In A Reverse Pluto Situation, A Planet We Didn't Think Was One, Actually Is

A decade after being discovered, the very first exoplanet candidate found by NASA's Kepler space telescope has finally been confirmed as a big ol' fiery planet.

You know what a planet is, I know what a planet is, and you can bet your bottom dollar NASA knows what a planet is -- but when it's floating out there some 2600 light years from Earth, it can take some time to confidently say 'now that's a planet'.

Just months after the telescope finished its mission, research from the University of Hawaii has concluded the first exoplanet candidate Kepler found is a hot Jupiter-like planet despite initially dismissing it as a false alarm.

An exoplanet is a planet found outside our own solar system, where it is hoped we will find planets not unlike our own and one day, fingers crossed, life.

"This is an exciting discovery," University of Sydney PhD student Isabel Colman, who was a part of the international team involved in the study, told 10 daily.

"What's most exciting about this, apart from that planets around sub-giants are quite rare as far as we've found, is that this was the first new signal of a planet detected by Kepler 10 years ago."

Now known as Kepler 1658-b, the planet was first spotted when it passed in front of the star it orbits -- Kepler 1658-- stopping a bit of starlight from reaching the telescope.

But after researchers underestimated the size of both the planet and its host star, it was quickly dismissed as a false positive following further analysis.

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"We had a kind of incomplete understanding of the host star. What this shows is just how important it is to understand the star if you want to understand the planet," Colman said.

We now know Kepler 1658-b is orbiting around a star three times bigger than our own sun, and with 50 percent more mass.

Sitting only about 8.2 million kilometres from Kepler-1658 -- a distance smaller than that between Mercury and our own sun -- the newly-discovered planet is orbiting closer to an evolved star than any others we know of.

If you were to stand on Kepler 1658-b, the star would appear 60 times larger in diameter than the Sun as seen from Earth.

An artist's rendition of the Kepler in space. Image: Getty

Despite being younger than our sun, having more mass means Kepler-1658 is already expanding towards becoming a red giant -- the penultimate stage of a star's life.

In time, our new friend Kepler 1658-b will spiral to its death, destroyed by its own star.

"It's kind of an ill-fated planet, so to speak," Coleman said.

"But we're talking hundreds of millions of years until that happens so it's got a while."

Since launching in early 2009, the Kepler telescope has found more than 2600 confirmed exoplanets and nearly 3000 further candidates.

It wrapped up its mammoth nine-and-a-half-year search for Earth-like planets in October last year, despite originally intended to only last three-and-a-half-years.