Category Archives: Exoplanet Discoveries

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

The Substellar Zoo: From Brown Dwarfs to Super-Earths

In this illustration by Frost all stars have planetary systems (which is about right) with regularly placed, circular orbits and planets like those in the solar system (mostly wrong).

In this illustration by Isaac Frost all stars have planetary systems (which is about right) with regularly placed, circular orbits and planets like those in the solar system (mostly wrong).

I particularly like Frost’s illustration from 1846 which shows how planetary systems were thought to look like in a post-Newtonian universe: in essence, Frost’s universe is filled with copies of the solar system – planets orbit each star. Interestingly, 130 years later the Star Wars universe was not that different: the desert planet Tatooine, the snow planet Hoth, or the forest moon Endor all strongly resemble Earth (not surprising for a movie shot mainly in California). From the vintage print of Frost to the last century’s most visionary sci-fi movie, exoplanetary systems remained just like the solar system and all planets remained similar to Earth (perhaps apart from the exotic collection of tentacled man-eating monsters).

Last week, when traveling from Budapest to Italy by train, I stopped briefly in Venice. Curiously, this charming town known for its canals, palaces, gondolas, and art exhibitions is also arguably the birthplace of cutting-edge exoplanet science: in 1995 in a single conference three important discoveries were announced. The discovery of the first brown dwarfs and the first extrasolar planet orbiting another star, 51 Pegasi b. These exciting discoveries marked the end of universe filled with Solas System ‘clones’ and brought about a reality that is much more interesting and often surprising. Now, there are enough different exoplanet types that they may appear to be a small zoo – but the main distinctions are simply mass, temperature, and composition.

Venice, the birthplace of the substellar zoo: The first exoplanet orbiting a star and the first brown dwarfs were announced here in 1995.

Venice, the birthplace of the substellar zoo: The first exoplanet orbiting a star and the first brown dwarfs were announced here in 1995.


 

So, what type of sub-stellar objects and exoplanets exist?

Brown Dwarfs: Brown dwarfs are gaseous objects that have too low mass too low (and therefore too low central pressure) to drive fusion reactions, from which stars get their energy supply. The stellar/substellar boundary is defined by the ability of the object to fuse hydrogen into helium and thus produce large amount of energy. The precise mass limit depends on the exact composition of the object, but it is about 70 Jupiter masses (or 0.08 solar masses or about 22,000 earth masses). Objects less massive than this but more massive than planets are called brown dwarfs. The lower boundary is often quoted as 13 Jupiter mass, which has been proposed as a natural break: objects more massive than this will very briefly fuse deuterium and generate some energy temporarily, while the less massive will never be able to drive fusion reactions.

Exoplanets: Objects less massive than brown dwarfs but larger than asteroid sizes that orbit other stars are called exoplanets (or extrasolar planets). Exoplanets are a very diverse group and come in many flavors.

Planetary-mass Objects: If Jupiter would be somehow ejected from the Solar System and become unbound, would it still be a planet? Not according to the current definition. We can now find in great number unbound gaseous objects that are lighter than brown dwarfs but as they do not orbit stars they are not planets. Lacking a better name the term planetary-mass objects has been coined for these.

It is interesting to note that many astronomers adopts a stricter definition for extrasolar planets: only objects that formed from material orbiting a star are called included. This definition addresses the status of the strange objects like 2MASS1207b – a planetary mass object that orbits a brown dwarf at such a large distance that it is not possible for it to have formed like a planet would.

The faint red companion 2M1207b is about 5-7 times as massive as jupiter. Although it has a mass low enough to be a planet, it is far enough from its host that it could not have formed from a disk, like planets do. Thus, it is an example for a planetary-mass object.

The faint red companion 2M1207b is about 5-7 times as massive as jupiter. Although it has a mass low enough to be a planet, it is far enough from its host that it could not have formed from a disk, like planets do. Thus, it is an example for a planetary-mass object.

Among exoplanets we often speak about very different objects:

Super-Jupiters: Exoplanets more massive than Jupiter but less massive than 13 jupiter masses, the planet/brown dwarf boundary. Examples include the four known planets in the HR 8799 system, Beta Pictoris b or the recently discovered GJ 504b.

Hot Jupiters: Gas giants planets with masses similar to Jupiter (which is 320 earth masses) that orbit very close to their host stars and thus have extremely high temperatures, usually well over 1,000 K (about 1,300 F). Examples include 51 Peg b, HD 209456b, and Corot-1b.

Hot Neptunes: Giant planets with masses around 20 earth masses that orbit their hosts stars on very short orbits (days) and due to the vicinity to their host stars they have very high temperatures, similarly to hot jupiters.

Super-earths: Planets with masses between 2 and 10 earth masses. Objects in this category may be fully rocky (i.e. jumbo versions of earth), may be ocean planets (with hundreds of times more water than earth has), but they have also enough gravitational pull to hold on to very massive gaseous envelopes (think of a mix between earth and neptune). Although the Solar System has no such planet, super-earths are now found in a rapidly increasing number and as they are easier to characterize than smaller planets, are set to be very important targets for astronomers in the coming years.

Earth-sized planets: Planets with radius similar to Earth. Note, that the density of earth-sized planets may cover a relatively large range and some of these planets may be more massive or  less massive than Earth. Even more importantly, an Earth-sized planet may have a similar size to Earth, and may be a much hotter dry rock or a deep-frozen icy body, depending on how close it is to its host star. Most such small planets known currently have been found by the Kepler space telescope; this transiting planet search mission can only measured the sizes of the planets and not their masses. 

Earth-like planets: An earth-like planet (or exo-earth) has very similar same size, mass, and temperature to Earth. If these key parameters are similar, there is a good chance that the conditions on the surface of the Earth-like planets is similar to those on Earth – but remember, that during most of Earth history the atmosphere and temperatures were very different from those on modern Earth. Nevertheless, finding and characterizing Earth-like planets is a key goal of astronomy and astrobiology.

Sub-Earths: In the Solar System two out of the four rocky planets are much smaller than Earth: Mars is 11%, while Mercury is only 5.5% of Earth’s mass! The Kepler space telescope‘s amazing accuracy has allowed the detection of planets smaller than Earth in a few exceptional cases, such as the three planets in the Kepler-42 system. Planet formation models predict that sub-earth-mass planets should be very common, even if we can currently only detect a few.

An amazing diversity, isn’t it?