The observation breaks new ground in the hunt for exoplanets — worlds that exist in other solar systems — although the planet itself is not "another Earth" as it is located in a scorchingly hot zone.
The planet swings close to the star Alpha Centauri B, one of a triple star system that is 4.3 light years away, which in cosmic terms is just next door to us.
The find was made thanks to a telltale wobble in the star's motion, tugged by the gravitational pull of the passing planet.
The signal is "tiny but real," said Xavier Dumusque of the Geneva Observatory in Switzerland. "It's an extraordinary discovery and it has pushed our technique to the limit."
The transiting planet makes the star move back and forth, in relation to Earth, by less than two kilometres (one mile) per hour, about the speed of a baby crawling.
The minute motion was detected using an instrument called HARPS, for high-precision spectrograph, installed on the European Southern Observatory (ESO) at La Silla, in the depths of the Atacama desert in Chile.
It took hundreds of observations, spanning more than four years, for the "wobble" to be teased out of the other light signals.
The study, published in the journal Nature, shows the technical advances in exoplanet research since the very first was spotted in 1995.
Since then more than 750 exoplanets have been confirmed as sightings and another 2,300 claims are pending.
None, however, is both Earth-sized and in a habitable area, something that would bolster theories that life can exist beyond our world.
The quest is for a rocky planet the size of Earth in the "Goldilocks zone."
This means a planet at a comfy distance from the star, enabling the temperature to be not too hot nor too cold, but just right to sustain liquid water, the stuff of life.
In the case of Alpha Centauri B's planet, its "year" — the time it takes to complete one orbit — is just 3.236 days, which means it is located just six million kilometres (3.75 million miles) from its star.
But other finds could follow, and an even more tantalizing discovery could be among them, the astronomers hope.
"Statistical studies of exoplanets suggest that low-mass planets are preferentially formed in multi-planetary systems," says the paper.
"There is therefore a high probability that other planets orbit Alpha Centauri B, perhaps in its habitable zone."
Alpha Centauri B and Alpha Centauri A are two stars that are similar to the Sun.
The third star in their system is somewhat more distant and cooler, and is named as Proxima Centauri.
These stars are very close to us in terms of cosmic distance, given that our galaxy, the Milky Way, is around 100,000 light years across.
Even so, they could never be reached by us using current chemical-based rocket technology.