Astronomers have detected what may be four roughly Earth-size planets orbiting Tau Ceti, the nearest sun-like star.

Two of the worlds appear to orbit within Tau Ceti's habitable zone, though a cloud of debris and asteroids may pose a threat to any life on them.

If the result is confirmed, independent astronomers say it would be "astonishing.

Astronomers may have just hit a crucial milestone in the search for other Earth-like planets. Scientists from the University of Hertfordshire and University of California, Santa Cruz announced they've discovered four planets orbiting a nearby, sun-like star — two of which may be habitable.

Researchers have turned up thousands of planet candidates in recent years, about 10 of which may be small, rocky, and habitable like the Earth. You'd think most of these worlds would orbit suns like ours, but that's not the case, since such stars are so big and bright that they easily drown out the faint signals of tiny planets.

That's why the new discovery of four roughly Earth-size worlds — some 12 light-years away from our solar system — is all the more exciting.

"If true this discovery is absolutely phenomenal — that one of our nearest neighboring sun-like stars might have rocky worlds," Sara Seager, a planetary scientist at MIT who wasn't involved in the research, told Business Insider in an email.

Worlds in the closest sun-like solar system

The suspected planets all orbit Tau Ceti, a star located 11.9 light-years away from us, according to a forthcoming study in The Astrophysical Journal. (You can read a pre-print version of the paper on arXiv.)

The star is about three-quarters the mass of the sun, but its brightness and color are very sun-like. It's the closest sun-like star to Earth.

The planets' sizes aren't known yet, but they're estimated to have about 1.7 times the Earth's mass. That would make them the smallest planets ever detected around a distant sun-like star, according to a press release emailed by the University of Hertfordshire.

Two of the four worlds orbit in a searing-hot zone close to Tau Ceti. The other two "super-Earths" seem to orbit within a "Goldilocks" habitable zone, where water on the surface can be liquid (rather than frozen solid or boiled away), according to a press release by the University of California Santa Cruz.

Two things make this particular discovery stunning to astronomers like Seager.

First, our solar system is the only place we know where life exists, which means that sun-like stars may be the best places to look for life (though there is some debate about whether smaller, cooler red dwarf stars could be better to explore).

Second, it's incredibly difficult to spy a relatively tiny planet in the figurative shadow of a sun-like star.

"Earth is so small in mass compared to its host sun-like star that finding the signal amidst the noise is really like finding the proverbial needle in a haystack," Seager added. "The authors have come up with a special technique to get rid of the noise to find the signal. It's always a tricky situation to look for very weak signals."

Scouting for tiny wobbles

Most planets are detected by a "wobble method," where a planet slightly jerks around its star.

Such wobbles or wiggles (technically called Doppler shifts) come from planets sharing a center of gravity that doesn't reside precisely in the middle of a star. Jupiter, for example, actually orbits a spot some 30,000 miles above the sun's surface — and not in a perfect circle.

A to-scale illustration of where the sun-Jupiter barycenter is located. NASA/SDO; Business Insider

This wobbling causes slight speeding up and slowing down of a star as a planet orbits it, leaving an imprint on the light the star emits.

In their new study, the team detected variations on the order of 30 centimeters per second — a gap of about 2/3 of a mile per hour. Astronomers think they'll need a sensitivity three times better (about 0.2 mph) to reliably detect Earth analogs around sun-like stars.

"We're getting tantalizingly close to observing the correct limits required for detecting Earth-like planets," Fabo Feng, an astrophysicist at the University of Hertfordshire who led the research, said in the press release. "Our detection of such weak wobbles is a milestone in the search for Earth analogs and the understanding of the Earth's habitability through comparison with these [new planets]."

The Keck I and Keck II telescopes on Mauna Kea in Hawaii. W.M. Keck Observatory To find the planets, it took Feng and his colleagues four years to sift through, analyze, and model the data gathered by two telescope instruments: the European Southern Observatory's HARPS spectrograph in Chile and the W.M. Keck Observatory's HiRes instrument in Hawaii.

Seager emphasized that "it would take a long time" for other astronomers to replicate and verify the discovery.

"There is no sugar-coating how hard it is to find an Earth-mass planet signal in a sun-like star, due to star noise and instrument noise," Seager said.

Two hits to habitability

Small, rocky worlds in a habitable zone aren't necessarily cozy environments for life.

One possible problem with Tau Ceti's two habitable-zone-worlds is their unknown size.

"They surely don't conclude that the four planets are Earth-sized, because the method they used to detect them provides no information on their sizes," Michaël Gillon, an astronomer at the Université de Liège who wasn't involved in the study, told Business Insider in an email. (Gillon helped with the discovery of the seven Earth-size planets orbiting the TRAPPIST-1 star system.)

"All they can say is that each planet is more massive than X Earth-masses," he said. In the case of Tau Ceti, this lower limit seems to be about 1.7 times as hefty as Earth, and it's uncertain if complex life could thrive on a planet like this.

Another problem is the debris.

NASA/JPL-Caltech

In fact, estimates suggests that the Tau Ceti system has roughly 10 times the mass of asteroids (and dust) than our solar system.

That's not a great sign, because asteroid impacts are known to wipe out vast numbers of species.

Still, Seager characterized the result as "astonishing." Once the planets are confirmed by other researchers, she added, it could be "a tremendous milestone en route to finding our Earth twin."