At least two nearly identical Earth-sized planets have been discovered orbiting one of our closest stars.

In space, over seven thousand confirmed exoplanets have already been identified. Most of the new worlds are discovered when they transit across the face of their star: the star dims slightly, providing information not only about the existence of the planet but also about its size. The frequency of these dimming events indicates how close the planet is to its sun: the more often it crosses in front of the star, the closer it is located.

Particular interest is drawn to a phenomenon that astronomers have dubbed the "pea pod": a collection of nearly identical rocky planets within the same system. The most notable example is the TRAPPIST-1 system, where some of these worlds are also situated in the potentially habitable zone—where conditions are neither too hot nor too cold, but just right for the existence of liquid water.

This raises the question of how these planets came to be so similar to one another and why there is such diversity in our Solar System. Recently, a team of astronomers from the USA, Australia, Chile, and other countries reported another such case: the HD 101581 star system, located in the southern sky, about 42 light-years away from us.

This star is roughly one-third smaller than the Sun; it is an orange dwarf with a temperature of about 4600 Kelvin, which is over a thousand degrees cooler than our yellow dwarf star. In a paper (available on the Cornell University preprint server), the scientists noted that two planets are definitely observed near this star, and a third is suspected. They were discovered using the aforementioned transit method.

All three represent those very "peas in a pod": each has a diameter nearly identical to that of Earth, while their masses are 3.6, 4.2, and again 3.6 times greater than that of our planet. This means these planets are nearly identical in "weight" as well.

Interestingly, they are located extremely close to their parent star: the first, HD 101581 b, is less than seven million kilometers away, the second, HD 101581 c, is eight and a half million kilometers away, and the unconfirmed third is just a bit farther. They complete a full orbit around their sun in four and a half, just over six, and nearly eight Earth days, respectively. For perspective, Mercury is about 58 million kilometers from the Sun and takes 88 Earth days to complete one orbit.

It is intriguing that these worlds are situated so close to their star—so densely packed, as if compressed. For example, previously Naked Science reported on an exoplanet located near its star, which, despite being one and a half times the size of Earth, has a mass eight times greater than Earth's. It was suggested that this might not be a full-fledged planet, but rather just the remaining solid core, with its entire gaseous outer layer having been "blown away" by stellar winds.

It is worth noting that the orange star hosting the "pea planets," HD 101581, is estimated to be nearly seven billion years old, making it significantly older than the Sun, which is 4.6 billion years old.