Date: February 2, 2012
Title: 150 Years of Exoplanet
Podcaster: Dr. Christopher Crockett
Organization: United States Naval Observatory
Description: While the discovery of planets around other stars has been ongoing for two decades, not many people realize the hunt really began nearly 150 years ago. Join us in this podcast as we take a tour of nearly a century and a half of false leads and dead ends that started with one wobbling star and has lead to the current cornucopia of known planetary systems.
Bio:Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets 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.
Sponsor: “This episode of 365 days of Astronomy is sponsored by John Cary. Thank you to all involved with these wonderful podcasts.
Transcript:
In 1584, the Dominican monk Giordano Bruno speculated that ‘[t]]here are then innumerable suns, and an infinite number of earths revolve around those suns…we do not discern the earths because, being much smaller, they are invisible to us”. Bruno, who was burned at the stake for such heretical musings, was a man well ahead of his time. Over four centuries would pass before Bruno’s remarkable insight could be vindicated. Since then, astronomers have discovered roughly 700 planets orbiting distant suns. What is often forgotten in the telling of this quest is that while the active discovery of extrasolar planets is very recent endeavor, the search began nearly 150 years ago.
Planets are very difficult to find. They are relatively tiny and orbit quite close around stars which are roughly one billion times brighter than they are. Trying to see a planet around a star just 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. Astronomers therefore resort to indirect methods of finding planets. Rather than look for the planet directly, we look for the effect that a planet has on its host star and infer the presence of another world.
The Earth does not simply orbit around the Sun. Rather, they both orbit around a common center. The combined gravitational tugs from all the planets in our Solar System causes the Sun to exhibit a fairly complex wobble as it gets pushed and pulled around by its family of planets. So one way to detect extrasolar planets is to look for stars that are wobbling in space. A wobbling star means something much fainter is orbiting around it.
It was the year 1844 when German astronomer Freidrich Bessel noticed the first wobbling star – the star Sirius, a brilliant blue-white star visible to the left of the constellation Orion and the brightest star in the sky after the Sun. He deduced, correctly, that the wobbling must be imposed by what he called a “dark companion” – an unseen object orbiting around Sirius. Nearly two decades would pass before American astronomer Alvin Clark would be the first to directly observe this faint companion and over fifty years would go by before the companion was identified not as a planet but as a “white dwarf” – the superhot remants of a long dead star where the mass of a Sun is squeezed into a volume no larger than the Earth.
Eleven years after Bessel’s discovery, Captain Jacob at Madras Observatory in India identified a wobble in the binary star system 70 Ophiuchi – two stars locked in an 80 year orbit sitting about 16 light-years away. Jacob claimed that perturbations in the orbits of the two stars were highly suggestive of a “planetary body in connection with the system”. It is here, in 1855, that we find the first claimed detection of another solar system! The claim received a boost in 1896 when American astronomer Thomas See reported that he had seen the same wobble in his own observations at the University of Chicago. Unfortunately, not only did subsequent observations by other astronomers fail to turn up any evidence of a wobble, but another astronomer, Forest Moulton wrote an article in the Astrophysical Journal pointing out that the claimed planet’s orbit would be highly unstable. In other words, Moulton argued that there was no way a planet *could* survive in the claimed orbit for very long. See’s letter to the Journal taking Moulton to task for his report was so scathing and abusive, that See was banned from publishing any future articles in that esteemed publication.
All remained quiet on the extrasolar planet front until 1943 when another claim for a planet around 70 Ophicuhi surfaced along with a possible detection around the star 61 Cygni – another binary star system which has the distinction of being the first star which had its distance from Earth measured – about 11 light-years. Dutch astronomer Piet van de Kamp in 1963 published data which suggested a planet 1.5 times more massive than Jupiter in orbit around Barnard’s Star; he even went on to postulate that the system had not one but two planets a few years later. Sadly, time has not been kind to the exoplanet discoveries of the mid 20th century. More observations with refined instruments and capabilities failed to turn up evidence for any of the claimed planets. By 1980, the number of known planets beyond our solar system had return to zero.
The trouble with finding planets in this way is that the wobble that astronomers are looking for is ridiculously small. 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 tug of the Earth, it 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 that’s just from a star next door. The further away our alien investigators get, the smaller the wobble appears in their sky. In fact, there has not yet been a single successful detection of a planet using this method. Rather, astronomers rely on something called the doppler shift – the same phenomenon that causes a train horn to change pitch as it races by you – to measure the speed of a star moving back and forth in space. As the star approaches the Earth, the color of the starlight is shifted slightly to become bluer than it would normally be; as it turns around and starts racing away, the star become slightly redder. The effect is extremely small – you can’t see it with your eyes. But it turns out that it’s easier to build instruments that can measure these slight color changes than it is to actually see the star moving.
And after nearly two decades of refining their instruments and their techniques, hope for finding new worlds was renewed. In 1988, a team of Canadian astronomers tentatively announced that they believed a planet about twice the mass of Jupiter was in orbit around the binary star system gamma Cephei – a star that one day will claim the title of the North Star as the Earth wobbles about its axis. Sadly, the status of that planet sat in limbo for fifteen years as other teams struggled to reproduce their findings. The discovery would not be vindicated until 2003 when better observations could be made and the claim confirmed. While gamma Cephei b therefore holds the title for the first planet discovered beyond our Solar System, a decade and a half would pass before its existance could be settled.
The first system of planets to be confirmed came from a most unexpected source. Astronomers using the Arecibo radio telescope were monitoring radio pulses coming from a pulsar – the rapidly spinning core of a supermassive star left behind after a fiery supernova explosion. Pulsars are generally remarkably steady clocks. They are the lighthouses of the cosmos: sweeping out a beam of intense radio energy as they spin at sometimes thousands of revolutions each second. Oddly, this pulsar showed very subtle variations in its timing; some pulses would come a bit early, others a bit late. This meant that there was something orbiting around the pulsar throwing off the timing. In 1992 they announced that two planets, one much smaller than our Earth, were in orbit around this long dead star. The announcement was met under an umbrella of intense skepticism as a similar claim for a different pulsar had been retracted just a week before their announcement. Continued monitoring not only brought the confirmation of the first exoplanetary system, but also turned up a third planet orbiting further out. These planets, and the handful of others like them, have long been enigmas. To be in orbit around a pulsar, they either would have had to survive the destrucive shockwave of their sun going supernova or they had to have formed out of the dying stars remnants. Either way, they occupy a unique place in exoplanet history.
As fascinating as planets calling a pulsar home was, the real hope was to find planets orbiting suns not too much different than our own. A century and a half of perservance finally paid off in 1995 when astronomers Geoff Marcy and Paul Butler made their now historic announcement: a planet with half the mass of Jupiter orbiting around the Sun-like star 51 Pegasus. This was quickly followed by the discovery of two more Jupiter-like planets orbiting very close to their stars the following year. And ever since, the pace of exoplanet detections has been relentless. 2012 marks 20th anniversary of the pulsar planet discovery. In just two decades we’ve gone from merely pondering on the existance of other solar systems to the realization that our corner of the Galaxy is littered with planets. And the exoplanet zoo never fails to surprise or disappoint. In the nearly 700 planets that have been confirmed, we’ve found planets much heavier than Jupiter flying around their host stars in mere days, hugely inflated by the intense heat of their suns. We’ve found planets going the wrong away around their stars, orbiting in the opposite direction that their suns rotate. We’ve found planets around old stars, young stars, and dead stars. We’ve found them nestled in disks of rocky debris left from over their epoch formation. We’ve even found planets with two suns in their sky. And very recently, the Kepler Space Telescope finally turned up evidence of a planet the size of the Earth orbiting a star not much different than our own. Though it’s much too hot for liquid water to exist on its surface and conditions there are likely too harsh for life, it’s a first step. One of many. One that started over 150 years ago with the dance of the star Sirius. And one that may very well ultimately lead to the finding of a sister Earth.
That planets are common may be one of the great discoveries of the past century. How many more are there? How many of them would we recognize and find hospitable? How many of them already harbor life on their own? And how many have their own telescopes pointing in our direction, seeing the dance of our own Sun, and have creatures wondering: is anyone else out there?
End of podcast:
365 Days of Astronomy
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