Grenoble and Planet Hunting with Adaptive Optics

In July I spent four exciting days at the Institut de Planetologie et d’Astrophysique de Grenoble (IPAG). Set in a picturesque green valley in the Alps where the rivers Drac and Ilsere merge, this scenic little town in Southern France is well-known for planet hunters.

A charming little town in the French Alps, a European center of adaptive optics development.  Image source: http://farm9.staticflickr.com/8346/8268563542_94b40c3690_b.jpg

A charming little town in the French Alps, a European center of adaptive optics development. Image source

The Grenoble group I visited is one of the world leaders in direct imaging of extrasolar planets: over the past decades Jean-Luc Beuzit and his group have developed more and more sophisticated instruments to enhance the sharpness of the images that can be obtained with the largest telescopes. Anne-Marie Lagrange and her group have used these adaptive optics systems to painstakingly search nearby stars (typically within a distance of 250 lightyears) for exoplanets. The group’s outstanding work has led to several exciting and now-classic discoveries: 2MASS1207b, AB Pictoris b, or the planet in the spectacular Beta Pictoris system, a project on which we worked together. During my stay in Grenoble I also enjoyed a memorable dinner hosted by Anne-Marie’s very friendly research group – everyone pinched in in the preparations and soon we enjoyed an amazing set of French dishes. Cheese, red wine, and exoplanets – what a pleasant way to spend an evening!

Planet around a Brown Dwarf?
Anne-Marie and her group made their discoveries with the European Very Large Telescope, a system of four 8m telescopes based at the Paranal Observatory in the Chilean Atacama desert. This telescope and its NACO instrument has discovered more directly imaged exoplanetary systems and planetary mass objects than any other telescope in the world.

Why do we need adaptive optics systems to image planets?

There are three factors that make it extremely difficult – really, almost impossible – to directly image planets around other stars:
1) Planets are inherently faint: unlike stars, planets have no stable internal heat sources apart from the decay of some radioactive isotopes. Therefore, planets are much cooler than stars – and being also much smaller, planets are typically ten million to a billion times fainter than their host stars. This means that the images must be very sensitive to detect them: very large telescopes must be used that can collect a lot of light.

2) By definition, each planet must orbit a star – and because stars are very distant, the planets appear to be extremely close to their host stars, which are always much brighter. Because of the large contrast between the star and its planets, we can only directly image planets that appear the farthest from their stars: those that are on very long orbits. For comparison, a planet at 5.4 AU separation (5.4 times the Earth-Sun separation or 1 times the Sun-Jupiter separation) will appear 5.4/10 = 0.54 arc second from its host star, the equivalent of the apparent size of a penny seen from about 5 miles. The great brightness difference between the star and its planet and the fact that they appear very close mean that an image showing exoplanets must have an extremely high contrast. And, in practice, the highest-contrast images are those that are taken with the highest quality and best-designed telescopes with near-perfect optics.

3) All ground-based telescopes must observe through the terrestrial atmosphere. As we are used to look through the atmosphere at first this may not seem to be a real problem, but it is: Just imagine trying to read time from a wall clock of a swimming pool from underwater – all you see are blurry images. Similarly, images taken even from the highest mountains are blurred by the turbulence of our atmosphere. Although observatories are built at sites that typically have clear skies and relatively calm air layers, the only ways to get truly sharp images is go above the atmosphere (think Hubble Space Telescope) or to build highly complicated adaptive optics systems that correct for the atmospheric turbulence (and even most imperfections of the telescope itself).

The astronomer slang for Adaptive Optics is AO. Although ten years ago astronomical AO systems were still a novelty, nowadays most major telescopes have them. But only the most powerful are capable of providing the image quality needed to hunt for planets.

The most capable systems can correct for changes in the atmosphere and telescope several hundred to a thousand times a second, providing incredibly sharp and stable images.

SPHERE AO system in Grenoble

SPHERE AO system in Grenoble

Perhaps the coolest thing I have seen in Grenoble was perhaps also the least exciting-looking. Tucked in a very large ground-floor laboratory was a room-sized pile of instrument boxes, all connected, and covered with green plastic covers and attached to large tubes and a thick string of cables. The tubes were blowing cold air under the covers to cool what is arguably the most complex adaptive optics system in the world: SPHERE. This highly complex instrument belongs to the next level in adaptive optics design: an Extreme AO system. Extreme AO systems (often called ExAOs) have been in the making for over a decade now with the first experimental one operating at the Palomar 5m telescope (Project 1640). The driver behind ExAO systems is to direct image extrasolar planets: to find them and to study their atmospheres. The European community has been building SPHERE for over 11 years, while the US astronomers have been working on their own ExAO system, the Gemini Planet Imager. Both systems are technological marvels – they are incredibly capable and will soon allow our telescopes to see and study dozens and dozens of new extrasolar planets. Both systems will likely deliver first science early next year and with that a friendly competition will begin for finding new worlds – and will bring us exciting new images and many surprising discoveries.

Saturn’s Super Storm

If you live in the US, you will remember the great February snowstorm of 2010 – which entered history as “Snowmageddon” – that covered the East Coast in thick snow and paralyzed cities and airports. It was one of the largest winter storms in recent history.

Snowmageddon
Yet, the same year in the outer solar system another storm developed that dwarfed Snowmageddon – in fact, it dwarfed all storms combined on our planet. This much larger, much colder, and arguable much more mysterious storm has developed in the atmosphere of Saturn.

This was not the first such storm on Saturn: roughly every Saturnian year (29.3 Earth years) a dramatic mega-storm develops. These storms have been observed three times by now, always occurring on the northern hemisphere on Saturn during its summer.

Saturn's 2010-2011 Great Storm as seen by Cassini's ISS camera. Once in about every Saturnian year (~30 years) a giant storm system develops which, in a few weeks, engulfs the northern hemisphere.

Saturn’s 2010-2011 Great Storm as seen by Cassini’s ISS camera. Once in about every Saturnian year (~30 years) a giant storm system develops which, in a few weeks, engulfs the northern hemisphere.


Although the mysterious storms have been seen before, what was different this time was that a spacecraft was present in the saturnian system. Cassini got a first row seat to observe the megastore develop, engulf the northern hemisphere and eventually dissolve, after several months.

The 2010-2011 Storm is the first one observed by a spacecraft in the saturnian system.

The 2010-2011 Storm is the first one observed by a spacecraft in the saturnian system.

Cassini‘s amazing images of the gigantic storm have been published before, but the nature of the storm remained unexplained. Now, in a a University of Wisconsin team led by Lawrence Sromovsky presents a detailed analysis of the storm. The group has worked on trying to figure out the composition of the material dredged up by the storm.

To understand this monster storm let me tell you a bit about Saturn itself. Saturn is a very cold world — at least its upper atmosphere which is visible to us. At 1 bar (the same pressure as at sea level on Earth) Saturn’s atmosphere is only 134 K. Saturn has as much mass as 95 Earths would have – and this massive, cold planet rotates fully around every 10.7 hours!

Like the Solar System’s other gas giant, Jupiter, Saturn is mostly made up of hydrogen and helium, the most common elements in the universe. Most of Saturn’s hydrogen is in its molecular form (H_2), concentrated to the upper layers of the atmospheres (down to about 2 million bars!). Below these immense pressures hydrogen is thought to be compressed to its metallic form, in which electrons are stripped from individual hydrogen atoms and can wander freely among the protons, like they would in “regular” metals.

Based on observations of the previous storms decades ago it was suspected that the storms may dredge up gas that is of different composition than the molecular hydrogen that dominates Saturn’s upper atmosphere. However, lacking detailed observations the actual components could not be identified. This time was different: Cassini’s VIMS (Visible and Infrared Mapping Spectrometer) obtained spectra of the storm head and its vicinity. Sromovsky and colleagues compared the spectra of the gas from the storm’s head to the “ambient” spectra to figure out what components does the storm carry with it.

An infrared color composite image of Saturn's Giant Storm obtained by Cassini's VIMS instrument. The instrument also obtained spectra at the locations 1-6, which are used to explore the composition of the material dredge up by the storm. Locations 1 and 2 are in the storm head, while the other points sample Saturn's atmosphere outside the storm. From Sromovsky et al. 2013.

An infrared color composite image of Saturn’s Giant Storm obtained by Cassini’s VIMS instrument. The instrument also obtained spectra at the locations 1-6, which are used to explore the composition of the material dredge up by the storm. Locations 1 and 2 are in the storm head, while the other points sample Saturn’s atmosphere outside the storm. From Sromovsky et al. 2013.

Comparison of the in- and out-of-storm spectra showed a prominent difference at 3 micron: Sromovsky and his team use sophisticated atmospheric models to try to figure out what causes the difference in the spectra. They conclude that this feature must be caused by small particles present in the storm, but not found otherwise in Saturn’s upper atmosphere. The detailed analysis of the spectra suggests that Sromovsky’s team has observed ice particles, made of a mixture of water and ammonia (which gives urine its smell). Water ice has never been seen in Saturn’s atmosphere previously and thought to exist in Saturn at depths of 200 km and below!

So, how large is Saturn’s Super Storm? It has emerged from a depth of at least 200 km and covered at least 7 degrees latitude when it was first seen in the atmosphere. And that 7 degrees at mid-latitude Saturn corresponds to about 1 Earth radius – making this a monster storm compared to Snowmageddon, which only covered part of the US and did not even smell that bad.

How to Get Your Own Exoplanet?

If you have been reading about exoplanets, you know that they all have boring names, such as GJ 876b, 51 Peg b, or WASP-19b (not to speak about the likes of KOI-762.02). Up to a few days ago the official names of exoplanets had to be the catalog identifier of their host star plus a letter assigned in the order the planet was discovered in the given system. Yes, while most sci-fi writers used exotic names for their planets, astronomers shied away from anything more exciting than GJ 436b. These were the rules – even if you discovered a new planet, you could not give it a proper name!
But this has all changed with a new decision by the International Astronomical Union, the entity that represents astronomers worldwide.

The names of exoplanets are currently a combination of the host star name and a letter in the order the planet was discovered in a given system. The first planets are assigned 'b', the second 'c', etc.

The names of exoplanets are currently a combination of the host star name and a letter in the order the planet was discovered in a given system. The first planets are assigned ‘b’, the second ‘c’, etc.


To understand the changes let me tell you more about the background of naming celestial objects and, in particular, extrasolar planets.

Let’s start with stars. All bright stars on the sky have names, often derived from their Arabic names (such as Altair or Deneb). Nowadays these names
are often used by amateur astronomers, but professional astronomers tend to use a simpler scheme: in each constellation the brightest star is named Alpha, the second brightest is Beta, etc. leading to names like alpha Persei (the brightest star of the constellation Perseus). However, most stars that we study are too faint for this system – there are not enough letters in the Greek alphabet to name the 13,234th brightest star in Orion!. We now simply use an identifier from one of the all-sky catalogs of stars. These names look like HD 172555, which is the 172,555th star in the Henry-Draper Catalog, a list of stars and celestial positions compiled in 1924. Similarly, a name like GJ 436 is the 436th star in the catalog of nearby stars compiled by German astronomers Wilhelm Gliese and Hartmut Jahreiss. Because the catalogs have the coordinates of each star, astronomers do not need to know anymore in which constellation it belongs to.

Sounds simple, right? But as astronomers began to study in detail the stars in the catalogs, many of the apparently single stars turned out to be two stars orbiting around each other (more precisely: they really orbit around the center of mass of the system). Of course, once you spent 15 years cataloging around 200,000 stars and numbered them, you don’t want to renumber all of them just because the third star turned out to be a binary, right?

Instead, astronomers decided to make a logical change to the system: if a star turns out to be a binary star, we keep the catalog number, but call the two stellar components as A and B. Some systems even turned out to have four components, leading to letters A through D.

Then came planets and with their discovery another change was needed. You could envision to add numbers for each planet, but astronomers decided that the simplest solution is to use lower-case letters for the planets: GJ436 b, for example, is the first planet discovered around the 436th star of the Gliese-Jahreiss catalog. Why is not planet “a” the first planet? Astronomers thought that “a” could lead to confusion with the star itself, so the planets start from b.

This system was simple and worked well. But as the number of planets rapidly grew and they became frequent subjects of the news in all media, there was more and more pressure to introduce more interesting names. The interest was so high that many companies decided to sell planet names – just as some companies used to sell land on the moon.
Eventually, the IAU decided to allow the public to propose planet names.

This is a welcome decision: Allowing everyone to propose names for new worlds means sharing the excitement of discovery.

So, how do you name your own planet?

Understandably, the IAU wants to proceed carefully and wants to avoid names that are controversial (xkcd collected an interesting set of possibilities). So, there is a somewhat complex submission and approval process, but the key points are that you need to identify a suitable name and gather enough support. The name should be, of course, not offensive and should not aim to lead to any financial or political advantages. The next step is to convince a large number of people to support the proposal and then submit it to the IAU for approval. The process is overall very similar to that used to name minor planets in the Solar System.

With this opportunity open, we will surely see interesting and original planet names popping up in large numbers!

The Wildest Clouds in The Universe

Flying on a Delta MD90 jet on my way back from Munich, Germany to Tucson among gorgeous towering clouds glowing in exotic shades of yellow, orange, and purple. Amazing view – especially interesting is the all the different clouds we see are made of water.

How would clouds on exotic other planets look?

Clouds seen from a jet on planet Earth - somewhere about Texas.

Clouds seen from a jet on planet Earth – somewhere about Texas.

Earth is special (in the Solar System, but not in the Galaxy) that it hosts three phases of water simultaneously: liquid, gas, and solid (ice). This allows some fun physics to take place and lead to the hydrological cycle we all know. In the terrestrial atmosphere most water clouds form as the rising moist air cools down and water condensates to condensation nuclei and forms droplets; this process forms the low clouds (~1-2 km). Higher up in the atmosphere, where temperatures are very low, ice crystals will form clouds (>6km).

But we don’t need to go far to find exotic clouds: Earth’s hostile twin, Venus, has a thick carbon dioxide atmosphere (with a crushing surface pressure as high as 900 meters underwater on Earth!). As Venus has lost all of its water in the past, it does not have water clouds – but still has very thick clouds. And these clouds are made of hot droplets of sulphuric acid (H_2SO_4)! The cloud layer is so thick that no sunlight reaches the surface directly. The thick yellowish cloud layer has blocked the views of surface from the early space probes.

This Cassini image of Titan shows the formation of clouds close to the south pole as the winter begins. The overall yellow color of Titan is given by a thick organic haze.

This Cassini image of Titan shows the formation of clouds close to the south pole as the winter begins. The overall yellow color of Titan is given by a thick organic haze.

Or think about the amazing Titan, Saturn’s large icy moon (larger than planet Mercury!). Titan is the only moon in the Solar System that has a significant atmosphere – in fact its atmospheric pressure is higher than that of Earth. But the surface temperature of Titan is so cold (only ~93 K) that it allows the three phases of methane on its surface. The Cassini orbiter, which keeps on returning amazing high-quality images from the Saturnian system, has photographed the formation and evolution of clouds in Titan’s atmosphere. The clouds of Titan are not well understood and remain an exciting field of research.

These planetary bodies are not exceptions: in fact, every Solar System planet that has an atmosphere has clouds, too. Depending on the temperature and the pressure (often called the “P-T profile”) and the composition of their atmospheres Solar System planets have different clouds. The very thin atmosphere of Mars allows the formation of tenuous water ice and carbon-dioxide clouds; Jupiter and Saturn sport clouds made of ammonia (NH_3), ammoniahydrosulfide (NH4SH), and water; while the upper atmospheres of the even colder ice giants Uranus and Neptune also have methane (CH_4) and SH_2 clouds.

This Voyager image from 1989 shows the great dark spot and surrounding whiter clouds on Neptune. Neptune's upper atmosphere is about 70 K and at these low temperatures most clouds are composed of methane.  Image Source.

This Voyager image from 1989 shows the great dark spot and surrounding whiter clouds on Neptune. Neptune’s upper atmosphere is about 70 K and at these low temperatures most clouds are composed of methane.
Image Source.

This bonanza of clouds raises the question: which are the craziest, most exotic clouds in the universe?
This is a difficult one, of course, as there are many candidates. My favorites are probably the clouds in hot super-Jupiters and brown dwarfs (see post on Substellar zoo). These gaseous planets and brown dwarfs have no solid surfaces but have extremely high pressures in their interiors. In their uppermost atmospheres, the layers visible to us, temperatures can exceed 1,800 K. In this class of objects iron can exist in two phases: in the hotter layer deeper down as gas and in the slightly cooler, lower pressure upper layers it will form droplets. Just as water droplets form massive clouds on Earth that can pour down heavy rain, hot super-Jupiters will have iron clouds that will drive heavy rain of hot molten iron droplets… Gives a whole new perspective on bad weather!

Back on the airplane, somewhere above New Mexico I miss another opportunity to get free pretzels, but still recall the beauty of our simple water clouds.

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?