Introduction

The Universe has always been a mysterious figure to us, nevertheless, our curiosity never stopped us from pushing the limit of our understanding. In this article, we will talk about a few fascinating discoveries of "Super-Earths" that we made in recent years. Moreover, we will be briefly looking at a few Super-Earths that are potentially habitable for us. If you are interested in astrophysics, then you should keep reading to find out more.

What is meant by “Super-Earth”?

Let's begin by discussing what it means for a planet to be a Super-Earth. "Super-Earths" refers to planets outside our solar system which are larger in size than Earth but much smaller than ice giants such as Uranus and Neptune. It provides a reference to the size of exoplanets, but it doesn't mean they are necessarily as habitable as Earth.

Kepler-438b

On the 6th of January, 2015, Kepler's space telescope discovered another planet named "Kepler-438b".  Surprisingly, scientists claim that its ESI is about 0.88, which means that it's one of the planets most similar to the Earth we have discovered thus far.

However, does this imply that it's a very suitable choice for us? More factors need to be taken into account. For instance, distance is one of the most significant issues that we face: it is located 470 light years away from our planet, which means that we can't migrate to Kepler-438b regarding the current pace of our technology.

The ESI Formula

Previously we mentioned the ESI, which stands for Earth similarity index. In simple words, it is a measure of how similar a planet is to our Earth based on multiple features. The concept was first proposed in 2011 by Prof/Dr. Schulze-Makuch. The calculation involves the interior aspects of a planet - the mean radius and the density; and the exterior aspects - the escape velocity (the minimum velocity required to leave the gravitational influence of a celestial body) and the surface temperature.

Kepler-438b's Conditions

Calculations have shown that the surface temperature of the planet is approximately 3°C, which suggests that there might be liquid water on the planet, providing more possibilities for life.

In addition, the planet also lies in the circumstellar habitable zone (the range of orbits around a star within which the planet has sufficient atmospheric pressure and liquid water on the surface) of Kepler-438.

All of this suggests that it is an appropriate candidate for our next home, excellent discovery, right?

The Plot Twist and Uncertainty

Just like a gripping story, there is usually a plot twist. In November 2015, NASA announced that it was very likely that there were no more possibilities of life on Kepler-438b.

Why though?

Research has shown that the star that the planet orbits explodes once per 100 days, as a result, regarding the relatively small distance between the star and Kepler-438b, the solar wind produced from the explosion would very likely evaporate all the liquid water on the planet and easily blow away the atmosphere, in which case all living organisms would be exposed to the strong UV, ultimately making it inhabitable.

However, there is one last hope we have for Kepler-438b, if there is a strong magnetic field around it, it would protect the planet from the solar wind and potentially allow us to reconsider the chances of moving there. But further investigation is required to support the proposal.

Kepler-1649c

On the 14th of April 2020, NASA's Kepler space telescope discovered Kepler-1649 c. It is an Earth-sized potentially habitable terrestrial exoplanet that is located 301 light years away from Earth, orbiting within the habitable zone of the red dwarf star Kepler-1649, ranked 5th in ESI in December 2020.

The Habitable Zone

We have previously stated that Kepler-1649c is orbiting within the habitable zone of the red dwarf star. However, what does the habitable zone exactly refer to?

According to the luminosity or radiation power emitted from the parent star (in our case, the Sun, and in Kepler-1649c's case, the red dwarf star), the region where liquid water may exist can be roughly calculated, this region is called the habitable zone.

The Relationship Between Kepler-1649c and Its Nearby Planets

Interestingly, there's a planet (Kepler 1649-b) near Kepler 1649-c, which has played a significant role in stabilizing its orbit around the red dwarf star; Kepler 1649-b and Kepler 1647-c possess a relationship just like Earth and Venus!

Kepler-1649c’s Condition

Estimation shows that Kepler-1649c's radius is approximately 6753.3 km, which is only 1.06 times that of Earth, suggesting that it is a perfect example of Super Earth.

Furthermore, Scientists have found out that Kepler-1649c receives 75% of the amount of sunlight that we receive on Earth. It is estimated that its surface temperature is about -39 degrees celsius (not taking into account other factors such as the atmosphere). However, if we use the same method to measure the Earth's average surface temperature, it will be around -18 degrees celsius.

Therefore, it is really likely that there is liquid water in some regions of Kepler-1649c, which suggests that there may be life on this planet!

Proxima Centauri b

"Proxima Centauri b" is yet another Super-Earth that we have explored. It has an ESI of 0.88, which is considerably high among all of the celestial bodies that we have discovered in the past. As you may tell from the name of the planet, it orbits the star named "Proxima Centauri", which is a small and low-mass star that is found to be about 4.24 light years away from the Earth. If we compare the distance to that of the celestial bodies previously being discussed, we realize that Proxima Centauri is much closer to our planet and thus much easier for possible migrations in the future.

Here is some sad news: the planet receives a vast amount of radiation in the form of UV and X-rays from Proxima Centauri (about 10-60 times what we get on the Earth), which means that living organisms on the planet are exposed to severe health risks like cancer due to cell mutation.

KOI-4878.01

In the previous paragraphs, we introduced many Super-Earths with very high ESI values. But next, we'd like to introduce you to the closest Earth-sized Super-Earth ever explored: KOI 4878.01!

According to the data from the Kepler space telescope, its ESI reaches 0.98, that is to say, it is 98% similar to the earth in terms of mass, size, escape speed, and surface temperature.

You might wonder, if KOI 4878.01 was also discovered by the Kepler space telescope, why does its name not start with "Kepler"?

The reason is that we have not yet completed the manual identification of its data, but once scientists can accurately confirm it, KOI 4878.01 will officially be named Kepler 4878-b and announced as the most Earth-like planet observed by scientists and researchers (yet).

KOI-4878.01's Conditions

First of all, its radius is only 1.04 times that of Earth, and besides, its mass is 0.99 times that of Earth. Therefore, it can be regarded that in size and mass, KOI-4878.01 is very similar to the Earth we now inhabit.

In addition, its parent star has nearly the same brightness and size as Earth's sun, it is also located nearly the same distance away from its parent star as Earth is from the sun. The average surface temperature of KOI-4878.01 reaches -15 degrees Celsius, while Earth's is -18 degrees Celsius, almost indistinguishable in the vastness of the universe. (The temperature calculation here does not take into account many other factors, such as the influence of the atmosphere, so it will be much lower than the actual surface temperature).

Combining these factors, we get an astonishing 0.98 for the ESI value.

Unfortunately, KOI-4878.01 is about 1075 light-years away from the Earth, which means that even if our technology reaches the speed of light, we will still need 1075 years to reach its surface.

Factors affecting habitability

One of the most important things that are required for living organisms' survival is liquid water. Living things need water for chemical reactions in the body to take place. As a result, the habitability of a planet greatly depends on the surface temperature, as high temperatures would mean that the water would evaporate and thus not be in the form of a liquid.

Apart from liquid water, the Sun that the planet orbits must have a long life, in other words, it must be stably present for at least a few billion years. You don't want the Sun to explode after a few years of moving there, do you?

A suitable natural satellite is essential. Gravity causes tidal friction, which takes some of the Earth's energy out and slows its rotation down. This is beneficial as it reduces the chances of storms and therefore contributes to the habitability of the planet. Moreover, the tidal forces cause the liquid iron inside the Earth to flow, creating current, and therefore producing a magnetic field.

This then leads to the significance of magnetic fields. As discussed earlier, a strong magnetic field would protect the planet from the solar wind.

Additionally, an active and high-temperature core as it provides a source of thermal energy for the outer layer and maintains the magnetic field around the planet which has many purposes.

Finally, illumination and a balanced climate are worth noting as well; the former allows for basic processes like photosynthesis, and the latter is closely associated with the safety of the species.

Conclusion

We are living things no more than 10 feet, but we always have the desire to inspect celestial bodies that are several million times our size and reach far beyond our Solar System. Throughout history, from scientific researchers to students like us, people were always intrigued by what's ahead of us on the unlit path of the exploration and observation of the Universe.

The Universe is an enchanting place with unlimited possibilities. Our exploration never stops, and we'd likely discover thousands more of these Super-Earths in the future.

Well, we have so many places to consider for migration, so is the Earth still critical to us anymore?

Undoubtedly yes. In the near future, there is no guarantee for efficient and large-scale migration, and therefore we should not put the entirety of our hopes onto other Earth-like planets.

The Earth had nurtured trillions of lives including us, and not only should we appreciate it, but you should also act upon it by contributing your part to save our common mother from our own hands.