The Coolest Exoplanet Imaged – The Discovery of GJ 504b

Exciting news for planet hunters: Working with the 8m Subaru telescope at Mauna Kea the international SEEDS team announced the exciting discovery of a new directly imaged planet – this new planet is exciting not only because very few planets have been directly imaged yet, but also because this one is different from all others seen until now.

Japan's 8.2m Subaru telescope at the summit of Mauna Kea, Hawaii.

Japan’s 8.2m Subaru telescope at the summit of Mauna Kea, Hawaii.

In short, this planet is cool because it is cool; and it is also exciting because it orbits a sun-like star. Let me explain why do these properties make this an exceptional discovery.

The Coolest Planet Yet Imaged
Exoplanets are very hard to image: they are faint and always close to very bright stars. In fact, this task is so difficult that right now we can only see the easiest planets: the ones that are the largest, furthest away from their host stars, and brightest. As you would expect, the hotter a planet is the brighter it shines, especially in the infrared, where direct imaging searches are conducted. Not surprisingly, planets imaged until now are all hot super-jupiters, as these are the brightest possible planets.
Unfortunately, these planets are rare and the vast majority of stars imaged by the few competing teams (including ours) do not seem to have such planets.

Most planets directly imaged until now had temperatures 800-900 K or higher (some as high as 1,800 K!). This is much hotter than Jupiter (170 K), Earth (288 K), and even warmer than Venus (737 K). Once a planet is imaged we can start exploring its atmosphere in detail – as we have done for over a dozen directly imaged hot planets.
The new planet GJ 504b is a record-holder: its temperature is only 510 K! Although still hotter than Earth, this planet is much cooler than the previous directly imaged planets and now offers an opportunity to explore how the atmosphere of such a warm giant planet look like!

Super-Jupiters around Sun-like Stars

Like all other directly imaged exoplanets, GJ 504b is a super-jupiter, but it differs from all the others in an important aspect: GJ 504b is the first such planet to orbit around a sun-like star. Most previously imaged exoplanets, curiously, orbit around much more massive stars (A-type stars). Although over 300 sun-like stars have been searched for large-separation super-jupiters, until now none has been found, suggesting an important difference in the planetary systems between sun-like and more massive stars (more on this interesting topic later). GJ 504b also turns out to have lower mass than all but one directly imaged exoplanet.

The newly discovered planet around the nearby star GJ 504. The image was taken by the international SEEDS team using the Subaru telescope.

The newly discovered planet around the nearby star GJ 504. The image was taken by the international SEEDS team using the Subaru telescope.

Although lighter than its counterparts around more massive stars, GJ 504b is still between 3-9 times more massive than our own Jupiter! Not only is it massive, the radius of this planet’s orbit is at least 44 AU. This is again surprising if you consider that Jupiter, the most massive planet in the Solar System, is only 5.4 AU from the Sun and at orbits this long the Solar System has virtually no mass in planets – which probably has to do with how much mass was available to form planets that far from the Sun.
So, the puzzle is: How can a star seemingly similar to the Sun be able to form such a massive planet so far out?

Interestingly, GJ 504 also joins the small set of known planet host stars that are visible to the naked eye. At a dark site you will be able to see GJ 504 (it is 5.2 magnitude) without a telescope in the Virgo constellation, although of course you will not be able to see the planet.

Fortunately, this exciting planet is equatorial and thus visible from telescopes at both the northern and southern hemispheres. The planet will be again visible from February next year and surely will be among the hottest targets for all adaptive optics systems.

The detection of GJ 504b is a very exciting next step toward cooler and lower-mass planets and very soon we will learn how the atmosphere of such a warm planet works! Congratulations to the SEEDS team!

Paper by Kuzuhara on arXiv

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