How Many Planets Orbit Our Nearest Neighboring Star?

Some of the most promising places to look for extraterrestrials have remained, so far, mostly hidden from astronomers. Now a game-changing instrument called NIRPS (Near-Infrared Planet Searcher) is leading the search for the most tantalizing targets in the cosmos: potentially Earth-like worlds around nearby red dwarf stars.

Red dwarfs, or M dwarfs, are the most tempting places to seek alien Earths because they’re the most abundant and enduring stars. They make up the majority of the stars in the Milky Way and shine with a slow thermonuclear simmer that should allow them to live exponentially longer than most—even, say, for 14 trillion years, or 1,000 times the current age of the universe.

But M dwarfs are also the smallest, dimmest stars, so they and their planets can be difficult to detect and inspect. Enter NIRPS, an instrument custom-built to tease out subtle signs of otherwise hidden worlds by making unmatched high-precision measurements of M dwarfs, which emit most of their light in infrared and near-infrared wavelengths.

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“I think we are in the golden age of M dwarfs, where they offer a huge amount of possibility—they are the best place to detect small Earth-sized planets,” says Natalie Hinkel, a planetary astrophysicist at Louisiana State University, who is not a member of the NIRPS team.

The M Dwarf Frontier

NIRPS can find planets via the radial velocity (RV) method, which looks for the tiny gravitational tug they exert on their stars. This tug ever-so-slightly changes the star’s velocity, which in turn almost indiscernibly changes the color of its light. When an orbiting planet tugs its star closer to Earth, the star’s light shifts slightly to the blue end of the spectrum in our telescopes; when the planet tugs its star away, the starlight shifts slightly to the red end. The cyclic blue-to-red stellar wobble that is the RV signature of a small world orbiting an M dwarf corresponds to a velocity shift of less than a meter per second. M dwarfs are especially well suited for RV planet surveys because their low masses can result in larger, more obvious tugs from any accompanying tiny worlds.

René Doyon, a professor at the University of Montreal and co-principal investigator of NIRPS, contextualized the breakthrough in a press release: “For the first time, we can reach sub-meter-per-second radial velocity precision in the infrared.” This means that NIRPS can view a distant star zooming through space and discern a change in its velocity that’s equivalent to the speed of a leisurely stroll through the park.

NIRPS and other RV-based projects are spectrographs, akin to prisms attached to existing telescopes. Much like a prism spreads white light into a rainbow of colors, a spectrograph splits incoming starlight into its constituent wavelengths, producing a rainbowlike spectrum. “Fingerprints” of different atoms and molecules in a star’s atmosphere can be imprinted on its spectrum, and they serve as reference marks to planet-hunters looking for minuscule velocity shifts.

Last month, in the first NIRPS science releases since the beginning of its operations in April 2023, researchers reported their painstaking observations of the red dwarf Proxima Centauri, the solar system’s nearest neighboring star, located just 4.2 light-years away. NIRPS helped confirm the presence of Proxima Centauri b, a roughly Earth-mass planet in the star’s habitable zone where liquid water might exist. It also confirmed another, smaller planet, Proxima Centauri d, which is only one third the mass of Earth. Finally, NIRPS refuted the existence of another potential planet that was purported to be in the system, Proxima Centauri c.

Although two confirmations—and one dismissal—of previously claimed worlds may not make many headlines, the NIRPS result is a remarkable RV feat and a major step forward in understanding the true nature of the planetary system closest to our own.

Yet Proxima is a single case, and improved RV instruments like NIRPS are set to discover a bigger, richer treasure trove of worlds around nearby M dwarfs. Of such stars within our own galaxy, astronomers estimate that perhaps one in five harbor planets in their habitable zones.

This embarrassment of planetary riches means that NIRPS can’t stand alone at the forefront of RV surveys. Much of its work is augmented by complementary observations from another spectrograph called the High Accuracy Radial Velocity Planet Searcher (HARPS), which is attached to the same telescope as NIRPS at La Silla Observatory in the Chilean Atacama Desert. HARPS has been engaged in an RV hunt for planets since 2003, although it looks in optical light rather than near-infrared, as NIRPS does. Together they can distinguish real planets from false positives caused by a star’s flares, spots and magnetic activity.

It’s no wonder, then, that the NIRPS-HARPS duo is the most oversubscribed instrument at La Silla Observatory, with a “combined request of 3277 hours” over a seven-month observation period, according to François Bouchy, an associate professor in the Department of Astronomy at the University of Geneva and NIRPS co-principal investigator.

Expanding the Exoplanet Catalog

So far, whether in near-infrared or optical, the RV technique has revealed more than 1,100 of the nearly 6,000 currently known exoplanets. It’s second only to the transit planet-detection method, which looks for dips in a star’s light as an orbiting planet passes in front of it, like a circling moth silhouetted against a flame. Overall, the transit method has yielded nearly 4,500 exoplanets—almost 75 percent of the worlds cataloged in NASA’s Exoplanet Archive.

But while transits may now eclipse RV as a planetary discovery method, RV is still an important tool for follow-up studies. NIRPS and its kin can winnow the wheat from the chaff for thousands upon thousands of planetary candidates found via transits, separating the true planets from the myriad potential false positives masquerading as transiting worlds. These next-generation RV instruments are also vital for revealing crucial details—most importantly, a planet’s estimated mass—which transits can’t typically resolve.

Mass measurements can be critical for distinguishing between newfound similarly sized planet types to reveal whether they’re water-rich, gassy, rocky or some surprising combination of these.

Yet NIRPS is just the most recent in an ever-growing family of RV planet-finders. It’s joined in its exploratory endeavors by others such as NN-EXPLORE Exoplanet Investigations with Doppler Spectroscopy (NEID), another extremely precise near-infrared spectrograph sponsored by NASA and the National Science Foundation and operating on the 3.5-meter WIYN Telescope at Kitt Peak National Observatory in Arizona. Similar to NIRPS, much of NEID’s work involves validating possible planets provided by other facilities; the instrument has recently confirmed candidates sourced from NASA’s Transiting Exoplanet Survey Satellite mission (TESS) and the European Space Agency’s (ESO’s) Gaia mission.

NEID observes a narrower wavelength range than NIRPS, though it spans the visible-light range from deep blue to near-infrared. It can measure changes in a star’s velocity that are on par with the crawling speed of an infant, almost 30 centimeters per second—though this does not include NIRPS’s broader range across the infrared. Yet, as Doyon explains, “detecting the [RV signal of] Earth around the sun” would require an even sharper precision of around 10 centimeters per second.

Reaching such extreme precision is not yet commonplace, but an existing RV instrument is getting close: ESPRESSO (Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations), which is installed on the ESO’s Very Large Telescope (VLT) in Chile, can achieve a sensitivity of around 20 centimeters per second. That’s precise enough to find worlds smaller than Earth, including Barnard b, which orbits Barnard’s Star, the nearest single-star system to Earth. (Proxima Centauri is part of a triple-star system.)

Barnard b is unlikely to be much like Earth, though: its orbit is 20 times smaller than Mercury’s, giving it a year of about three days and an estimated temperature of around 125 degrees Celsius.

Otherworldly Horizons

Although NIRPS can find worlds aplenty, because it’s typically observing a planet’s host star rather than a planet itself, the instrument can’t seek out any direct signs of alien life. But it demonstrates that astronomy, like history, is cyclical. Just as other observatories picked out candidates for NIRPS and other RV spectrographs to explore, so too will these instruments select promising planetary targets for atmospheric studies by powerful facilities such as the James Webb Space Telescope.

NIRPS will also identify interesting exoplanets for next-generation observatories like NASA’s Nancy Grace Roman Space Telescope, which may launch as early as 2026, and ESO’s upcoming Extremely Large Telescope (ELT), which is being built on the summit of Cerro Armazones in Chile and will become the world’s largest ground-based telescope later this decade.

Like every revolutionary technology, NIRPS is already inspiring successors: namely, a follow-up project called ANDES (Armazones High Dispersion Echelle Spectrograph), which is being developed for the ELT with the goal of parsing the light not only from planet-hosting stars but also from some potentially habitable planets themselves and potentially probing them for biosignatures. “ANDES is NIRPS on steroids,” Doyon says.

Farther over the cosmic horizon, planets picked up by NIRPS may be scoured by next-next-generation projects, such as NASA’s Habitable Worlds Observatory, which is envisioned as the first ever space telescope specifically designed to answer science’s most pressing question: How alone are we, really?