May 30, 2019, 00:00 PDT
The planet hunter and the ‘unseen giant'
It was March 1988, and astronomer David Latham was working into the night, puzzling over an odd result from an experimental instrument at Harvard’s Oak Ridge Observatory in Massachusetts.
At the time, planets around other stars were an unproven – if thrilling – idea. The decades since have revealed them in stunning variety. Thanks to space telescopes such as NASA's Kepler spacecraft, we now know that there are more planets than stars in the galaxy. Nearly 4,000 planets that orbit stars other than our Sun – exoplanets – have been confirmed.
But first, pioneering scientists had to lay the foundations, chasing clues to the possible presence of these distant worlds when the technology was still in its infancy.
By 1988, Latham and his colleagues had been searching for signs of “extrasolar planets” for years, and had come up dry. Teasing out the presence of planets by tracking the wobbling motions of stars was turning out to be extremely difficult. But now he was seeing something remarkable: a spike in the side-to-side motion of a star, suggesting a companion in orbit around it. The data had been gathered using an experimental fiber feed – now standard equipment – from the “digital speedometer” instrument at Oak Ridge, which measures stellar motions.
In an email on a primitive system, which took two hours to reach his colleagues in Geneva, Latham described his findings. He ended with a modest observation: that it would be “very exciting” if the anomaly he’d seen “was due to an unseen giant planet in an orbit similar to Mercury’s.”
The result, published the following year, was HD 114762 b, one of the earliest known potential planets beyond our solar system. That publication marks its 30th anniversary in May 2019.
The early days of planet hunting were filled with such moments – more “That’s funny” than “Eureka” in Isaac Asimov’s famous phrase. In August 1988, a buzz of press excitement greeted Latham’s description of his finding at a science conference. Another planet detection had been announced by a Canadian team in 1987, then withdrawn – only to be reconfirmed more than a decade later.
While his 1989 paper identified the new discovery as a “probable brown dwarf” – a kind of failed star that is not considered a planet – Latham wrote that the object “may even be a giant planet.”
Latham ran immediately into scientific headwinds. The planet he seemed to have found was just too strange – too unlike anything in our solar system. The astronomical community was not convinced.
“It’s my three strikes analogy,” Latham said recently. “The first strike is, it had an eccentric orbit (tracing an elongated ellipse around its star). Everybody was convinced giant planets had to have circular orbits; it’s that way in our solar system.”
Strike two: The planet’s “year” was much too short – just 84 days to go once around its star, comparable to Mercury’s orbit in our solar system. In those days, “everybody knew” giant planets had to be formed much farther out, Latham said. Such a big planet just couldn’t be that close to its star.
And this was a really big planet: at least 11 times the mass of Jupiter.
Strike three. Theorists at the time couldn’t find a way for nature to make a planet more than about twice the mass of Jupiter.
Just a few years later, all that cautious reasoning would be cast to the wind. That’s when 51 Pegasi b (51 Peg for short) arrived on the scene. It was one of the first discoveries of an exoplanet to capture the world’s attention. And it was very strange.
About half the size of Jupiter, 51 Peg had a scorching 3.5-day orbit. A number of such “hot Jupiters” have been discovered since. About 1 percent of Sun-like stars are estimated to host a hot Jupiter.
51 Peg showed that big planets could, indeed, hug their stars tightly – in fact, far more tightly than the possible planet Latham had found.
The discovery also validated the planet-hunting method Latham and others employed: watching the stretching and compressing of light from a star as an orbiting planet tugs it one way, then another. Light from stars moving away from us is “Doppler shifted” and appears more red; from those moving closer, light is shifted toward the blue.
This technique is called radial velocity, or simply the “wobble” method, and it’s yielded hundreds of exoplanet discoveries. In the years since those early finds, it’s been surpassed only by the “transit” method, which looks for the tiny dip in starlight as a planet passes in front of its star. Confirmed transiting planets number in the thousands; many of these were confirmed using radial velocity.
The discovery of 51 Peg is not without irony. Michel Mayor, who made the 1995 discovery with Didier Queloz, had been a co-author with Latham on his 1989 paper.
“Michel Mayor said, ‘No, it can’t possibly be a planet,’” Latham said. Mayor did, however, use an approach similar to that of Latham’s team to devise the instrument that would reveal 51 Peg.
Early detections such as these were tantalizing, suggesting there were more planetary systems out there, just waiting to be uncovered. As scientists continued their exoplanet searches into the 1990s and early 2000s, they became ever more certain that new technologies, especially in space, could pave the way to even more discoveries.
This allowed NASA to invest confidently in increasingly sophisticated space missions, to discover and characterize these worlds in far greater numbers and with far greater sensitivity. More than 2,500 exoplanets were found with Kepler, which launched in 2009 and ended its mission in 2018. Groundbreaking observatories such as NASA's Transiting Exoplanet Survey Satellite, TESS, and the upcoming James Webb Space Telescope, have been developed because of these advances, and will teach us even more about our galactic neighbors.
And Latham’s discovery?
Though the astronomical community now catalogs the unseen companion as a likely planet among thousands of others discovered since, Latham still considers his 30-year-old find a candidate. Still at Harvard and still hunting for exoplanets, he is awaiting data from the European Space Agency’s Gaia probe to help further pin down the object’s true size. Oak Ridge, which yielded his early, exciting result, closed in 2009.
Based on the initial reactions to his historic discovery, Latham reaches farther back into history for advice to astronomers of the future. In 1963, he attended a final lecture by astronomer Cecilia Payne-Gaposchkin. She found in 1925 that stars were mostly made of hydrogen; her work was roundly rejected before, much later, being proved correct. The thrust of her talk still echoes.
“Be prepared for surprises,” Latham remembered her saying. “And recognize things that look anomalous. Try to understand what it is, make the best case for what it might be, then go ahead and publish. If someone is so upset that they don’t believe it, they will take up the experiment to show you where you were wrong. Sometimes all that does is show you where you were right.”
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