외계 행성학의 시대: 행성 크기 차이에 대한 이해

### '외계 행성 지름 간격'의 미스터리가 밝혀졌습니다.
* 발견된 대부분의 외계행성은 **슈퍼지구(지구 크기 또는 지구 지름의 1.5배)**와 **미니 해왕성(지구 지름의 2~3배)**의 두 가지 크기로 분류되며, 그 사이에는 상당한 차이가 있습니다.
* 🔭 이 차이를 설명하는 두 가지 주요 이론은 **별의 강렬한 빛으로 인한 광증발**과 **행성 자체의 열로 인한 핵에 의한 질량 손실**입니다.
* ⏳ 광증발은 **1억 년 이내에 빠르게 일어나는** 반면, 핵에 의한 질량 손실은 **50억~200억 년이 걸리는 느린 속도**입니다.
* 🛰 케플러 우주 망원경은 임무의 어려움에도 불구하고 **젊은 플레이아데스 성단과 하이데스 성단 등 다양한 성단에 있는 외계 행성에 대한 중요한 데이터**를 제공했습니다.
* 🌌 젊은 성단의 외계 행성은 대부분 **미니 해왕성**인 반면, 오래된 성단은 **암석 행성**이 더 많아 **핵에 의한 질량 손실**이 지배적인 크기 감소 메커니즘임을 뒷받침합니다.
* 🌌 이 연구 결과는 외계 행성 연구가 개별적인 특성에서 통계적 추세로 전환되고 있음을 시사하며 **"외계 행성학"의 시대**가 열렸음을 알립니다.
* 👩 "우리의 천문학은 이미 그 수준을 넘어섰습니다. 이제 우리는 시간이 지남에 따라 축적된 방대한 양의 외계행성 데이터를 수집하여 **외계행성이 보여주는 추세를 분석**하고 외계행성의 탄생과 진화 과정을 보다 통계적으로 이해하기 위해 노력하고 있습니다."
* 외계행성 직경 격차의 발견은 **행성의 진화와 외계행성 집단을 형성하는 요인**을 이해하는 데 중요한 단계입니다.

천문학자들은 외계 행성의 크기 차이가 별빛보다 핵심 과정에 의해 형성된다는 사실을 밝혀내면서 외계 행성 연구의 초점이 바뀌고 있습니다.

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.

왜 지구보다 두 배 큰 행성은 존재하지 않을까?! | 외계행성 크기의 놀라운 비밀!

우주를 사랑하는 먼지들의 시간 ✨

우주먼지의 현자타임즈

지난 30년 동안 약 1만 개의 외계 행성이 발견되었으며, 이제는 개별적인 특성보다는 전체적인 특성에 초점을 맞추고 있습니다.

이러한 외계 행성은 크게 슈퍼 지구와 미니 해왕성의 두 가지 유형으로 분류할 수 있으며, 이들 사이에는 현저한 크기 차이가 있습니다.

이 직경 차이의 미스터리, 특히 지구 직경의 1.

5배에서 2배 사이의 행성이 희귀한 이유는 아직 설명되지 않았지만, 최근 케플러 탐사선의 데이터를 이용한 연구를 통해 그 비밀을 밝히려는 시도가 진행되고 있습니다.

5~2배 크기인 행성은 크기와 중력이 모호하여 가스 대기를 유지하기가 어렵기 때문에 관측하기 어렵습니다.

가스 거성의 외부 대기가 손실되어 암석 핵이 노출되는 것은 강한 별빛과 항성풍에 의한 광증발 또는 행성 내부 핵에서 방출되는 열을 통해 발생할 수 있습니다.

두 가지 방법 모두 궁극적으로 가스 대기의 분산과 행성의 수축으로 이어지지만, 이 과정이 완료되는 데 걸리는 시간은 다릅니다.

대기 탈출이 외계 행성 크기에 미치는 영향

외계 행성에서 대기가 빠져나가는 과정은 1억 년 이내에 빠르게 일어나는 반면, 열에 의한 탈출은 50억 년에서 20억 년이 걸립니다.

2009년에 발사된 케플러 우주 망원경은 2013년에 K2 임무를 통해 시야를 넓혀 다양한 별을 관측할 수 있는 도전에 직면했습니다.

플레이아데스 성단과 하이데스 성단에 있는 별들의 나이가 젊다는 것은 주변 행성이 대기를 잃고 지구보다 작은 슈퍼 지구 크기의 행성이 발견되었을 수 있음을 시사합니다.

항성 열이 외계 행성 대기에 미치는 영향

외계 행성에서 대기가 점진적으로 빠져나가는 것은 50억 년에서 20억 년에 걸쳐 열 방출에 의해 이루어집니다.

플레이아데스 성단과 하이데스 성단의 젊은 별들은 미니 해왕성형 행성의 비율이 79~107%에 달할 정도로 매우 풍부합니다.

반면, 나이가 많은 별들은 미니 해왕성의 비율이 20%로 낮고 작은 암석 행성의 빈도가 더 높으며, 이는 핵에 의한 질량 손실로 인한 대기 손실 과정이 더 느리다는 것을 나타냅니다.

외계 행성에 대한 인류의 관점은 개별적인 특성에서 추세와 패턴 분석으로 변화하고 있습니다.

지구와 닮지 않았지만 지구와 비슷한 특성을 가진 외계 행성의 발견은 더 이상 희귀하거나 흥미로운 사건이 아닙니다.

외계행성의 형성과 진화를 이해하기 위해 외계행성 데이터의 통계적 분석에 초점을 맞춘 외계행성학이라는 새로운 천문학의 시대가 열리고 있습니다.