Rare 'super-Neptune' exoplanet found orbiting nearby star

Astronomers using telescopes from around the world have discovered an exoplanet more massive than Neptune, orbiting a star cooler than the Sun at an orbital radius similar to that of the Earth.

Around cool stars, this orbital region is thought to be the birth place of gas-giant planets, according to the researchers at National Astronomical Observatory of Japan.

The results, published in the Astronomical Journal, suggest that Neptune-sized planets could be common around this orbital region.

Because the exoplanet discovered is closer than other planets discovered by the same method, it is a good target for follow-up observations by telescopes like the Subaru Telescope, the researchers said.

In 2017, amateur astronomer Tadashi Kojima in Gunma Prefecture, Japan reported an enigmatic new object in the constellation Taurus.

Astronomers around the world began follow-up observations and determined that this was an example of a rare event known as gravitational microlensing.

The researchers explained that Einstein's Theory of General Relativity tells us that gravity warps space.

If a foreground object with strong gravity passes directly in front of a background object in outer space, this warped space can act as a lens and focus the light from the background object, making it appear to brighten temporarily.

In the case of the object spotted by Kojima, a star 1600 light-years away passed in front of a star 2600 light-years away, the researchers said.

By studying the change in the lensed brightness, astronomers determined that the foreground star has a planet orbiting it.

This is not the first time an exoplanet has been discovered by the microlensing technique, the researchers noted.

However, microlensing events are rare and short lived, so the ones discovered so far lie towards the Galactic Center, where stars are the most abundant.

In contrast, this exoplanet system was found in almost exactly the opposite direction as observed from the Earth, according to the researchers.

A team led by Akihiko Fukui at the University of Tokyo in Japan using a collection of 13 telescopes located around the world observed this phenomenon for 76 days and collected enough data to determine the characteristics of the exoplanet system.

The researchers found calculated that the host star has a mass about half that of the Sun.

The exoplanet around it has an orbit similar in size to Earth's orbit, and a mass about 20 per cent heavier than Neptune, the found.

This orbital radius around this type of star coincides with the region where water condenses into ice during the planet formation phase, making this place theoretically favourable for forming gas-giant planets, the researchers said.

This exoplanet system is closer and brighter as seen from Earth than other exoplanet systems discovered by microlensing, they said.