Astronomers using the Subaru Telescope on Mauna Kea have found the lowest-mass planet ever detected around a sunlike star using direct imaging techniques.

Most so-called exoplanets are found when they pass in front of their star, making it dim ever so slightly, or cause small gravitational wobbles in the star that can be seen in a lightwave shift.

The planet is several times the mass of Jupiter and similar in size, and is glowing a dark magenta from the heat of its formation only 160 million years ago, the research team said Monday.

While direct imaging, in this case in near infrared wavelengths, is arguably the most important technique for observing planets around other stars, it is also the most challenging.

“Imaging provides information about the planet’s luminosity, temperature, atmosphere and orbit, but because planets are so faint and so close to their host stars, it’s like trying to take a picture of a firefly near a searchlight,” Masayuki Kuzuhara at the Tokyo Institute of Technology, who led the discovery team, said in a statement Monday.

Actually, the planet is much farther away from its star than most gas giants, raising questions about its origin.

Recent research indicates that, with a few known exceptions, gas giants form relatively close to their parent star. Jupiter, for instance, is at five times the Earth-sun distance (93 million miles), a common yardstick called an astronomical unit. 

Uranus, at 19 AU, and Neptune, at 30 AU, are unusually distant.

The new planet, dubbed GJ 504b, is at an estimated 43.5 AU.

According to widely accepted theory, called the core-accretion model, Jupiter-like planets get their start in the gas-rich debris disk that surrounds a young star. A core produced by collisions among asteroids and comets provides a seed, and when this core reaches sufficient mass, its gravitational pull rapidly attracts gas from the disk to form the planet.

While this model works for planets out to Neptune’s distance, it’s more problematic for worlds farther out, astronomers say.

“This is among the hardest planets to explain in a traditional planet-formation framework,” said team member Markus Janson, a postdoctoral fellow at Princeton University in New Jersey. “Its discovery implies that we need to seriously consider alternative formation theories, or perhaps to reassess some of the basic assumptions in the core-accretion theory.”

The research is part of the Strategic Explorations of Exoplanets and Disks with Subaru, a project to directly image extrasolar planets and protoplanetary disks around several hundred nearby stars using the Subaru Telescope on Mauna Kea, Hawaii. The five-year project began in 2009.

The star, GJ504, also known as 59 Virginis is 57 light-years away and visible to the naked eye in the constellation Virgo.

The discovery will be detailed in an upcoming issue of the Astrophysical Journal.