August 06, 2013

Using the Subaru telescope on Hawai'i, a group of astronomers including scientists from the Max Planck Institute for Astronomy have taken a portrait of the Jupiter-like planet GJ 504b – the first planet that orbits a sun-like star. GJ 504b, which is the name of the celestial object, is the coldest and lightest planet yet imaged. Astronomers have thus taken an important step towards finding proof of distant planetary systems that are similar to our own Solar system.

To date, astronomers have confirmed the existence of nearly 930 exoplanets (that is, planets orbiting stars other than the Sun). Almost all of them have been detected only indirectly, by the pull they exert on their host star, or when they block part of their host star's light.

But if, say, extraterrestrial astronomers were to observe our own Solar System with those indirect methods, they would miss out on a number of its most intriguing aspects. In particular, they would be likely to miss the slow-moving outer planets. This is where methods such as direct imaging of exoplanets – observations producing images on which the planet is clearly identifiable – can help. Such images also give information about a planet's temperature and atmosphere. Direct spectroscopy (currently in its infancy) would yield detailed information about the atmosphere's chemical composition.

That is why astronomers (here on Earth) are so keen to develop methods for direct imaging of exoplanets – they are a key element of the strategy to detect and examine distant analogues of our own Solar System. Obtaining direct images is, however, a difficult task. Stars are much brighter than their planets (typically by a factor of a billion or more); using traditional observational techniques, the planet will be hidden in the glare of its host star. Only with sophisticated tricks – such as blocking out the star's light mechanically (coronagraphy) and combining several images in just the right way to reduce spurious signals – can astronomers obtain these images at all.

Now a group of astronomers led by Motohide Tamura (National Observatory of Japan [NAOJ] and Tokyo University), which includes several astronomers from the Max Planck Institute for Astronomy (MPIA), has come an important step closer to detecting far-away analogues of our own planetary system. Using the Subaru Telescope in Hawai'i, they were able to obtain direct images of a Jupiter-like planet, GJ 504b, orbiting the star GJ 504 in the constellation Virgo, about 60 light-years from Earth. The distance of GJ 504b from its star amounts to 44 times the distance between the Earth and the Sun (44 AU). GJ 504b is thus comparable to that of Neptune within our own Solar System.

Gallery of images of disks around young stars, imaged in the near-infrared by the Subaru telescope as part of the SEEDS project.
Understanding these disks is an important part of understanding how planets form around stars.
© NAOJ

This is the first image of a planet orbiting a Sun-like star (spectral type G); previous observations had targeted more luminous stars that tend to host more massive planets, which tend to be hotter and thus more easily detectable. Estimates of the planet's mass are linked to estimates of the age of its host star. The estimate favoured by most of the researchers would put the mass in the region of 3 Jupiter masses, which would make this the lightest planet yet imaged. Going by the observations, this is also the coldest exoplanet yet detected in an image.

The discovery was made as part of the SEEDS project ("Strategic Explorations of Exoplanets and Disks"). It comes at a time when SEEDS is looking back on completing the first half of the project's observing program – which has yielded remarkable images not only of planets, but also of the disks of gas and dust that surround young stars.

The Max Planck Institute for Astronomy was one of the co-founders of the SEEDS survey. MPIA director Thomas Henning explains: "With the unique experience in observing strategy, data reduction for this type of high-contrast imaging, and physical modelling of exoplanets, it was natural for MPIA's Planet & Star Formation Department to join the SEEDS project – and it is gratifying to see the great progress the project has made over the last few years!"