NASA has announced that its planet hunter satellite TESS discovered an Earth-sized world within the habitable range of its star, which could allow the presence of liquid water.

The planet, named "TOI 700 d", is relatively close to Earth: only 100 light years away, NASA's Jet Propulsion Laboratory said on Monday during the annual American Astronomical Society meeting in Honolulu, Hawaii.

"TESS was designed and launched specifically to find Earth-sized planets orbiting nearby stars," said Paul Hertz, NASA astrophysics division director.

TESS initially misclassified the star, which meant the planets appeared larger and hotter than they actually are. But several amateur astronomers, including high school student Alton Spencer - who works with members of the TESS team - identified the error.

"When we corrected the star's parameters, the sizes of its planets dropped, and we realised the outermost one was about the size of Earth and in the habitable zone," said Emily Gilbert, a graduate student at the University of Chicago.

The discovery was later confirmed by the Spitzer Space Telescope.

Habitable zone

A few other similar planets have been discovered before, notably by the former Kepler Space Telescope, but this is the first one discovered by TESS, which was launched in 2018.

TESS stabilises on one area of the sky to detect whether objects - planets - pass in front of stars, which causes a temporary drop in the stars' luminosity. This allows TESS to infer the presence of a planet, its size and orbit.

Star TOI 700 is small, about 40 percent of the size of the Earth's Sun and only about half as hot.

TESS discovered three planets in orbit, named TOI 700 b, c and d. Only "d" is in the so-called habitable zone, not too far from and not too close to the star, where the temperature could allow the presence of liquid water.

It is about 20 percent larger than Earth and orbits its star in 37 days. Planet 700 d receives 86 percent of the energy that Earth receives from the Sun.

It remains to be seen what 700 d is made of. Researchers have generated models based on the size and type of star in order to predict 700 d's atmospheric composition and surface temperature.

In one simulation, NASA explained, the planet is covered in oceans with a "dense, carbon dioxide-dominated atmosphere similar to what scientists suspect surrounded Mars when it was young".

The planet is tidally locked to the star, meaning that one side always faces the star, as is the case with the Moon and Earth.

This synchronous rotation meant that, in another model, one side of the planet was constantly covered in clouds.

A third simulation predicted an all-land world, where winds flow from the planet's dark side to its light one.

Multiple astronomers will observe the planet with other instruments, in order to obtain new data that may match one of NASA's models.