Since the establishment of the heliocentric model of our world and our understanding of our own solar system, astronomers have had many challenging tasks. The first one, is the discovery of planets outside our own solar system. Discovery of planets in other stellar systems would improve our knowledge in planetary physics and evolution of such systems, since it can provide us with information about the formation and evolution of planets and planetary systems. Additionally, it would remove the dogma of the uniqueness of our own solar system. The second task, which is more important, is the discovery of Earth-like planets and the frequency they occur. We have always been wondering if there is life in other planets, thus discovery of Earth-like planets would help verify this.
When was the first exoplanet discovered
This made astronomers around the world to search for planets in nearby stars, but for many decades the outcome was not fruitful. The first confirmed detection of a planet in another system (not our own solar system) was in 1992, when several planets were detected orbiting the pulsar PSR B1257+12. A pulsar is a highly magnetized spinning neutron star and it’s usually the remnant from a supernova explosion of a star with mass at least eight times that of the Sun.
Three years later a giant planet was identified orbiting the nearby star 51 Pegasi. Since then the number of detected planets has increased significantly and the current known exoplanets are around 4200. Most of these have been detected through NASA’s Kepler mission, where around 20% of them have been characterized as Earth-like planets.
List of Earth-like planets
The term Earth-like planet is used to classify planets that their characteristics, match those of the Earth (mass , radius, distance from the host star and surface temperature). Till the end of 2019, around 50 Earth-like planets have been detected. A table with some recently discovered Earth-like planets hosted in the nearby star TRAPPIST-1 is given below, which are believed very good candidates of Earth-like planets.
(Note that the equilibrium temperature, is the temperature of a planet without considering the presence of an atmosphere. The mean temperature on Earth by considering its atmosphere too, is 14°C.)
New Earth-like planets discovered
On the 3rd of January 2020, NASA through observations with Transiting Exoplanet Survey Satellite (TESS), announced the discovery of new Earth-like planets, around TOI 700, a red dwarf star, located in the constellation of Dorado at a distance of 100 light years away.
The host star TOI 700, is an M-type dwarf star, which has around 40% of the Sun’s mass and size and half its surface temperature (3200 °C). Even though dwarf stars are not favoured to host planetary systems that can support habitable zones, TOI 700 does not show white-light flares. Additionally, the low rotation rate of the star indicates low stellar activity.
In total three new planets (b,c,d) were detected orbiting TOI 700 (see Figure 1). The discovery was also confirmed also by NASA’s Spitzer Space Telescope. The detection of planets around stars requires long time observations. Thus, a series of images is obtained in order to measure variations in the stellar brightness caused by an orbiting planet. This phenomenon is known as transit.
(Figure 1: An illustration of the newly discovered planetary system around TOI 700. Planets TOI 700b and TOI 700c are too close to the host star. It is clear that TOI 700 d is located within the habitable zone, thus making it a good Earth-like planet candidate.)
From the current knowledge on this planetary system, it seems that planets b and d, are more likely to be rocky in nature, just like our Earth, while planet c is possibly like Neptune.
Planet b is believed to have a similar mass and radius with our Earth and orbits around the host star in around 10 days at a distance of 9.5 million km (this is only 6% of the distance between the Earth and the Sun). For planet c the mass has not been estimated with high accuracy, but it is believed that it has a radius 2.5 times that of the Earth. It completes an orbit around the host star in 16 days at a distance of 13.8 million km from the host.
As seen in figure 1, only planet d lies in the habitable zone, which is the range of distances, where conditions are those to allow the presence of liquid water on the surface. The planet is believed to have a radius 20% larger than the Earth, while its mass is 1.7 times that of the Earth. The planet orbits the host star at a distance of 24.3 million, while it receives 86% of the energy that the Earth receives, with the mean temperature being around -4°C. Finally, the planet needs approximately 37 days to complete an orbit around its host.
Through observations with TESS, three new planets have been detected in the nearby star TOI 700. We saw that there are two planets that are clearly Earth-like in nature. The main difference though between the two, is that planet d is located on the habitable zone, which may favor conditions for the presence of liquid water on its surface.
Based on the current knowledge, TOI 700 d, is considered to be a robust candidate Earth-like planet. Follow up observations are necessary on this planetary system, in an attempt to obtain more information, specifically on TOI 700 d, and verify if the planet has an atmosphere, thus the necessary conditions to support life.
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