Date: October 9, 2009
Title: The Search for Distant Worlds
Podcaster: Christopher Crockett
Organization: Lowell Observatory
http://www.lowell.edu/users/crockett/
Description: In the past 14 years, we have gone from knowing of no other planets around other stars to cataloguing over 300 of them. The search for extrasolar planets is one of the hottest and most rapidly growing fields in astronomy as we seek out not just other planetary systems but race towards the Holy Grail of planetary science: another Earth! Listen today for a discussion on how astronomers find and characterize these distant worlds, what the future brings for exoplanetary exploration, and how we might be able to detect life orbiting an alien sun.
Bio: Christopher Crockett is a University of California, Los Angeles graduate student currently working as a predoctoral fellow at Lowell Observatory. His research involves searching for planets and brown dwarfs around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the Harford County Astronomical Society in Bel Air, Maryland. We dedicate this podcast in memory of one of our founders, Leo Heppner. Leo shared his love of astronomy with thousands through teaching and demonstrations while encouraging them to seek what they loved about astronomy whether it be science aspects, astrophotography or stargazing. Please visit us on the web at http://www.harfordastro.org/.
Transcript:
Hello. This is Christopher Crockett from Lowell Observatory.
“Innumerable suns exist; innumerable earths revolve about these suns… living beings inhabit these worlds.” These were the words of Giordano Bruno, a 16th century Dominican monk who was later burned at the stake under charges of heresy. He was an early champion not only of Copernicus’ idea that the Earth revolved around the Sun, but also of the infinite nature of the Universe. He believed that the stars in the sky were distant Suns and that these Suns harbored Earths, and possibly living creatures, of their own. He was a man far ahead of his time. It would be nearly 400 years after his death before the first planets orbiting other stars would be detected.
As Bruno’s musings indicated, the notion that there are other solar systems in the Galaxy and beyond is not a new idea. Astronomers have long suspected the existence of alien planetary systems. But success at finding them as long eluded us. The available technology caught up to our imaginations only in the past couple of decades. Since the discovery of the first extrasolar planets in 1992, our understanding has exploded and made the search for distant worlds one of the hottest fields of modern astrophysics.
Why are planets so difficult to find? Generally, they must be found using indirect means. That is, with some recent exceptions, we cannot directly image them. The reason that planets are notoriously difficult to see is the combined effects of contrast and apparent separation.
By contrast, I am referring to the difference in the amount of light coming from the planet and the star around which it orbits. The planet’s light is reflected light from the star. Because the planet only intercepts a tiny fraction of the star’s total light, there isn’t much to reflect. What’s more, a planet doesn’t reflect all of the light it receives; it often absorbs some of it. The amount of light a planet reflects into space is therefore much less than how much light its host star emits. If we restrict ourselves to visible light – that is, light that our eyes can detect – a star is roughly one billion times brighter than any planets orbiting it! That’s a one followed by nine zeros. To help wrap your brain around that, one billion pennies would encompass the same volume as five school buses or, if stacked, would tower four times higher than the orbit of the space shuttle.
So, that’s one problem.
But the difficulties are compounded when you consider the apparent distance between a planet and its host star. Consider the Earth and the Sun. The Earth is, on average, 93 million miles from the Sun – a pretty respectable distance! If you were to travel just over 2.5 billion miles away – roughly to the orbit of Neptune – the apparent distance between the Earth and the Sun could be covered by your thumb. Go one light year away (a distance of about 6 trillion miles) and the gap between the Earth and the Sun could be covered by a dime seen edge-on from 300 feet away. As seen from the nearest star to our Sun, Proxima Centauri – a distance of 4.2 light years – the Earth-Sun distance now subtends less than one second of arc (1/3600 of a degree). In other words, take that same dime and place it just over 1000 feet away.
Perhaps now you can start to see our problem. We’ve got two objects, one a billion times brighter than the other and separated by very small angles. Trying to see a planet around a star 10 light years from Earth is like trying to see a candle 20 feet from a searchlight in Washington, D.C. while standing in Los Angeles. That’s really hard! Because of these inherent difficulties, direct imaging of planets is limited to very large planets orbiting very far from their suns. If we want to locate solar systems more like our own, we’re going to need another way. Fortunately, nature has provided an alternative. We can look instead for how the presence of an orbiting planet affects the motion of its host star.
A planet stays in orbit around a star because of the gravitational force exerted by the star. But a planet also gravitationally tugs on its sun. As the planet swings around on its orbit, it gently nudges the star around causing the star to wobble in space. How much wobble depends on the relative masses of the planet and star. A very massive planet, like Jupiter, will induce a larger wobble than a small rocky world like Earth. What if, instead of looking for planets directly, we instead look for wobbling stars?
The trouble now is that the amount of wobble is pretty small. The Earth only makes the Sun wobble back and forth a total distance of roughly 600 miles. When you consider that the Sun itself is nearly 1.5 million miles in diameter, you realize that isn’t very much! If an astronomer on a hypothetical planet orbiting the closest star to our Sun, about 4 light years away, were to attempt to measure the wobble of our Sun resulting from the movement of the Earth, she would be trying to measure a wobble that subtended only 4 millionths of an arcsecond! That would be like trying to measure the thickness of a dime located 175,000 miles away – roughly 3/4 the distance to the Moon!!
And, remember, this is from a vantage point of only 4 light years away, basically next door. Most of the stars we’d want to measure are hundreds or even thousands of light years from Earth.
Unfortunately, no current telescope can come close to doing that. It gets easier if instead of trying to find Earth-like planets, you instead try to look for more massive Jupiter-like worlds with larger orbits. This technique favors finding massive planets on wide orbits around stars which are relatively close to Earth. But it’s still very difficult with the current generation of telescopes – only one positive detection has been reported thus far.
At this point, things may seem hopeless. Planets don’t lend themselves to imaging, and the wobbles they impart on their host stars are just too small to measure. Fortunately, there is a way to look for wobbling stars without trying to directly see the stars’ motion. In fact, it’s the same method police use to hand out speeding tickets. We’re talking now about Doppler shifts.
If you were to take a flashlight and shine it through a prism, you’d get the familiar rainbow. But if you now place a clear container filled with, say, helium gas between the flashlight and prism, the rainbow would change. You would see “gaps” in the smooth continuum of colors, places where the light has gone missing. The helium atoms are tuned to absorb very specific colors of light. This shows up in the rainbow as missing colors – what astronomers refer to as “absorption lines”. Replace the helium with oxygen and you’ll get a completely different pattern of absorption lines. Every atom and molecule has a distinct absorption “fingerprint” that allows astronomers to tease out the chemical makeup of distant stars and galaxies.
When we take starlight and pass it through a prism (or similar device), we see a rainbow marked by the absorption lines of hydrogen, helium, sodium, and so on. However, if we take that star and send it hurtling away from us, all of those absorption lines “shift” towards the red part of the rainbow – a process called “redshifting”. If the star turns around and now comes flying towards us, the opposite happens. This is called, not surprisingly, “blueshifting”.
So, what if we find a star where, over time, the lines regularly alternate between blueshift and redshift? That implies the star is moving towards us and then moving away and then towards… over and over and over. It tells us: the star is wobbling back and forth in space! This could only happen if there was something we couldn’t see near the star pulling it around. By carefully measuring how much the absorption lines shift, an astronomer can figure out how massive the unseen object is and how far away it is from the star. Now that’s how to find a planet!
This technique of finding planets via the so-called Doppler shift is responsible for finding the overwhelming majority of known planets. As of the day I’m recording this podcast, 347 of the known 374 extrasolar planets have been found this way. This technique generally favors larger worlds on short period orbits because these planets produce the largest Doppler shifts. But astronomers have greatly improved on the technology over the past decade. This, combined with decade-long monitoring programs, has allowed us to start finding planets several times the mass of the Earth and worlds on multi-year orbits. While a large number of planets we’ve found thus far are Jupiter-sized worlds orbiting very close to their stars, this is predominately a limitation on the techniques we use and isn’t necessarily an accurate picture of the exoplanetary zoo.
Occasionally, one of these distant solar systems will be aligned in such a way that the orbits of their planets pass directly between us and their parent stars. When that happens, the planet momentarily blocks some of the light of its sun, and we find ourselves in the shadow of a far away world. Astronomers call such an event a “transit.” Transits have the potential to offer a wealth of information. Unfortunately, they are pretty rare. You need just the right alignment of astronomer, exoplanet, and parent star.
When a transit does occur, astronomers can directly measure the size of the planet by carefully measuring how much starlight gets blocked. If you’ve also calculated the mass based on Doppler shift measurements, you can combine the two pieces of information to figure out the planet’s density. This is an important first step in determining the bulk composition of the planet and is crucial in distinguishing gas giants like Jupiter from rocky worlds like our own.
Secondly, when the planet transits its star, some of the starlight passes through the planet’s atmosphere while en route to Earth. By comparing the spectrum of a star before, during, and after a planetary transit and looking for what changes, astronomers can measure the chemical makeup of exoplanetary atmospheres! Our first indication of life in the Universe will probably not come from a radio signal blaring across the Galaxy, but from the subtle chemical effects of biological activity on a distant planet’s skies.
The big disadvantage to relying on transits is that they are very rare. Not only will most exoplanets never transit their stars, but the ones that do only do so briefly. If a planet takes years to orbit its star, the transit event itself may only last for hours or days. That means, in order to be successful, astronomers must stare at the same stars for years at a time if they want to increase their chance of success. This is the primary justification of the recently launched Kepler space mission. It has begun to stare at one patch of sky in the constellation Cygnus, monitoring the brightness of roughly 100,000 stars for four years. The payoff could be huge: it is currently the only instrument sensitive enough to detect the presence of an Earth-sized planet in an Earth-like orbit!
We have now been carried to the brink of a revolution in understanding the innumerable worlds that orbit other suns. Within five years time, we may very well uncover one of the holy grails of modern astronomy: the detection of another Earth in our neck of the Galaxy! We have now, within our reach, the ability to vindicate the words that Giordano Bruno wrote 425 years ago: “… there is a single general space, a single vast immensity which we may freely call Void; in it are innumerable globes like this one on which we live and grow. This space we declare to be infinite, since neither reason, convenience, possibility, sense-perception nor nature assign to it a limit. In it are an infinity of worlds of the same kind as our own.”
End of podcast:
365 Days of Astronomy
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