What Other Worlds Are Out There?

NIST has played a role in the discovery of more than 500 exoplanets — roughly 10% of all planets that have been found outside our solar system.

Credit: JPL/NASA

For millennia, humans have looked out at the night sky and wondered: Are we alone?

In 1995, scientists took a big leap toward answering this question. That’s when they announced the first discovery of a planet orbiting a star similar to our own Sun.

But this planet was nothing like our familiar Earth. It’s half the size of Jupiter and orbits extremely close to its star, exposing its surface to blistering heat. The odds of life evolving in such conditions seem slim.

Nevertheless, the 1995 discovery, recognized with the 2019 Nobel Prize in Physics, launched the field of exoplanet astronomy. Researchers have since found more than 5,500 planets outside the solar system. Of those discoveries, NIST has played a role in more than 500.

There are two main ways to look for exoplanets. In the “transit” method, scientists train telescopes on a field of stars and look for ones that periodically dim slightly. This dimming indicates an orbiting planet crossing in front of its star from our vantage point here on Earth. These searches are best done from space-based instruments such as NASA’s Kepler and Transiting Exoplanet Survey Satellite (TESS), which have spotted thousands of exoplanets in our galaxy.

In the “wobble” method, scientists look for evidence that a star is moving slightly because of a planet’s gravitational tug. This is where NIST comes in.

What Is NIST’s Role?

NIST has helped astronomers scan the skies for exoplanets by pioneering ways to precisely measure subtle fluctuations in starlight.

The wobble method relies on the Doppler effect. Think about how an ambulance siren’s pitch rises as it comes toward you and falls as it moves away. The same effect stretches and compresses light waves coming from a star when it shimmies toward or away from Earth.

Why might a star shimmy? In short: gravity. When we talk about planets orbiting stars, we really mean that both bodies revolve around their common center of mass, as the star and planet tug on each other with equal but opposite forces. (See animation.) The more massive a planet, the more its gravity moves the star. When a star moves toward Earth, its light waves compress to become slightly shorter and bluer; when the star moves away, light waves stretch to become a bit longer and redder. 

Finding Exoplanets Using the “Wobble” Method

These shifts are extremely small. For example, Earth’s gravitational pull jiggles the Sun — with 330,000 times more mass — at about 10 centimeters (4 inches) per second, roughly the speed of a spider walking across the floor. A Jupiter-sized planet could move its star at the speed of a car on a residential street. To detect such subtle motions in gigantic objects, scientists must measure the Doppler shift of starlight to a few parts in a billion. 

Stars emit a spectrum of many colors of light, with “lines” produced by different chemical elements in the star. 

To look for subtle shifts caused by planets, astronomers use an instrument called a spectrograph, which separates the star’s spectrum into individual colors — like a prism. If all colors collectively shift red or blue, that could indicate a moving star.  

But there’s a problem: Astronomers must measure slightly shifting starlight from a location — Earth — that is itself moving through space. 

“Doppler spectroscopy is a very flexible way of detecting planets,” said NIST physicist Gillian Nave. “But everything is moving ― the star, the Earth, your telescope. So, what you need is some reliable, fixed reference” to compare to the light from the star.

One answer to this problem involves cells filled with iodine molecules. When placed between a telescope and a spectrograph, iodine molecules absorb hundreds of specific and well-known wavelengths, subtracting them from the star’s incoming light. The iodine absorption spectrum does not change as the starlight Doppler shifts. So if the star’s spectral lines all shift in tandem relative to the iodine absorption lines, astronomers can be certain that the shift is caused by the star moving and not something else.

By carefully measuring the amount and frequency of those Doppler shifts, scientists can locate a planet 100 or more light-years away and calculate its mass.

NIST plays a crucial role by calibrating the iodine cells before they go on the telescopes. In NIST’s spectroscopy lab, starlight is replaced by light from a high-intensity xenon lamp, producing a smooth, white-light spectrum. All spectral lines come from iodine absorption. NIST’s spectrometer can measure the positions of the spectral lines to within a few parts in a billion, the same precision an exoplanet hunter needs. 

Once calibrated, a cell can serve as a reference on a telescope for decades. NIST-calibrated iodine cells have been installed on telescopes in California, Hawai‘i, Chile, Australia and South Africa. These telescopes have helped astronomers discover more than 500 exoplanets — roughly a tenth of all exoplanets known today. (Astronomers estimate that our galaxy hosts billions of exoplanets, the vast majority of which have not yet been discovered.)

Telescopes with NIST-calibrated iodine cells are also used to study planets discovered using the transit method. While the space telescope TESS is the best instrument ever created for discovering exoplanets, it cannot determine a planet’s mass or other properties — including how Earthlike a planet is. But ground-based telescopes based on the wobble method can. For example, the 6.5-meter Magellan II Telescope at Las Campanas Observatory in Chile has a NIST-calibrated iodine cell installed on its Planet Finder Spectrograph and is following up on planets discovered by TESS to look for a Doppler signal.

Seeking Our Planet’s Twin

The ultimate prize would be an “Earthlike” exoplanet — a rocky world orbiting its star in the “habitable zone” where surface water could remain liquid, considered a likely prerequisite for life. To aid in that search, NIST scientists have pushed their quest to precisely measure Doppler shifts to a new level, using something called a frequency comb.

A frequency comb is a specialized laser that shines in thousands of precisely calibrated colors at once. (Each color can be thought of as one of the “teeth” that make up the comb.) The comb acts as a ruler for light, enabling scientists to precisely measure the color or spectrum of a light source. 

To measure small Doppler shifts in distant starlight, astronomers strive to measure subtle changes that happen at the same time, and to the same degree, in many different colors of the star’s spectrum. The frequency comb, with thousands of teeth each representing a different, well-calibrated light color, is ideal for this kind of massively parallel measurement. During the past two decades, astrocombs have become increasingly popular for high-precision spectroscopy. 

To date, no Earthlike exoplanet has been spotted. And to do so, exoplanet hunters will likely need to overcome other challenges. For example, astronomers will need to filter out of their data random jitters in stars’ light spectra that can mimic Doppler shifts. But in the past few years, an astrocomb and a light-filtering device called an “etalon” designed and built at NIST have contributed to several exciting discoveries that have advanced the field of exoplanet astronomy.

Click on the links below to learn more.

• Hobby-Eberly Telescope
• WIYN Telescope