Kepler's final exoplanet discovery revealed

Just months after its mission ended and a decade after its launch, glimmers of data detected by NASA’s Kepler space telescope in 2009 have been confirmed as evidence for a large, hot-Jupiter-like planet orbiting a star 2600 light years from Earth.

That 10-year mission to find distant planets around distant stars has shown that the universe is literally teeming with planets. There are more than 2300 confirmed exoplanets, ranging from huge gas giants to rocky worlds, perhaps not dissimilar to Earth.

It is likely, maybe, that we are not alone.

Now, an international team of astronomers has solved Kepler’s first mystery: the curious case of KOI-4, ‘Kepler object of interest – four’.

Evidence gleaned a decade ago, from the mission’s initial sweep of data, prompted KOI-4 as the first candidate for a new planet discovered by Kepler. It was quickly dismissed as a false positive – just a two-star binary system giving hope where there was none.

However, some galactic sleuthing led by University of Hawaii PhD student, Ashley Chontos, has meant that Kepler’s uncertain first finding has become its latest confirmed discovery.

That work was assisted by University of Sydney PhD student Isabel Colman, who helped rule out that the light from the host star Kepler 1658 was contaminated by light from a nearby star.

“Much of what I do is work on close analysis of Kepler pixel data,” Ms Colman said. “Looking at the signal from KOI-4, we could see a rotating sun spot. Using a technique called difference imaging to pinpoint the pixel source, I could confirm the signal came from KOI-4, giving us more information about the star.”

Star bigger than first thought

The work was further supported by Ms Colman’s supervisor, University of Sydney Professor Tim Bedding. His work on astroseismology – analysing the internal processes of stars – helped confirm the star was much bigger than first thought, meaning the mass of the suspected exoplanet was large enough to explain the cyclical dips in light from stars caused by orbiting planets.

“It is a happy spin-off that the same Kepler observations used to reveal exoplanets can also be used to measure the size of the host star,” Professor Bedding said.

Piecing this information together with work from other researchers, Ms Chontos in Hawaii could conclude that the object was, in fact, a Sun-like star with an orbiting ‘super-hot Jupiter’ planet, now known as Kepler 1658-b.

“Our new analysis, which uses stellar sound waves observed in the Kepler data to characterise the star, demonstrated that the star is in fact three times larger than previously thought. This in turn means that the planet is three times larger, revealing that Kepler 1658-b is actually a hot Jupiter,” Ms Chontos said.

Compared to the Sun, Kepler 1658 is 50 percent more massive and three times larger. While it is younger than the Sun, having more mass means it has burned through its hydrogen fuel faster and is already expanding towards becoming a red giant, the penultimate fate of our home star.

The planet orbits just 8.2 million kilometres from Kepler 1658, which is even closer than Mercury is to our Sun. This means it is one of the closest planets found orbiting a star. Eventually it will be destroyed as it spirals into its host star.

“Kepler 1658 is a perfect example why a better understanding of exoplanets’ host stars is so important,” Ms Chontos said. “It is also telling us that there are many treasures left to be found in the Kepler data.”

Ms Colman agreed. “There are still thousands of potential exoplanet hosts in the Kepler data, so there is plenty more to be found.”

The discovery was announced today by Ms Chontos at the fifth Kepler/K2 Science Conference held in Glendale, California.