An international team of astronomers has discovered two Earth-sized planets around a nearby star only 12.5 light-years away. They are some of the closest exoplanets to our Solar System and both orbit their star in the habitable zone, the region where water may be found in all three states.

As reported in Astronomy & Astrophysics, the star in question is called “Teegarden’s Star”, a red dwarf only discovered in 2003 that's about 10 percent the mass of the Sun. It’s fairly dim and cool with a surface temperature of 2,700°C (4,900°F).

Due to its lower temperature compared to the Sun, the habitable zone is much closer. The two planets, Teegarden b and Teegarden c, orbit in 4.9 and 11.4 days respectively and are slightly more massive than our own planet and Venus, the largest rocky planets in the Solar System.

"The two planets resemble the inner planets of our Solar System," lead author Mathias Zechmeister, from the University of Göttingen, said in a statement. "They are only slightly heavier than Earth and are located in the so-called habitable zone, where water can be present in liquid form.”

Previous observations suggested that the planets might be present around this star, but there was not enough data to confirm it. To do so, the team used the CARMENES high-resolution spectrograph at the Calar Alto Observatory and looked for periodic variations in the light of the star over the last three years.

The method employed to find these planets is known as Doppler-spectroscopy, or the radial-velocity method – about one-third of all exoplanets discovered so far have been detected in this way. The instruments look at the small motions of the star based on the movement of the planets orbiting it. The team thinks there is a chance the system has more undiscovered planets. 

"This is a great success for the Carmenes project, which was specifically designed to search for planets around the lightest stars," said Professor Ansgar Reiners of the University of Göttingen, one of the scientific directors of the project.

Thanks to the many observatories both ground-based and in space, we have now discovered more than 4,000 planets beyond the Solar System. While Earth-sized planets are still a minority given the difficulties in detecting them, dedicated instruments like TESS are expected to bring a plethora of new worlds to the fold.