our Solar System. And so it was nice to see it a second time. Some ideas were clarified in my mind, some things I mis-heard last time I caught this time, and a few new ideas were introduced. And I learned where there are still some things to be explained and some interesting avenues of future research. Maybe 5 years from now we'll find that this idea just doesn't work. Or maybe it will be confirmed. I'll await the next re-run to see.

Congratulations, Fergal!

Image Credit: McDonald Observatory

Today another graduate student has earned a doctoral degree in astronomy. Today's lucky winner is Fergal Mullally, an Irishman here at the University of Texas. Fergal, like myself, studies white dwarfs, the ashes of stars that have burned all of their nuclear fuel.

Fergal has spent the last several years looking for planets around white dwarfs. Due to complex physics that is not well understood, white dwarfs of very specific temperatures "pulsate," getting brighter and fainter as the atmosphere sloshes around. This sloshing is very steady, however, and is almost as steady as the most accurate atomic clocks on Earth.

If a pulsating white dwarfs has a planet around it, the planet's gravity will pull on the white dwarf, causing it to move in its own small orbit. Our sun slowly moves in such an orbit due to the pull of the planet Jupiter. As the white dwarf moves, sometimes it will be a little closer to us, and sometimes it will be a little further away. The light that it emits will then take either a little shorter or a little longer time to get to us. So, if we see the white dwarf's pulses arriving a little early or a little late, and this happens in a very regular fashion, there might be a planet there!

Fergal's results are very interesting. Most white dwarfs don't show any evidence of a planet, but one is very interesting. Fergal still needs a little more data to tell what's going on. And once he knows, I'll let you know.

So, congratulations, Fergal! Fergal will be leaving Texas this summer to take a job as a postdoctoral researcher at Princeton, helping out with some massive amounts of astronomical pictures they've been taking.

A Snowball in hell?

A news story making the rounds for the past few days contains claims that astronomers have found a hot planet (called "GJ 436b") made out of ice. How can this be?

First, let me state that I am far from convinced about this claim. What do we really know about the planet? We know its mass, which we have measured by the planet's gravitational pull on its parent star. We also know its diameter, because the planet passes in front of its star as we see it from Earth, and we can measure how much light it blocks out. Because we know its diameter and its mass, we also know its average density. In these ways, the planet is very close to a twin of Neptune.

But this is really all we know for certain. We can guess how hot GJ 436b must be based on how close it is to its parent star (this works well in our solar system for Mercury and Mars, it's close for Earth, and fails miserably for Venus). But we have no direct data on what the planet is made out of. The claim that the planet must be made out of "hot water ice" is a guess. An educated guess, but just a guess. So don't start buying beachfront real estate on GJ 436b yet.

Why would astronomers guess that the planet is made of "hot ice?" First, in our solar system, the planets Uranus and Neptune, the planets closest in size and average density to GJ 436b, are thought to be made of ices (including water ice) covered by an extraordinarily thick atmosphere of methane. So, it is not unreasonable to guess that all planets the size and density of Neptune and Uranus look like Neptune and Uranus. But, again, this is just an educated guess.

How can ice exist if the planet is hot? Look at the phase diagram for water below, which shows what form water will take as a function of its temperature and its pressure. For temperatures and pressures on the Earth (about 250-310 degrees Kelvin and a pressure of about 100,000 Pascals), water is near the point where all three phases of water -- gas (tan), liquid (green), and ice/solid (blue) can exist. On the diagram, notice that even at extremely hot temperatures, like 600 Kelvin (by chance, about 600 degrees Fahrenheit), water can be in the form of ice if it is at high pressures -- at least 10 million times the pressure on Earth's surface.

Image credit: Martin Chaplin / London South Bank University

So, if GJ 436b has a very thick atmosphere, like Uranus and Neptune, the pressure inside the planet can be high enough to make water turn into ice, even though it would be very hot! This ice is not quite like ice on Earth -- the structure of ice crystals on Earth is not the same structure of ice crystals at high pressure. And, if this hypothesis is right, most likely the atmosphere of the planet would go through a gradual transition from air to liquid to solid. This is quite unlike the Earth, where there is an abrupt transition from the atmosphere to the sea and again from the sea to the sea floor. On GJ 436b, there is probably no real "surface" to the planet.

So, as I've said before, take this new press announcement with a grain of salt. Until we have observations showing us what a planet around another star is made out of, we can only make educated guesses. And the makeup of this "snowball in hell," GJ 436b, remains just that -- an educated guess.

Have we found a new Earth?

In a fairly big news story today, Swiss astronomers announced that they found an Earth-like planet orbiting a red dwarf star. The news stories are speculating about water and life on this planet. But let's step back just a bit before we start planning the State dinner for the ambassador of Gliese 581c.

First, let's look at what we know for certain. Gliese 581 is a red dwarf star -- a star smaller and cooler than the sun. Gliese 581 is about 21 light-years away and was already known to have a Neptune-mass planet (Gliese 581b) orbiting the star every five and a third days. Astronomers knew this because the star is wobbling as the gravity of Gliese 581b pulls on it, and we can measure this wobble. But one planet didn't fully explain the observed wobbles. After more observations, the European astronomers figured out that the system must also have a planet at least five times the mass of the Earth orbiting the star every 13 days.

Really, that is all we know. Anything else is guesswork, though these guesses have some scientific basis and are not just idle speculation. If the Gliese 581c is made of rock, its diameter will be about 1.6 or times that of the Earth, and the pull of its gravity will be about 1.6 times stronger than that of the Earth, meaning that if you weigh 180 pounds on the Earth, you'd weigh 290 pounds on Gliese 581c. However, it could be made of ice, and therefore be larger, or it could have a lot of gas (like Uranus or Neptune), in which case it could be quite a bit larger. We don't know.

We also know that, at the distance that Gliese 581c is from its parent star, it lies in a "sweet spot" where temperatures are just about right for water to exist as a solid, liquid, and a gas, as it does here on the Earth. Scientists call this the "Habitable Zone," since life as we know it could live there. However, just because a planet is in the Habitable Zone around a star doesn't mean the planet could support life. Maybe it doesn't have any water. Maybe it has an atmosphere like Venus and is a thousand degrees on the surface. Maybe it doesn't have an atmosphere at all. We don't know.

Another issue with Gliese 581c is that it is so close to its parent star that the same side of the planet almost certainly faces the star, just like the same side of the moon always faces the Earth. This would make one side of the planet fairly hot, and the other side frigid cold (although an atmosphere could help mitigate this effect). It may be that all the water on the planet is locked in giant ice caps on the side of the planet away from the star. We don't know.

In short, Gliese 581c is a very interesting discovery, and the astronomers who found it have made a very important find. But the find illustrates how little we know about planets in other solar systems. It will likely take years before we learn much more about Gliese 581c, and it may be decades or longer before we can ever separate its light from the light of its parent star (separating the light would make it easier to study its atmosphere). But you can be certain that astronomers will continue to study this planet and to search for more and more Earth-like planets around other stars!