Doppler spectroscopy

The Exoplanet That Changed How We See the Solar System, and the Universe

The Nobel Prize for Didier Queloz and Michel Mayor is in recognition of their willingness to stand by their data, seeing a planet where others didn’t.

In 1992, Aleksander Wolszczan and Dale Frail became the first astronomers to publicly announce that they had discovered the first planets outside the Solar System, orbiting the dense core of a dead star about 2,300 lightyears away. This event is considered to be the first definitive detection of exoplanets, a portmanteau of extrasolar planets. However, Michel Mayor and Didier Queloz were recognised today with one half of the 2019 Nobel Prize for physics for discovering an exoplanet three years after Wolszczan and Frail did. This might be confusing – but it becomes clear once you stop to consider the planet itself.

51 Pegasi b orbits a star named 51 Pegasi about 50 lightyears away from Earth. In 1995, Queloz and Mayor were studying the light and other radiation coming from the star when they noticed that it was wobbling ever so slightly. By measuring the star’s radial velocity and using an analytical technique called Doppler spectroscopy, Queloz and Mayor realised there was a planet orbiting it. Further observations indicated that the planet was a ‘hot Jupiter’, a giant planet with a surface temperature of ~1,000º C orbiting really close to the star.

In 2017, Dutch and American astronomers studied the planet in even greater detail. They found its atmosphere was 0.01% water (a significant amount), it weighed about half as much as Jupiter and orbited 51 Pegasi once every four days.

This was surprising. 51 Pegasi is a Sun-like star, meaning its brightness and colour are similar to the Sun’s. However, this ‘foreign’ system looked nothing like our own Solar System. It contained a giant planet much like Jupiter but which was a lot closer to its star than Mercury is to the Sun.

Astronomers were startled because their ideas of what a planetary system should look like was based on what the Solar System looked like: the Sun at the centre, four rocky planets in the inner system, followed by gas- and ice-giants and then a large, ringed debris field in the form of an outer asteroid belt. Many researchers even thought hot Jupiters couldn’t exist. But the 51 Pegasi system changed all that.

It was so different that Queloz and Mayor were first met with some skepticism, including questions about whether they’d misread the data and whether the wobble they’d seen was some quirk of the star itself. However, as time passed, astronomers only became more convinced that they indeed had an oddball system on their hands. David Gray had penned a paper in 1997 arguing that 51 Pegasi’s wobble could be understood without requiring a planet to orbit it. He published another paper in 1998 correcting himself and lending credence to Queloz’s and Mayor’s claim. The duo received bigger support by inspiring other astronomers to take another look at their data and check if they’d missed any telltale signs of a planet. In time, they would discover more hot Jupiters, also called pegasean planets, orbiting conventional stars.

Also read: Famous Exoplanet Detected In 1995 Found to Have Water In Its Atmosphere

Through the next decade, it would become increasingly clear that the oddball system was in fact the Solar System. To date, astronomers have confirmed the existence of over 4,100 exoplanets. None of them belong to planetary systems that look anything like our own. More specifically, the Solar System appears to be unique because it doesn’t have any planets really close to the Sun; doesn’t have any planets heavier than Earth but lighter than Neptune – an unusually large mass gap; and most of whose planets revolve in nearly circular orbits.

Obviously the discovery forced astronomers to rethink how the Solar System could have formed versus how typical exoplanetary systems form. For example, scientists were able to develop two competing models for how hot Jupiters could have come to be: either by forming farther away from the host star and then migrating inwards or by forming much closer to the star and just staying there. But as astronomers undertook more observations of stars in the universe, they realised the region closest to the star often doesn’t have enough material to clump together to form such large planets.

Simulations also suggest than when a Jupiter-sized planet migrates from 5 AU to 0.1 AU, its passage could make way for Earth-mass planets to later form in the star’s habitable zone. The implication is that planetary systems that have hot Jupiters could also harbour potentially life-bearing worlds.

But there might not be many such systems. It’s notable that fewer than 10% of exoplanets are known to be hot Jupiters (only seven of them have an orbital period of less than one Earth-day). They’re just more prominent in the news as well as in the scientific literature because astronomers think they’re more interesting objects of study, further attesting to the significance of 51 Pegasi b. But even in their low numbers, hot Jupiters have been raising questions.

Also read: What Have We Learnt About Exoplanets From the Kepler Mission?

For example, according to data obtained by the NASA Kepler space telescope, which looked for the fleeting shadows that planets passing in front of their stars cast on the starlight, only 0.3-0.5% of the stars it observed had hot Jupiters. But observations using the radial velocity method, which Queloz and Mayor had also used in 1995, indicated a prevalence of 1.2%. Jason Wright, an astronomer at the Pennsylvania State University, wrote in 2012 that this discrepancy signalled a potentially deeper mystery: “It seems that the radial velocity surveys, which probe nearby stars, are finding a ‘hot-Jupiter rich’ environment, while Kepler, probing much more distant stars, sees lots of planets but hardly any hot Jupiters. What is different about those more distant stars? … Just another exoplanet mystery to be solved…”.

All of this is the legacy of the discovery of 51 Pegasi b. And given the specific context in which it was discovered and how the knowledge of its existence transformed how we think about our planetary neighbourhoods and neighbourhoods in other parts of the universe, it might be fair to say the Nobel Prize for Queloz and Mayor is in recognition of their willingness to stand by their data, seeing a planet where others didn’t.

Famous Exoplanet Detected In 1995 Found to Have Water In Its Atmosphere

Its discovery is a testament to the method that the astronomers used, whose usefulness in studying the properties of exoplanets was established only in 2010.

This article was originally published in February 2017, and was republished on October 8, 2019, at 3:50 pm on the occasion of Didier Queloz and Michael Mayor being awarded one half of the 2019 Nobel Prize for physics.

The exoplanet called 51 Pegasi b was discovered in 1995 to international acclaim. Though the first exoplanet had been discovered three years prior orbiting a pulsar, 51 Peg b was the first found to be orbiting a main sequence star. Main sequence is astronomy speak for a star’s being positioned on the Hertzsprung-Russell diagram, which categorises stars according to their brightness and colour. If a star is said to be on the main sequence, then it’s simply a conventional star. Indeed, 51 Peg b orbits 51 Pegasi, a star much like our Sun.

At the time, Michael Mayor and Didier Queloz detected the planet by studying how it was making its host star wobble – using changes in the measurement of the star’s radiation as a proxy. Their measurements indicated that 51 Peg b was a ‘hot Jupiter’, a giant planet orbiting much closer to its star than astronomers at the time thought was possible. Encouraged by Mayor’s and Queloz’s efforts, however, planetary astronomy soon pivoted to looking for more planetary hot Jupiters, also known as pegasean planets. Most of them have been identified using the same radial velocity method, also called Doppler spectroscopy, that the duo used. According to scientists at NASA’s Jet Propulsion Lab, “Astronomers now believe that large planets may form far from their stars and ‘migrate’ closer to the stars over millions of years.”

Over two decades after the discovery, a group of Dutch and American astronomers have made superior observations of the same planet, getting a better fix on its mass, orbital properties and atmospheric contents. Specifically, they have found that 51 Peg b’s atmosphere is 0.01% water – a sizeable proportion – and that it orbits its star at 133 km/s and weighs 0.476-times as much as Jupiter (give or take 7%). Their results were uploaded to the arXiv preprint server on January 25.

More than the specific measurements themselves, the significance of the astronomers’ accomplishment lies in their methods – as it did 21 years ago. When a planet orbits a star, the planet’s gravity will cause the star to start wobbling, as if it was moving in a tiny orbit around a slightly displaced centre of motion. When observed from Earth, the star will be discernible by a corresponding back-and-forth motion, understood in terms of the star’s radial velocity, the velocity at which it undertakes this motion.

“After 21 years, the detailed nature of 51 Peg b is beginning to reveal itself, yet it remains an intriguing and extreme solar system.”

When radiation is emitted by this star and observed from Earth, a ‘back’ motion will cause the radiation to become shifted ever so slightly to the redder end of the spectrum; a ‘forth’ motion will offset it towards the bluer end. These red- and blue-shifts, a consequence of the Doppler effect, are measured by a spectrometer to then determine how much the star is wobbling.

As it happens, the back-and-forth motion is also applicable to the planet as it moves in its orbit around the star. However, this is much harder to study because the star’s emissions are three to four orders of magnitude stronger and could overwhelm the planet’s own radiation when observed from telescopes on Earth.

Nonetheless, advancements since 1995 have allowed astronomers to build extremely sensitive telescopes – even better than the ELODIE spectrograph in southeast France that Mayor and Queloz used and which could pick up on a radial velocity of 7 m/s. The CRIRES instrument, located in north Chile and used by the Dutch-American team, is sensitive to velocities of 5-10 m/s. An upgraded version, called CRIRES+, aspires to a range of 2-3 m/s once it is ready in 2018. At the same time, CRIRES boasts of a resolution more than twice that of ELODIE’s.

Interestingly, Ignas Snellen, one of the authors of the new study on 51 Peg b, was among the first astronomers to demonstrate how the radial velocity method could be used to study the properties of planets, in 2010.

Getting, cleaning and mining the data

In the new study, Snellen and his colleagues studied 51 Pegasi for four hours in the infrared spectrum of radiation, as the planet ‘b’ swung around in its rapid four-day orbit. They gathered data about radiation emanating from the star as well as radiation bouncing off the planet from the star. Once they were done, they had to clean the data in two steps. The first was to remove all the information that corresponded to Earth’s atmosphere itself. The second, more difficult step was to find out which radiation in their data was coming off the planet and which from the star. The astronomers were aided in this exercises by two things, one a natural phenomenon and the other a tool.

The natural phenomenon is that planets usually possess those gases that strongly absorb radiation of a particular wavelength emitted a star. So when a spectroscope observes the radiation emitted by a star after it has bounced off such a planet, it will notice that a part of the radiation is missing. As a result, knowing what kind of radiation a star emits and which parts of it are absorbed by different substances can be used to determine what makes up the planet’s atmosphere.

Next, the tool was called cross-correlation. Essentially, Snellen and co. came up with a list of plausible compositions of 51 Peg b’s atmosphere and what their radiation would look like to a spectroscope. Then, they scanned the radiation signature of each composition against their CRIRES data to find the closest match. When this process was repeated for different combinations of water, gases and other materials, optimising at each step to accommodate other positive comparisons, the team arrived at the most likely and eventual scenario: 0.01% water, and no carbon dioxide or methane that CRIRES could detect.

The presence of water does not immediately suggest the presence of life, however. That will require further study, especially of 51 Peg b’s composition, surface characteristics, what other gases are present in the atmosphere and, finally, of the presence of biomarkers.

Their paper concluded thus: “After 21 years, the detailed nature of 51 Peg b is beginning to reveal itself, yet it remains an intriguing and extreme solar system.” And the radial velocity method – which allows us to study the surface-level properties of an object 473,000 billion km away – is only just getting started, its relevance controlled mostly by the ability of human engineering to build more sensitive instruments. Recently, the method was used to predict that the three exoplanets in the Wolf 1061 system 13.8 lightyears away were likely uninhabitable; to establish Proxima Centauri’s orbit is ‘bound’ to Alpha and Beta Centauri’s; and to understand how often brown-dwarf stars form. It seems radial velocities are set to feature in the answers to some of the most fascinating questions astronomers have to ask about the universe.

James Peebles, Didier Queloz and Michael Mayor Win 2019 Physics Nobel Prize

When asked if he had expected to win the prize, Peebles said that it wasn’t something he had planned for.

The 2019 Nobel Prize for physics has been awarded to James Peebles, Michael Mayor and Didier Queloz “contributions to understanding the evolution of the universe and Earth’s place in the cosmos”.

Peebles, who takes half of the prize, “has been decorated for theoretical discoveries in physical cosmology”. According to a presentation at the press conference, “he developed theoretical tools to uncover the universe’s dark components” and that, “with his help, cosmology evolved into a science of precision and matured into physical cosmology.”

Mayor and Queloz have been cited “for the discovery of an exoplanet orbiting a (extrasolar) Sun-like star”. This planet is designated 51 Pegasi b, orbiting the star 51 Pegasi about 50 lightyears away. To quote an older report on The Wire:

At the time, Michael Mayor and Didier Queloz detected the planet by studying how it was making its host star wobble – using changes in the measurement of the star’s radiation as a proxy.

Peebles has made significant contributions to cosmologists’ understanding of the Big Bang event at its dawn, followed by contributions to theories of dark energy and dark matter. When he won the Shaw Prize in 2004, his citation stated that “he laid the foundations for almost all modern investigations in cosmology, both theoretical and observational, transforming a highly speculative field into a precision science.”

Mayor and Queloz are Swiss and Peebles is Canadian-American. When asked if he had expected to win the prize, Peebles said over the telephone at the conference that it wasn’t something he had planned for. “Prizes are charming, much appreciated, but that’s not part of your plans,” he added, referring to aspiring scientists. “You should enter science because you’re fascinated by it.”

Thus far, 212 people have won the physics prize, including this year’s winners. Of these, only three have been women: Marie Curie in 1903, Maria Goeppert-Mayer in 1963 and Donna Strickland in 2018. None of these women won the whole prize but had to share it with two men.