The Frontiers of Astronomy

DISCOVER's panel of top astronomers and astrophysicists discuss some of the biggest questions in the universe.

Sunday, May 10, 2009

What lies beyond the edge of the solar system? Does life exist on other planets? Why is the universe constructed the way it is? Drawing on new insights and technologies, scientists are probing these unknowns in ways that were unimaginable just a few years ago. This month, DISCOVER presents a three-part investigation of the astonishing results. First up: In conjunction with the National Science Foundation, Caltech, and the Thirty Meter Telescope, DISCOVER brought together four leading astronomers to describe their studies of wayward comets, alien worlds, black holes, and the expanding universe. The panel discussion was held at Caltech’s Beckman Auditorium and moderated by DISCOVER’s Bad Astronomy blogger, Phil Plait. The four-part video from the panel is included in the two-page article below; to see video interviews with the four panelists, see the homepage for the event.

Also see the other articles in the three-part package on what's out there in the universe:
The Biocentric Universe Theory: Life Creates Time, Space, and the Cosmos Itself
▪ A Scientist's Guide to Finding Alien Life: Where, When, and in What Universe (to be published tomorrow)

Phil Plait: When each of you started your research, you were trying to answer some big questions. Can you explain those initial questions and tell us if you have different big questions now?

Saul Perlmutter: I couldn’t imagine being as lucky as I was, to be able to study whether or not the universe is going to last forever and whether it is infinite or finite. In 1998, when we discovered that the expansion of the universe isn’t slowing down—it’s speeding up—we realized this could tell us something fundamental about how physics works. Now we’re hoping that the explanation for why the universe is accelerating might address questions like “How do you tie gravity into the other forces in physics?” In that sense, the questions have really changed, but they’re still in the same category of fundamental, deep questions that I love.

Debra Fischer: One of the questions in the beginning was “Are there other planets out there?” When the first extrasolar planets were found, they seemed so different from those in our own solar system, so the question quickly morphed to “How does our solar system compare with other solar systems?” The one thing they turn out to have in common—and this is still not completely appreciated—is that in solar systems with multiple planets, these multiple planets fill all the gravitationally stable niches around the star. I used to wonder why our solar system didn’t have more planets, but then people who run solar system simulations tried to drop more planets in and found that all the other planets became gravitationally unstable. We started out with a solar system where many planetesimals were forming, and that evolved into a system where all the stable niches are filled. To me that’s one of the most exciting discoveries in this field.

Mike Brown: One thing that I did not set out to study initially, but which has become important, is that most of the objects we find in the outer solar system are quite small. You hear about the big ones, such as Eris [the largest known body beyond Neptune], because it led to the very justified death of Pluto as a planet...

Plait: Hey!

Brown: ...but for a lot of the very important science, it’s the small objects that really matter. The small ones are little particles that sit in the outer solar system, and they’re gravitationally swept around by planets. The analogy I like is that these objects in the outer solar system are the blood splattered on the wall after some horrendous murder. I love this analogy—it’s disturbing, but I love it. As Debra just suggested, there might have been additional planets that used to be here in our solar system [but were ejected due to gravitational instability]. The bodies have all been removed. I like to study the blood that’s left around. I didn’t know that this was where the field was going when I started. I thought I was just very interested to know what are the largest things out there. Is there something bigger than Pluto? What can we say about the outer solar system? But as you go farther and farther, you realize the richness of everything you’re finding and how it preserves this forensic record of what happened in the distant past.

Andrea Ghez: I didn’t know astronomy was so gory!

Plait: Yeah—how do you follow an answer like that?

Ghez: The question that I started off with was, I thought, very simple. It was just “Is there a massive black hole at the center of the Milky Way?” But one of the things I love about science is that you always end up with new questions. What happened with my research is that the stars we studied to prove that there was a black hole turned out to be very young. Young stars have absolutely no right to be next to a black hole because a black hole should shear them apart. We have no idea how these stars formed. So that’s one of the major questions we’re trying to address today: “How do baby stars form next to this completely inhospitable object?”

Plait: One of the advantages of being a blogger is that I get to read about the kinds of research people are doing today and think about how much things have changed since I was a kid. I used to look at dingy pictures of galaxies and planets in Time Life books. And now we have these gorgeous pictures from Hubble and Keck. So that leads me to the next question: How have changes in technology in the past few years helped you in searching the limits of your knowledge about what you study?

Brown: Is there anybody in this audience who does not own a digital camera? Almost everybody has a digital camera—in your cell phone, at home, or somewhere. This is one of the few things in my field where commercial technology has done an incredible job of trickling down to astronomers. The same technology that’s in your digital camera has led to people like me being able to take pictures of wide swaths of the sky at a time. This seems trivial, the fact that you can have a better camera every year, but for me it has been transformative.

Perlmutter: Then you couple camera technology with computers. Consumer demand meant that we finally could afford computers that could scan through millions of images to find the rare things that we’re looking for. In our case, we were looking for supernova explosions to use as distance markers.

Ghez: For me it’s the exact opposite. I know exactly where I want to look, so I don’t need a big field of view. Instead, I want to see as much detail as possible. So the technology that’s changed so much for me is the ability to see this very fine detail. Astronomers are obsessed with building larger and larger telescopes. There are two promises that we make with bigger telescopes: that they can see fainter things and that they see more detail. But it’s been really hard to follow through on that second promise because of atmospheric distortion. The atmosphere is great for people—it allows us to survive—but it’s a real headache for astronomers. We lose about a factor of 20 in detail because of the atmosphere. The technology that has made immense strides in the last decade has been methods for overcoming atmospheric distortion [such as using adaptive optics to cancel out the blurring].

Plait: We have some great questions for the panel that were sent to the DISCOVER magazine Web site. Here’s one for Mike: We’re finding more and more Kuiper belt objects. How soon before we make a direct detection of Oort cloud objects?

Brown: The Kuiper belt is what I spend most of my time studying. It’s the region of space outside of Neptune’s orbit that extends out a good ways. It includes Pluto, it includes Eris and all the other smaller things that I’ve been talking about. The Oort cloud is much, much farther away. It’s a hypothetical region of space where comets come from, about halfway between us and the nearest star. It’s incredibly far away compared with everything else we’ve ever seen. When my students and I have had way too much coffee, which happens daily, we sit around and think, “How are we gonna ever detect something in the Oort cloud?” It’s kind of our holy grail. If the coffee fails, we switch to wine. Still doesn’t help! We cannot figure out any way to make that happen. I would be willing to bet anybody, because I’ll never have to pay off, that there are things the size of Earth, and larger, in the Oort cloud. But I don’t know that in my lifetime we’re ever going to have a way to find out. If anybody here has any ideas, let me know.

Plait: Andrea, I’ll give you an easier question: How does a black hole affect the evolution of a galaxy?

Ghez: These black holes at the centers of galaxies are big [as black holes go]. But compared with a galaxy, they’re really small. So they don’t have that much of an effect; we have nothing to fear. They only affect the very closest stars to them. But recently it was observed that the mass of a central black hole correlates with the mass of the galaxy around it! Before that observation, we didn’t know if the black hole formed first and then the galaxy formed around it, or if the galaxy formed first and then the black hole formed from the galaxy. The correlation means that the black hole and galaxy had to form together. They couldn’t be separate events because a black hole can’t affect an object as big as a galaxy. Whatever gave rise to that galaxy had to give rise to the black hole.

Plait: Now a question for Saul: The discovery that 96 percent of everything out there is dark matter and dark energy would seem to indicate that we need a new theoretical model of the universe. What are the leading candidates?

Perlmutter: What’s amazing is that theories of how to explore dark matter used to come out relatively slowly. But as soon as dark energy came into the picture 10 years ago, the field just exploded. Every three days for the past decade there has been a new paper proposing what dark energy might be. What’s interesting, though, is that I doubt almost any of the authors of those papers would stand up and say, “I think I’ve got the answer.” Almost every one of them would say, “I’m just expanding the range of ideas we have to consider.” They are really throwing the ball back into the court of observers like us to give them some more clues. The theory of everything is unknown—we’re still pretty clueless.

Audience member: Do you think there could be life on other planets?

Fischer: Now we’re playing the role of theorists, imagining what will be. To me, it’s hard to imagine that there isn’t life somewhere else. We look at our galaxy and other galaxies and we see that they are glowing with molecules called polycyclic aromatic hydrocarbons. They become the building blocks of proteins. So the raw material is out there. Biologists look here on Earth at life in extreme environments and say, “Wherever there’s water, there’s life.” I’ll bet you a hundred dollars there’s life—though I don’t know if it’s the kind of life that is scum in a pond or if it’s the kind of life that walks up to a microphone and asks questions!

Brown: I would be willing to bet that hundred dollars that in your lifetime, somebody will actually get to Mars and discover some sort of microbe.

Audience member: The Big Bang started from a pinprick. Could there be something leaking from another side into our universe that is causing the expansion of the universe, and that’s dark energy?

Perlmutter: There’s been a long-standing tradition of using the analogy of an expanding balloon for the expansion of the universe. If you use that analogy and work your way backward in time from now, you end up imagining everything coming from a point—a pinprick. I’ve been on a campaign to get people to stop talking about balloons, because current data indicate that we live in an infinite universe. That being the case, now start working backward in time. In the simplest cosmological theory, you just suck space out between each one of you. As you go back in time, eventually everybody’s right on top of each other, but the universe is still infinite. You can still go as far as you want in any direction. It’s just very dense. Our understanding of physics happens not to work in that very hot, dense regime. That’s as far back as we know. We don’t know what happened before that, and that’s what we’re calling the Big Bang. We’re working really hard to invent ways of figuring out what happened just before that moment, and just before that. This picture is not as fun as the image of a little dot exploding, but I think it’s probably our best picture of what we’re talking about.

Audience member: What kind of mass makes up a black hole?

Ghez: The answer is that we don’t know. We have this interesting problem with black holes. What is a black hole? It is a region of space where you have mass that’s confined to zero volume, which means that the density is infinitely large, which means we have no way of describing, really, what a black hole is! The fact we say things like “the density is infinitely large” means we don’t have our description of physics quite right. I think the question really is about our description of the physical world, as opposed to the matter content of the black hole.

Audience member: What do you think about the existence of wormholes?

Ghez: They’re an interesting idea—that you’ve got black holes that take things in, and maybe something comes out on the other side [possibly in another place and time]. That gets us into the realm of science fiction because we can’t test the idea. So we let it be, without worrying about whether or not it really is possible. It’s mathematically possible, but we’ll never know whether or not it’s true. That’s probably not the answer you want, but it’s an answer.

Audience member: If a person did fall into a black hole, where would he go?

Ghez: The problem with falling into a black hole is that you would never make it. You’d get sheared apart. As you fell in your feet would feel a pull so much greater than your head that you would be torn apart. It would be a really bad ride.

Brown: Like blood spattered everywhere!

Ghez: There’s that gore again.

Fischer: It’s a theme tonight!

Audience: What are your hopes for this year’s International Year of Astronomy?

Brown: If there is anything I can convince people to do, I want people to not just sit here and listen to astronomers and think about astronomy but to look at the sky. So what I want everyone to do when you walk out tonight is to look up. You’ll see Orion, you’ll see Sirius. Just look up at the sky for a minute and think about what’s out there. That’s what I want.

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