From 0 to 5,000 Planets in Exactly 20 Years

By Corey S. Powell | October 6, 2015 9:57 am
Illustration of 51 Pegasi b circling its star. Twenty years ago it was the first exoplanet detected; this year, it was the first seen directly by its reflected light. (Credit: ESO/M. Kornmesser/Nick Risinger)

Illustration of 51 Pegasi b circling its star. Twenty years ago it was the first exoplanet detected; this year, it was the first seen directly by its reflected light. (Credit: ESO/M. Kornmesser/Nick Risinger)

Twenty years ago today, an invisible object circling an obscure star in the constellation Pegasus overturned everything astronomers knew about planets around other stars. No, the fallout was even bigger than that. The indirect detection of 51 Pegasi b—the first planet ever found around a star similar to the sun—revealed that they had never really known anything to begin with.

At the time, even the most adventurous minds blithely assumed that our solar system was more or less typical, a template for all the others. 51 Peg b threw a big splash of reality in their faces. The newfound world was bizarre, a Jupiter-size world skimming the surface of its star in a blistering-fast “year” that lasted just 4.2 days. Its existence ran counter to the standard theories of how planets form and evolve. It answered one big question: Yes, other planetary systems really do exist. But it raised a thousand others.

Michel Mayor still sounds giddy as he recalls the profound confusion sparked by his discovery. In the fall of 1994, he and Didier Queloz, his graduate student at the University of Geneva, were trying out a brand-new spectrograph they had built, called ELODIE. It was designed to split up light in a way that can reveal the very precise motions of stars. Mayor and Queloz used ELODIE to search for subtle back-and-forth stellar dance steps caused by the gravitational tug of a planet or brown dwarf (bigger than a planet but not big enough to shine) orbiting around it. They succeeded almost right away, but the signal they detected from the star 51 Peg was like nothing they anticipated.

Left: Michel Mayor and his post-doc, Didier Queloz, in 1995 when they found 51 Pegasi b. Right: That modest-looking device is the ELODIE spectrograph that ferreted out the planet. (Credit: Mayor)

Left: Michel Mayor and his post-doc, Didier Queloz, in 1995 when they found 51 Pegasi b. Right: The ELODIE spectrograph that ferreted out the planet. Doesn’t look like much, but it sure did the job. (Credit: Mayor)

“It was so strange that we decided to wait until the next observing season,” Mayor recalls. The motion of the star was far too fast and too extreme to be explained by a planet that was at all like the ones in our solar system…and yet the signal was undeniably there. After another round of observations yielded the exact same result, Mayor and Queloz wrote up a paper—only to have it rejected by one of the reviewers at Nature, who found its claims too outlandish. The editor in chief interceded, Mayor popped a bottle of champagne, and on October 6, 1995, he announced his findings at an Italian astronomy conference, to a chorus of amazement and incredulity.

I shouldn’t sound so judgmental. It’s easy to forget now, but twenty years ago there was no convincing evidence—none—of planets around other sunlike stars. There were just science-fiction fantasies, a handful of discredited claims, and some broad-brush inferences based on the only example scientists had to draw on: our own solar system. (The sole clue about the true diversity of planets was the discovery, three years earlier, of three small objects circling a pulsar, the dead remnant of a supernova explosion—a finding so bizarre that researchers are still puzzling over it.)

...and that's how you find a planet without seeing it. At left, a starlight spectrum from ELODIE. At right, the measured motions of the star 51 Pegasi, which betrayed the pull of an unseen planet. (Credit: Mayor/Queloz)

…and that’s how you find a planet without seeing it. At left, a starlight spectrum from ELODIE. At right, the measured motions of the star 51 Pegasi, which betrayed the pull of an invisible planet. (Credit: Mayor/Queloz)

Mayor and Queloz opened the floodgates. They showed exactly how to detect planets, and what to look for; they proved, right out the gate, that many of those alien worlds are nothing like the ones in our own solar system. By 2000, astronomers had found dozens of such “exoplanets,” using an expanding set of search techniques. A European satellite called CoRoT pioneered the approach of looking for transits, miniature eclipses created when a planet passes in front of its star. Then in 2009, the Kepler Space Telescope took this technique and pushed it much further, to stunning effect.

Since its launch, Kepler has dominated the planet-hunting business, overwhelming the number of worlds that Mayor and his colleagues could find from the ground. The current tally lists more than 5,400 likely exoplanets, the majority from the Kepler dataset, including about 1,600 that have been confirmed by follow-up observations. Those follow-ups are crucial, though, because Kepler is great at detections but lousy at analysis. It is, as Mayor puts it, a machine of “fantastic statistics.” Making sense of those stats is long, hard work. But that hard work has paid off beautifully.

Continuing in the tradition of 51 Peg b, the exoplanet catalog now contains a dazzling variety of unfamiliar types of worlds: evaporating planets, puffy planets, diamond planets, backward-orbiting planets, and planets orbiting double stars—just like Tatooine in Star Wars, as many fans have eagerly pointed out. In 2004, Mayor’s team discovered a whole new class of planets called super-Earths: rocky worlds that are up to 10 times as massive as our own, and potentially habitable. Super-Earths turn out to be the single most common kind of planet around other stars. “It is very strange to see this huge population,” Mayor says, “since we do not have any planet in this range in the solar system.”

Mayor’s pioneering work in planet hunting has been somewhat obscured over the years by the onslaught of discoveries from Kepler and by a growing number of competing teams. His modest nature and richly accented English surely do not help. But if Mayor feels in any way slighted, he certainly does not show it. When I speak with him, he expounds as effusively about his current research as he does about the initial groping efforts that led him to 51 Peg b.

Nearby "super-Earth" planet HD 219134b passes in front of its stars, casting a slight shadow. This transit (mini-eclipse) will make it possible to study its atmosphere and composition in detail. (Credit: NASA/JPL-Caltech)

Nearby “super-Earth” planet HD 219134b passes in front of its stars, casting a slight shadow. This transit (mini-eclipse) will make it possible to study its atmosphere and composition in detail. (Credit: NASA/JPL-Caltech)

Just in the past year, Mayor and his team have made two more notable contributions to the exploration of worlds around other stars. In April they revisited 51 Peg b and this time were able to detect the reflected light of the planet itself. That is the first time anyone has observed a direct reflection off the cloudtops of an exoplanet—an incredible achievement given that the nearby star is millions of times brighter. Then in July, Mayor’s collaborators found a planet called HD 219134b, one of the closest super-Earths. It is located just 21 light years away, which will make it a valuable object of study for future space telescopes.

What Mayor and his now-sizable mob of colleagues have not yet managed to track down is the thing they want the most: Earth’s twin, another warm, wet, and welcoming planet that can (or does) support life. Filling in the landscape of exoplanets has been the great adventure of the last twenty years. Finding that one special place on the cosmic map is the great quest for the year 2035.

How will we do it? I’ll look at the plan of attack in my next post.

For more astronomy and space news, follow me on Twitter: @coreyspowell

  • johnumana

    5,400 exoplanets discovered in 20 years — bravo, Kepler team! You have to wonder whether some of those planets harbor folks looking back on Earth and asking whether they are alone. I tell you that the Milky Way galaxy is teaming with life and with intelligent life. There are great cities up in the stars. Go find them.

    • stevlich

      I’m sick and tired of wondering. In 50 years of SETI they can’t even find a microbiome. NASA is just teasing us. They don’t want to find life. Scientists are addicted to suffering and making others suffer. Nothing will ever convince those stone faces at NASA that it’s proof of alien life. They’re sadistic.

      • coreyspowell

        Oh, I assure you they want to find life. Anyone who uncovers the clinching evidence will become one of the most acclaimed and influential scientists in history. The problem is that finding life is extremely challenging, and making a misguided claim is extremely easy. Consider that we are still, even today, finding entirely new ecosystems and new types of life right here on Earth!

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  • Luke101

    It is now becoming quite apparent, that our little blue marble is quite extraordinary, and possibly unique in many qualities.

    • ziff

      Well, we can’t know yet. Detection methods are biased towards larger planets. That is, larger planets are easier to detect, so we naturally find those first. There may many earth-like planets out there, they’re just difficult to find at this point.

      • Luke101

        It’s pretty silly to say that there are many “earths” when we haven’t found any yet.
        You want that to be , but so far not.

        • ziff

          That’s not what i said. I’m not saying there are many earths. I’m saying you cannot say there aren’t. You can’t say we’re special or unique because we don’t know yet.
          I agree that earths are certainly rare in comparison to all exoplanets. Just how rare no one can say due to current detection limits.

          • Luke101

            If you are going to edit you posts, I cannot continue.

          • Jai Guru

            “You cannot say there aren’t”

            Well in a universe where the impetus is on proving a negative you’ll have a point. The fact is you made a positive claim and THAT is what logically must be prosecuted. Own your responsibility.

    • Torbjörn Larsson

      We can make some claims based on the Kepler statistics, such as:

      – Earth is not extraordinary. It has been boxed in by smaller and larger planets, by superhabitables (superEarths around red dwarfs, who should have longer HZ and plate tectonics lifetimes), et cetera.

      – Earth is unique, but so is each planet in the wide, dispersed statistics. On the other hand our own Venus show that this doesn’t say much, the difference in mass, orbit and composition between planets becomes small seen over the population.

      • Luke101

        No, I am speaking generally as to the various traits our planet has that offer a unique environment.
        Stable orbit with a large satellite. Protected status by the gravity wells of much larger planets on the outside of our orbit; very strong magnetic pole to radiation;
        Tremendous quantity of liquid water; Nitrogen atmosphere, and finally, perhaps the only reason we are here, the natural evolutionary progression was interrupted by asteroid/volcanic cataclysm – resulting in the decline of Reptilian life, and the rise of mammalian life.

        • ziff

          Remember, we only have one data point- Earth. So you can make any trendline out of a single data point.
          Since other star systems and planets are created by the same processes that created ours, maybe all evolutionary progression gets interrupted by asteroids and volcanoes, and thus maybe mammals are a natural outcome.
          I agree, time will tell, but i’d like to know sooner rather than later. :)

          • Luke101

            maybe all evolutionary progression gets interrupted by asteroids and volcanoes…

          • Torbjörn Larsson

            – “Speculation IS HYPOTHESIS …….duh”

            An hypothesis is a testable claim. [ ]

            – “The only part of it was about possible alien life as we may find it. The rest is known.”

            That was the point: other planets do not need to fulfill your random criteria set, and we know that your list is erroneous. For example, ” Protected status by the gravity wells of much larger planets on the outside of our orbit”: “It has long been thought that the presence of a giant planet is a pre-requisite for the development of life on potentially habitable planets. Without Jupiter, it was argued, the Earth would have been subject to a punishing impact regime, which would have significantly retarded or outright prevented the development of life on our planet. Although this idea is widely embraced, little research has previously been carried out to support it. Here, we present the results of several suites of dynamical integrations used to model the influence of Jupiter’s mass and orbit on the impact rate that would be experienced by the Earth. We find that, far from being a simple shield, Jupiter’s role in determining the terrestrial impact flux is significantly more complicated than previously thought. Far from being a simple friend, such giant planets are perhaps more likely to imperil the development of life on otherwise habitable planets.” [ ]

            “Also I never said any of your talking points, so you can get lost.”

            It was _your_ talking points. And I don’t see that you have stopped making them. =D

          • Luke101

            You are full of inconsistencies and very skilled at talking in circles. Also, you have edited your posts, so I cannot continue your nonsense.

          • Torbjörn Larsson

            If you forget that life is a process among populations you can’t say much. But same as we could predict from our own planetary system re its formation process that our system was one among 100 of billions in our own galaxy alone, we can predict from our own planet re its rapid emergence of life that our biosphere is one among many billions in our galaxy alone.

            Re mass extinctions, there have been at least 5 large and many more small over 4 billion years but life survived and diversified. So another prediction from observing the process rather than an event in a set: wholesale planetary extinctions will be rare.

          • Luke101

            I believe I already said that.

          • Jai Guru

            Mammals existed before the dinosaurs died. Their survival had more to do with their size, which was kept small due to large predators routinely preying on them, forcing them into a burrowing niche which was advantageous in the ruined world that followed the KT event. If the roles had been reversed, with large Dinosauresque mammals preying on small reptiles, the outcome may have been very different. My point is, you’re making an unfounded assumption here that mammals are somehow the logical outcome of this process.

        • Torbjörn Larsson

          “the various traits our planet has that offer a unique environment for life to develop, as we know it.”

          As I already noted, Earth is humdrum and not the best possible or even likely habitable.

          You then list the “Rare Earth” hypothesis criteria, which is a) untestable (you can always chose criteria to make your putative likelihood anything between 0 and 1) and b) the mentioned criteria are all now known to be wrong. E.g. the Moon is not involved in evolution, Jupiter throws more impactors our way than it deflects them, et cetera.

          And you finish off with something irrelevant re our planet. Biologists already know species are unique on any planet due to the contingencies of evolution, no one disputes that.

          • Luke101

            Speculation IS HYPOTHESIS …….duh
            Also I never said any of your talking points, so you can get lost.

      • Jai Guru

        You are using the term “Super Earth” incorrectly. The term does not have anything to say about habitability at all. It only speaks to size. There are no reliable methods for judging the habitability of even supposed Earth like planets at this time.

    • stevlich

      There’s nothing extraordinary about a universe that came up empty except for this one hodgepodge of patchwork flotsam and jetsam that we reside on.

      • Luke101

        The universe is big………

        • Chris Bingham

          and you sir appear to be somewhat closed minded

          • Luke101

            about what? I said the universe is beyond our imaginations large, and you call me closed minded? wth?

  • KMGuru

    Perhaps our Milky Way Galaxy has just one other planet with advanced life. To get where there are more…you have to go to Andromeda Galaxy and others….just a thought….

    • Jai Guru

      I think it’s very likely advanced life may be a one in X number of galaxies proposition.

    • KMGuru

      I found out there are more than one humanoid life forms in Milky Way…something you learn lately…

  • stevlich

    5000 exoplanets but not a stitch of life. Disgusting!!

    • coreyspowell

      …but keep in mind that almost all of the planets found so far are kinds that would be extremely unlikely to support life. And the most earthlike worlds that astronomers have detected so far are also the hardest to study. It is entirely possible that they have already found a living world, but just lack the means to know it.

  • Jai Guru

    The assumption that our solar system was an example of a standard model was not “Blithe”. It’s part of a highly valuable scientific mindset that precludes the “special place in the universe” mentality which poisoned human thought so thoroughly for the vast majority of our existence. It is a reasonable, stable platform from which we can reliably spring without making the variety of assumptions that lead to cognitive bias.

    By all means know something about what you speak before you write about it. That is writing 101.

    • coreyspowell

      You need to know the full history here to understand why I used that word. As early as 1980, detailed dynamic models had shown that planetary migration was not only possible but probable. Observational astronomers ignored those models, because the result did not conform to expectations. Cambpell & Walker, and later Marcy & Butler, had the capacity to find hot Jupiters but did not do so because they failed to consider orbital periods drastically shorter than those of the planets in our solar system. Even after Mayor & Queloz found 51 Peg B, many of their colleagues refused to believe it…again, because it did not conform to expectations.

      These responses did not reflect a Copernican principle of mediocrity. They reflected the opposite type of bias: the assumption that the way things are here must be the archetype of the way things are everywhere. When you have only a single example to work from, that is a reasonable place to start. But when you have good reason to question the assumption but repeatedly refuse to do so, that is instead a reflection of unconscious bias–the kind that can afflict even the greatest minds.


Out There

Notes from the far edge of space, astronomy, and physics.

About Corey S. Powell

Corey S. Powell is DISCOVER's Editor at Large and former Editor in Chief. Previously he has sat on the board of editors of Scientific American, taught science journalism at NYU, and been fired from NASA. Corey is the author of "20 Ways the World Could End," one of the first doomsday manuals, and "God in the Equation," an examination of the spiritual impulse in modern cosmology. He lives in Brooklyn, under nearly starless skies.


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