NASA's planet hunter TESS has just discovered a new distant world

Nasa's planet-hunting Transiting Exoplanet Survey Satellite (TESS) has made it's third discovery since it began surveying space in July, it has now spotted an exoplanet that is triple the size of Earth.

The rocky planet is only 53 light-years away and has been given the catchy title of HD 21749b for now.

An average temperature of 149 degress celsius on the planet's surface may seem rather hot, but given it's proximity to its sun it's actually considered quite cool.

The exoplant has a 36-day long orbit around a bright star in the Reticulum constellation.

It's the coolest small planet that we know of around a star this bright," said Diana Dragomir, a postdoctoral researcher in the MIT's Kavli Institute for Astrophysics and Space Research told CNN.

"We know a lot about atmospheres of hot planets, but because it's very hard to find small planets that orbit farther from their stars and are therefore cooler, we haven't been able to learn much about these smaller, cooler planets. But here, we were lucky and caught this one and can now study it in more detail."

She added: "We think this planet wouldn't be as gaseous as Neptune or Uranus, which are mostly hydrogen and really puffy. The planet likely has a density of water or a thick atmosphere."

It is also thought that a second planet could exist in this system with a 7.8-day orbit - this could be the first Earth-size planet dicovered by TESS.

The other two planets TESS has discovered over the past seven months are Pi Mensae c, a super-Earth that zips around its star in 6.3 days, and LHS 3844b, a rocky planet that flies around its planet in an 11-hour orbit.

Pi Mensae c is about twice the size of Earth and orbits the star (very similar to our sun in size), named Pi Mensae.

"We've confirmed three planets so far, and there are so many more that are just waiting for telescope and people time to be confirmed," Dragomir said. "So it's going really well, and TESS is already helping us to learn about the diversity of these small planets."