Category Archives: Nature of Planets

Climate Stability and a Hike along a Triassic Coral Reef

The breathtaking white-gray peaks in the Italian Alps. Photo by D. Apai

The breathtaking white-gray peaks in the Italian Alps.

After two hours of hike up on a rocky trail in the Italian Alps, finally I stand at an elevation just above 2,500 meters, staring at a breathtaking and unique mountain range, the Dolomites, that holds an exciting clue to the habitability of our planet.

One of the many streams along an Alpine trail.

One of the many streams along an Alpine trail. Photo by D. Apai

With gigantic sharp white-gray peaks emerging from the lush green of Alpine meadows, these mountains rise where the African continental plate has been slamming violently into the European plate for millions of years, forcing rocks up thousands of meters  — and giving birth to the geologically young Alps.

In a trip zig-zagging Europe — visiting observatories, universities, and workshops — I stopped briefly in South Tirol for a few hikes. The most picturesque of them took me up to the Three Peaks of Lavaredo (or Tre Cime di Lavaredo), three 3,000m-high peaks, one of the gems of the Alps. Dotted by rifugi (mostly little huts, but at the easier trails often with nice cafes) the trails are popular among both tourists and locals. They offer an incredible view ascending towards the peaks, before joining an old network of high-altitude Alpine hiking trails, many of which take a week to complete.

Alpine flowers in the Dolomites. Photo by D. Apai

Alpine flowers in the Dolomites. Photo by D. Apai

The Dolomites are a unique mountain range within the Alps: their composition and history is different from any other in the Alps. They also hold an exciting clue to the process that keeps our planet habitable. Named after a relatively rare form and unusually stable form of carbonate rocks, dolomite, the mountain range’s unique color and composition was noted long ago and, for some time, posed one of the mysteries of geology. Now we know that the majestic dolomite layers in the Dolomite mountains are — amazingly — the work of tiny organisms: it is a very thick layer of ancient coral reefs. During part of the Triassic period (about 255-199 million years ago) the region was part of a shallow sea, which was slowly pulled deeper and deeper. But corals, only capable of living in the upper photic zone of the sea (where enough light is present for photosynthesis), kept on building their reefs higher and higher, managing to always keep the top layer of the coral reef close to the sea surface. With the sea floor sinking and the coral reef growing higher, these tiny animals constructed one of the giant carbonate deposits of the Triassic period.

As most geological periods, the Triassic also did not end well: in fact, it ended with the Triassic-Jurassic mass extinction, one of the greatest extinctions known, which eradicated about 50% of the known marine species. This extinction — occuring just before the Pangea super-continent began to break apart — paved the way for dinosaurs to become the dominant land animals in the Jurassic period that followed. The giant coral reefs of the Dolomites sank further and were covered by sedimentary layers and laid in depth for the next two hundred million years.

The Tre Cimes are a striking triple peaks in the Dolomites.

The Tre Cimes are a striking triple peaks in the Dolomites. Photo by D. Apai

Only recently, when the African continental plate collided again with the European plate, were the ancient coral reefs forced to resurface again. Together with other Triassic layers these rocks — once a seafloor — were now pushed up thousands of meters to become the dramatic high peaks of the new-born Alps. Once exposed to snow, ice, rain, and wind, the layers began to rapidly erode, creating the picturesque formations I was able to see today.

But the Dolomites’s story also holds a clue to why we are here: amazingly, the process that formed them and destroys part of a grander process them keeps Earth habitable. The mean temperature of Earth and its local and seasonal variations — its climate — is relatively stable: although major global changes occurred in the past and will probably occur in the future, Earth’s mean temperature mostly remained close to the current temperature and has seen much smaller changes than Mars and Venus have.

The key long-term stabilizing mechanism that keeps Earth’s climate in the habitable range (allowing liquid water on its surface) is the carbon cycle: it is the journey of carbon through the atmosphere, the ocean, the rocks, and the volcanoes of our planet. It is a journey that may take hundreds of million of years for a given carbon atom to complete, providing a slow connection between key reservoirs of carbon in Earth: CO2 in the atmosphere and carbonate rocks in the lithosphere. What makes this journey a feedback cycle is that it is both sensitive to the temperature and able to regulate it: The amount of CO2 — a powerful greenhouse gas — in the atmosphere directly impacts Earth’s temperature: the more CO2 is in the air, the more of Earth’s own emission is captured by it and re-radiated back to Earth, just like a blanket would provide additional heating to our planet (by slowing its cooling) — just as glass windows do in a greenhouse. However, the higher the temperature, the higher the humidity in the air and the more condensation occurs — and the more it rains, the more CO2 is washed out from the atmosphere forming acidic rain. The rain then interacts with silicate-rocks and forms carbonate rocks in the silicate weathering process — or, in a planet that is so filled with life as ours, tiny organisms can grab the carbon-dioxide dissolved in the ocean to build shells or coral reefs. As the Dolomites also show, vast amounts of carbon dioxide can be captured (over long periods of time) in rocks. Slowly, the carbonate rocks will be eroded and carried by rivers to the oceans, deposited to the ocean floor and, eventually, subducted along the oceanic/continental plate boundaries. There, many kilometers deep, the carbonate rocks will be exposed to very high pressures and temperatures, converting the carbonate rocks back to the silicates and expelling CO2 and water — these gases will then find their ways to the surface through explosive volcanoes near the plate subduction boundaries.

Because the loss of CO2 from the atmosphere is temperature sensitive (higher temperature leads to more rain and more carbonate formation) but the source of the CO2 is temperature insensitive (volcanoes do not care about the surface temperatures), the whole cycle forms a net negative feedback cycle: higher temperatures will result in cooling and lower temperatures will result in warming. The negative cycle means that it is stabilizing the temperature of Earth: because the carbonate reservoirs are vast, the effect is powerful; but because it takes hundreds of millions of years to transport carbonate rocks to subduction zones via plate tectonics, the cycle is also very small. While it has kept Earth habitable on long timescales (~100 Myr), the cycle can’t work well on short timescales (<10-30 Myr).

How would this apply to other Earth-like planets? While on present-day Earth the carbonate formation is dominantly through organic processes (various shell-forming marine organisms are happy to make use of the CO2 dissolved in the ocean), in the early Earth and, presumably, in other Earth-like planets with little or no life the same process can occur inorganically, but somewhat slower, in silicate rock weathering.

Therefore, as long as the overall composition of other Earth-like planets are the similar to ours, we would expect them to sport a carbon cycle (either organic or inorganic), also providing a stable climate for them — as long as the planets remain within the temperature range where the carbon cycle can work.

This means that carbonate deposits should be common even beyond the Solar System — and, just perhaps, a few in the Galaxy will also match the majestic beauty of the Dolomites.

The Dolomites mountain range preserves a thick layer of Triassic coral reefs.

Daniel Apai at the Dolomites mountain range, that preserves a thick layer of Triassic coral reefs.

The Best Astrobiology and Exoplanet Books

I am often asked to recommend books on astrobiology, habitable exoplanets, and extraterrestrial life.

There are many great books in these exciting fields, but there are a number of stand-outs that I highly recommend. Below is a gradually growing list of my favorite ones.

Is your favorite book missing? Add other book suggestions in the comments below!

Astrobiology:

Cosmos
CosmosCarl Sagan
A classic 1980 book by Carl Sagan. Although missing some of the new developments, this book remains an excellent treatise on life in the universe (and Earth).

Rare Earth
Peter Ward and Don Brownlee

This is a classic book which provides an interesting overview of many key factors and problems that have made it difficult for complex life to evolve on Earth. Many of these factors apply to all habitable planets making, in the view of the authors, complex life extremely rare.
The “Rare Earth” hypothesis splits astrobiologists and it will take decades — if not centuries — until we will be able to decide if Ward and Brownlee are right. Nevertheless, the book provides a highly readable and interesting narrative of many exciting problems related to the development of simple and complex life.

Peter Ward is a Professor of Geosciences at the University of Washington, has led one of the NASA Astrobiology Institute nodes and an author of 16 popular science books.

Don Brownlee is a Professor of Astronomy at the University of Washington and the Principal Investigator of NASA’s Stardust mission.

Life as We Do Not Know It: The NASA Search for (and Synthesis of) Alien Life
Peter Ward

How to Find a Habitable PlanetKasting How to Find a Habitable Planet
James Kasting
Princeton University Press, 2010

James Kasting is one of the pioneers of planetary habitability studies and in this book he provides an insider’s view on what makes a planet habitable and how can we find planets suitable for life.

The 5th Miracle: The Search for the Origin and Meaning of Life
Paul Davies
Touchstone, 2000

Paul Davies’s book provides an exciting exploration of the possible origins of life, including the principles of biological systems.

Crowded Universe
Alan Boss
Basic Books, 2009

Alan Boss’s book offers an enjoyable insider’s view on the birth of the exoplanet field: from the first radial velocity discoveries until the launch of the Kepler mission, Alan gives a diary-like summary of the major new exoplanet discoveries and results, including the controversies, debates, and the impact of politics and space policies on the science of exoplanets.

Alan P. Boss is an astrophysicist at the Carnegie Institution for Science’s Department of Terrestrial magnetism and an expert on extrasolar planets and the formation of planetary systems.

Biology / Paleontology

Life on a young planet
Andrew Knoll
Princeton University Press, 2003

The book provides an interesting, in-depth, but very readable discussion of research on the earliest life on Earth and especially on microfossils. While the book does not focus on extraterrestrial life, the history of life on Earth is an absolutely fundamental part of astrobiology and this is a great introduction to it.

Exoplanets:

Distant Wanderers
Bruce Dorminey
Copernicus Books, 2002

A somewhat older, but excellent book on the beginning of the era of exoplanet discovery and characterization. The book includes great interviews with many of the prominent scientists in the field and provides a great introduction to the initial discoveries of extrasolar planets.

Strange New Worlds
Ray Jayawardhana

This book provides an exciting narrative of exoplanet exploration and discoveries, with clear explanation oft he techniques and peppered with anecdotes from the field.

Textbooks

Life in The Universe
Benneth Shostak Life In the Universe3rd Edition
J. Benneth, S. Shostak

A best-selling introduction to astrobiology, mainly aimed at non-science majors. This richly illustrated and entertaining textbook provides a well-balanced overview of how concepts from astronomy, planetary sciences, geosciences, and biology can be combined to search for life in and beyond the Solar System.

How to Build a Habitable Planet: The Story of Earth from the Big Bang to Humankind
Charles Langmuir and Wally Broecker

Princeton University Press, 2012

This well-written book follows Earth’s formation and evolution, including the overview of biological evolution. The book provides an interesting, geoscience perspective on these topics, which complements well most other books that approach the topic more from an astrophysics/planetary sciences perspective. Well suited for undergraduate courses.

Earth: Evolution of a Habitable World
Lunine Evolution of a HabitablePlanetJonathan Lunine
Cambridge University Press, 2013

An excellent undergraduate introduction to the formation and evolution of Earth and to the processes that made and keep our planet habitable.

See my review of the this book in Meteoritics & Planetary Sciences.

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.

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?