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Public Radio's Environmental News Magazine (follow us on Google News)

Alien Planets

Air Date: Week of

n 1995, astronomers discovered there are planets that revolve around stars other than our sun. These "extra" or "exo-solar" planets have not actually been seen or photographed but scientists are able to measure their gravity, and, just recently, two of the planets' light, as proof they exist. Astrophysicist Neil deGrasse Tyson heads New York's Hayden Planetarium and he joins Living on Earth host Steve Curwood to talk about what lies beyond our solar system.


CURWOOD: Now, most of us know that our solar system is made up of Earth, Mars, Jupiter and the other planets that orbit the sun. But, how much do you know about planets beyond our solar system? In 1955, the first of these planets was discovered and now 150 of so-called exosolar planets have been found orbiting other suns and the number continues to rise.

Now, until a few weeks ago, scientists weren't able to see directly or photograph exosolar planets. But, thanks to the Spitzer infrared telescope that was launched into space two years ago, two teams of astronomers have now been able to detect light from two of these alien worlds.

Joining me to talk about exosolar planets is Neil deGrasse Tyson. He's an astrophysicist and director of New York's Hayden Planetarium and a regular contributor to our program. Hey, welcome back to Living on Earth.

TYSON: Great to be back. Thanks.

CURWOOD: So, this is pretty big news, huh, that scientists actually have seen light from two exosolar planets, planets that exist beyond our solar system? How did this come about?

TYSON: Yeah, what's great news about it is not that we've discovered an exosolar planet. We've got more than 150 in the can right now. What's new about this is the way in which they were measured. Up until now, the best means we had about knowing about planets around other stars was detecting the effect of their gravity on the host star. In this particular case, the measurement is made of the light.

Now, it's a little tricky; we're not actually measuring the light directly. What we're doing is combining the light of the host star with the light of the planet. They're combined in the measurement and you get a certain number. Write that down. Turns out, in these particular systems the planet orbits behind the star, comes out the other side and then moves in front of it. We have planet eclipses in these particular systems. So, what you do is take a picture when the planet is to the side of a star. Take another picture when the planet is behind the star. Because in one you have light, planet plus star, the other you have the light of just the star. Subtract those two measurements and what you're left with is the light of the planet.

If you have the light of the planet, you can do things like measure, you can get a spectrum of the light of the planet. You get a spectrum, it's great because you can start probing the chemistry of the atmosphere of that planet. Start asking questions: does, are there biomarkers embedded in that planet's atmosphere? Is there evidence that on that planet's surface there is life producing oxygen and methane just as it takes place here on Earth? So, it opens up a whole new vista in the search for exosolar planets.

CURWOOD: Now, how is it that you can use gravity to tell that there's a planet going around the star?

TYSON: This was the first method by which exosolar planets were known. Normally when we think of planets orbiting a star, we think of the star being fixed in the center of the system and then immovable; and with planets orbiting around; that's not really how it works. The way it works is the star and the planet orbit their common center of gravity and, obviously, the bigger orbit is the planet and the tinier orbit is the sun. But, that little orbit that the sun takes around the center of gravity is measurable from here on Earth. And once you see the reaction of the host star to the gravity of what's orbiting around it, you can infer all kinds of information about that other object. How long it takes to orbit, what its mass is and from its distance to the star. You can even get an estimate of what temperature it might have. So, this tally, while it doesn't involve photographs of the surfaces of planets, we're basically, if you're going to think of it this way, photographing their gravity and thereby deducing their existence.

CURWOOD: You're telling me the stars wiggle?

TYSON: They wiggle and jiggle, yes. And the bigger, the more massive the planet in orbit around the host star, the more that host star jiggles. And so, it's easy to detect the really massive planets. It's very hard to detect the puny little planets, like Earth-size planets around these other stars and that remains a frontier in this business.

CURWOOD: How close would one of those big planets have to be to a star for us to see it wiggle?

TYSON: The first of these exosolar planets that were discovered was a Jupiter-sized planet. Jupiter is extremely massive in our own solar system. If you add up the mass of all the other planets combined, it still wouldn't equal the mass of Jupiter. We're finding Jupiter-sized planets in orbit around these other stars, except orbiting really close. Like, as close as Mercury or Venus is in orbit around our star. That's how close these Jupiter-sized planets are orbiting their stars. And, that originally defied all explanations for the formation of a solar system.

CURWOOD: Yeah, I mean, how could a big planet be that close to a sun, to a star?

TYSON: We had no idea at the time. We, you know, we have this bias and the bias normally gets us pretty far. The bias is that we're not special. It's a scientific bias. The human non-scientific bias is that we are special and that gets us into trouble all the time. But, a very successful bias is that we're not special and if we're not special, other solar systems ought to look like us. So, everybody came up with theories on how to make solar systems. And, the results of each of these theories had a Jupiter-sized object kind of where our Jupiter is and lower mass rocky planets kind of closer in, there's a little variation, but nothing fundamentally different as we're now finding. So, all the theorists have to go back to the drawing board. And, we have some tentative ideas of how you could move big planets in and out, but there's still argument on that frontier about how this all happens.

CURWOOD: Wait a second, you're saying that the orbits of planets change?

TYSON: Oh yeah, that's the fun part. Because we still pretty much agree you can't form a Jupiter planet that close to the host star. There'd be too much competition for material and the star would win and you'd end up as a puny little rocky thing the way Earth is. So, you'd have to form it much farther out and then have the planet migrate toward its host star.

By the way, not only did technology enable us to discover these planets, computing power enabled us to calculate the birth and evolution of our solar system, and all evidence suggests that our solar system might have started with many more than the number of planets we now tally.

CURWOOD: Now, we've barely begun to explore the planets in our own local solar system. Why look for new planets to explore?

TYSON: That's a great question. Well, the ones in our own solar system, yes, we spent centuries looking at them first with the naked eye and then with telescopes and now we're visiting them. So this is, sort of, the natural order of things. We're a long way from visiting planets in orbit around other stars. They're much too far away given any propulsion technologies that we now enjoy. For example, the fastest spacecraft we've ever launched, unmanned spacecraft, at their speed it would take 70,000 years to get to the nearest planet. And, usually planetary geologists, they want their experiment to finish before they die. So, the experiment that takes 75,000 years before you get your results are not popular in the scientific community. So, it's far out of our reach and so right now, the best we can do is just try to measure them with our telescopes.

CURWOOD: Dr. Neil deGrasse Tyson is an astrophysicist and director of New York's Hayden Planetarium. His latest book is called, "Origins, 14 Billion Years of Cosmic Evolution" and there's a chapter in that book about planets that aren't part of our solar system. Thanks for taking this time with me today, Neil.

TYSON: It's a pleasure. Thank you.

[MUSIC: Bob Dylan "Dirt Road Blues" Time Out Of Mind (Columbia) 1997]



The Extrasolar Planets Encyclopedia


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