Just 30 years ago, the only planets known to humanity were the planets of our solar system orbiting the Sun. Naturally, it was assumed that there would be exoplanets orbiting other stars, but their existence could not be confirmed. Since the discovery of the first exoplanet orbiting a pulsar in 1992, nearly ten thousand exoplanets and candidate celestial bodies have been discovered. In the early days, when exoplanets were being discovered one by one, each exoplanet was considered special and precious. Some resembled our Earth, while others were planets with seas of bubbling magma. For a while, we focused on the characteristics of individual exoplanets. However, now that we have entered the era of ten thousand exoplanets, we no longer pay attention to the characteristics of individual exoplanets. Instead, we gather them together to statistically analyze the overall characteristics of exoplanets.

Astronomers have discovered an interesting fact here. Most of the exoplanets found so far can be broadly categorized into two types based on size. One type consists of rocky planets that are about the size of Earth or approximately 1.5 times the diameter of Earth. These planets are referred to as super-Earths. The other type includes gas planets that are slightly smaller than Uranus and Neptune, with diameters ranging from about two to three times that of Earth. These planets are called mini-Neptunes. While they are covered by a thick gaseous atmosphere, it is presumed that they have a solid rocky core at their center. What's intriguing is that exoplanets with sizes between 1.5 and 2 times the diameter of Earth, which would fall between super-Earths and mini-Neptunes, are found with a frequency nearly ten times lower. The overwhelming number of these planets is significantly less, leaving a gap between super-Earths and mini-Neptunes.

The gap in the sizes of exoplanets, specifically the diameter gap and radius gap, is still not perfectly explained as to why exoplanets are not commonly found in this range. Particularly puzzling is the fact that in our solar system, there are no super-Earths or mini-Neptunes. We only find smaller rocky planets or larger gas giants. The question arises as to whether the diameter gap is caused by the processes of growing or shrinking planets, and what secrets might be hidden in those processes. Unfortunately, we have had no opportunity to examine this closely. However, recent astronomers have found intriguing clues to unravel the mystery of this diameter gap by reusing the past observational data from the retired space environment mission Kepler, which finished its mission in 2018 and has since become space debris.

What secrets are hidden in the process? Why is it that planets with a size approximately 1.5 to 2 times that of Earth are particularly not being discovered? This is a story about the remarkable void of exoplanets that the universe shows us. The size that a planet can attain is determined by its gravity. If a planet is formed with a relatively small mass, it often becomes a small, solid rocky planet like Earth. However, if a larger mass of material is formed into a heavy planet, it can capture lighter gaseous materials more easily due to stronger gravity. This allows for a thicker atmosphere, as seen in Jupiter and Saturn. Yet, planets with a size that is only about 1.5 to 2 times the diameter of Earth have a somewhat ambiguous size, and their gravity...

As the gas atmosphere surrounding the rocky core becomes insufficient, it may be difficult to retain it for a long time. Gradually, the gas atmosphere may disperse into space, causing the planet to shrink until only the solid rocky core remains. The process by which a gas giant, which initially has a larger size, loses its outer atmosphere and exposes its rocky core can occur in two main ways. 

One way is due to the strong starlight and stellar winds emitted by the central star, which can blow away the gas atmosphere surrounding the planet. This phenomenon is called photoevaporation. The powerful ultraviolet light emitted from the star ionizes the gas molecules in the planet's atmosphere, breaking them down into smaller and lighter particles. These lighter atmospheric particles move more rapidly and can more easily escape the planet's weak gravity, allowing them to escape into outer space.

Another method is through the heat emitted by the planet's inner core. The temperature inside the planet can increase due to various causes, such as radioactive decay or volcanic activity in the rocky core. As a result, the temperature of the atmosphere surrounding the planet's rocky core also rises, causing the movement of atmospheric molecules to accelerate. Gradually, the atmospheric molecules escape the planet's gravity and fly into outer space. This process is referred to as mass loss driven by the planet's core, or core-powered mass loss. Ultimately, both methods result in the outer atmospheric layer being blown away, leaving only the rocky core behind. However, there is a significant difference in the total time it takes for the entire process to complete. The intense starlight at the center ionizes the stellar atmosphere, and the phenomenon of light evaporation occurs as the planet...

After completion, the process occurs relatively quickly within about 100 million years. In contrast, the second method, driven by heat from the planet's core, causes the atmosphere to heat up and gradually leads atmospheric particles to escape into space over a much longer period, approximately 5 to 2 billion years. By utilizing the difference in the total time for these processes, it is possible to determine which of the two mechanisms is the primary factor influencing the size of exoplanets that exist in the universe. The exoplanet-hunting mission Kepler, which was launched into space in 2009, concluded its long search after approximately 99 years. Originally, Kepler aimed to focus on a specific area of the sky, just a small patch that could be covered by the palm of a hand, in the constellation Cygnus, to intensively search for exoplanets in that direction. However, once the mission began,

Three years after its launch, in 2012 and 2013, two out of the four reaction wheels that adjust the direction and attitude of the telescope broke down. As a result, Kepler became unable to focus on a specific direction in the northern constellation of Cygnus. After much deliberation, astronomers initiated the K2 mission in 2013, which aimed to widen the field of view of the Kepler space telescope and implement a new strategy. It was no longer possible to perfectly fixate on a specific direction. Instead, the telescope began to orbit the Sun along with Earth, allowing its field of view to rotate over time. The Kepler space telescope started observing various stars that entered its field of view along the ecliptic plane. Among the stars observed during this period were the Pleiades cluster and the Hyades cluster, both of which are approximately 600 million to 800 million years old.

There is a star cluster composed mainly of very young stars. Furthermore, the cluster is primarily made up of stars that were born around the same time, making them peers. Therefore, the planets surrounding the stars in these two clusters can all be considered relatively young, having been formed recently. This young age is crucial. What if most of the planets are experiencing a runaway greenhouse effect due to intense starlight, causing their atmospheres to be stripped away? This process occurs rapidly within a hundred million years. The planets orbiting the young stars in the Presepe and Hyades clusters should have already undergone significant changes. Consequently, the planets discovered in these clusters should have nearly lost their atmospheres, leaving behind only rocky remnants, with primarily smaller super-Earth-sized planets being found. Conversely, if most planets have their own planetary cores...

What if the atmosphere is gradually escaping due to the heat being released? This process occurs slowly over 5 to 2 billion years. Young stars, like those in the Pleiades and Hyades clusters, have very little time. Many exoplanets found around the stars in these two clusters are still maintaining their large sizes without losing their outer atmospheres, resembling mini-Neptune-type planets. So, what do the exoplanets discovered in these clusters actually look like? They are primarily mini-Neptunes, and they are overwhelmingly abundant. The ratio of mini-Neptune planets found around the stars in these two clusters is about 79 to 107%. For reference, the reason this can exceed 100% is that there can be two planets orbiting a single star. On the other hand, these two young clusters are much older.

The stars that can be seen in the field star, or star cluster, have varying densities in the vastness of space. The ratio of mini Neptunes discovered around older stars is only 20%. Instead, smaller rocky planets, which are presumed to have lost their atmospheres, are frequently found. This is likely because they have been slowly orbiting around older stars for a long time, gradually undergoing the process of atmospheric loss through mass loss from the planet's core. In other words, this discovery confirms that the main mechanism by which exoplanets reduce in size is not the light pressure effect from the central star, but rather a much slower process driven by their own core, known as core-powered mass loss. However, this finding is not just a simple issue of exoplanet radius.

There is an amazing discovery that shows how humanity's perspective on exoplanets is gradually changing. We no longer focus solely on the characteristics of each individual exoplanet. While it is still fascinating to discuss whether a specific exoplanet has oceans or has a warm temperature like Earth, such news is no longer rare. The recent discovery of an exoplanet that does not contain Earth-like qualities, yet resembles Earth in many ways, no longer requires us to be overly excited. Our astronomy has already surpassed that level. Instead, we are now gathering a vast amount of exoplanet data that has accumulated over time to analyze the trends they exhibit and to understand how exoplanets are born and evolve in a more statistical manner.

We have reached a point where we need to consider new methods for research. Today, a new era of astronomy, called exoplanetology, is dawning. This discovery is clear evidence that the new era of astronomy is already at the very center of astronomical research. I will conclude today's story here. Thank you, cosmic dust. Please subscribe and like as well. Goodbye, cosmic dust.

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