(CNN) -- NASA's new planet-hunting TESS telescope has found an exoplanet three times the size of Earth only 53 light-years away. It has also discovered a super-Earth and a rocky world, making three exoplanet discoveries in the first three months since it began surveying the sky in July.
The nearby exoplanet, HD 21749b, orbits a bright neighboring star in the Reticulum constellation, with a 36-day orbit and a surface temperature of 300 degrees Fahrenheit. That's actually quite cool, considering how close the planet is to its star.
The discovery was announced Monday at the 233rd meeting of the American Astronomical Society in Seattle.
"It's the coolest small planet that we know of around a star this bright," said Diana Dragomir, a postdoctoral researcher in the Massachusetts Institute for Technology's Kavli Institute for Astrophysics and Space Research. "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."
Its size, three times that of Earth, makes it a sub-Neptune. But despite the exoplanet's size, it has 23 times the mass of Earth. This means it is most likely a gaseous planet, rather than rocky like Earth. But the gas that makes it up it is probably more dense than that of Uranus or Neptune.
"We think this planet wouldn't be as gaseous as Neptune or Uranus, which are mostly hydrogen and really puffy," Dragomir said. "The planet likely has a density of water or a thick atmosphere."
There is also evidence that a second planet could exist in this system with a 7.8-day orbit -- which could be the first Earth-size planet discovered by TESS.
The Transiting Exoplanet Survey Satellite launched in April to take up the planet-hunting baton from the Kepler space telescope as that historic mission ended.
The other two planets it has discovered 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 a whopping 11-hour orbit.
Pi Mensae c is about twice the size of Earth and orbits the star Pi Mensae, 60 light-years away in the Mensa constellation. The star is very similar to our sun, both in mass and in size.
"This star was already known to host a planet, called Pi Mensae b, which is about 10 times the mass of Jupiter and follows a long and very eccentric orbit," said Chelsea Huang, a Juan Carlos Torres Fellow at the MIT Kavli Institute. "In contrast, the new planet, called Pi Mensae c, has a circular orbit close to the star, and these orbital differences will prove key to understanding how this unusual system formed."
LHS 3884b is 49 light-years away in the Indus constellation, orbiting a cool M-dwarf star that's only about one-fifth the size of our sun. It's probably a "lava world," Huang said, given how close some of the planet's rocky surface is to its star.
"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."
During this initial survey, TESS also witnessed six supernova explosions, recorded before ground-based telescopes saw them.
"Some of the most interesting science occurs in the early days of a supernova, which has been very difficult to observe before TESS," said Michael Fausnaugh, a TESS researcher at the MIT Kavli Institute. "NASA's Kepler space telescope caught five of these events as they brightened during its first four years of operations. TESS found as many in its first month."
How TESS works
TESS is surveying an area in the sky that is 400 times larger than what Kepler observed, including 200,000 of the brightest nearby stars. Over the course of two years, the four wide-field cameras on board will stare at different sectors of the sky for days at a time. This will enable scientists to survey nearly the entire sky.
These findings are from just the first few sectors TESS has observed in the Southern Hemispheres, beginning in July.
TESS will look for exoplanets using the transit method, observing slight dips in the brightness of stars as planets pass in front of them. Bright stars allow for easier followup study through ground- and space-based telescopes.
NASA expects TESS to allow for the cataloging of more than 1,500 exoplanets, but it has the potential to find thousands. Of these, officials anticipate, 300 will be Earth-size exoplanets or double-Earth-size Super Earths. Those planets could be the best candidates for supporting life outside our solar system. Like Earth, they are small, rocky and usually within the habitable zone of their star, meaning liquid water can exist on the surface.
TESS is considered to be a "bridge to the future," finding exoplanet candidates to study in more detail.
These exoplanets will be studied so that NASA can determine the best targets for future missions, like the James Webb Space Telescope. That telescope, launching in 2021, would be able to characterize the details and atmospheres of exoplanets in ways scientists have not been able to do.
Scientists are already working on followup observations for more than 280 planet candidates that TESS has found.