As stated on NASA’s Exoplanet Exploration website, more than 4,000 exoplanets have been discovered since the identification of the first confirmed exoplanet orbiting a sun-like star in 1995. The NASA Kepler Space Observatory, launched in 2009 to determine the prevalence of Earth-like planets in the Milky Way galaxy, has played a pivotal role in advancing our understanding of exoplanets.

Astronomers have long dreamed of finding the first true “Alien Earth,” and recent discoveries of exoplanets indicate that small, rocky worlds like our own are common throughout the cosmos. For a planet to be considered potentially life-friendly, it must reside in the “Habitable Zone” of its star, which is the region where liquid water can exist on a planet’s surface. Future advancements in telescope technology will allow astronomers to consider additional factors, such as a planet’s atmosphere and the activity level of its parent star.

What is an Exoplanet?

In our solar system, every planet orbits the sun. Exoplanets, on the other hand, are planets that revolve around stars outside our solar system. They are extremely challenging to observe directly using telescopes because the intense light from the stars they orbit often obscures them.

To find and study these distant planets, astronomers employ indirect methods. They look for the gravitational effects that exoplanets exert on their parent stars, which can reveal their presence.

How do Exoplanets Form?

Planets form from circumstellar disks of gas and dust surrounding young stars, known as protoplanetary disks. These disks are difficult to observe because they are typically obscured by a star that is 100,000 times brighter than the disk itself. However, the Hubble Space Telescope has successfully captured images of these disks thanks to its superb sensitivity, resolution, and high-contrast imaging techniques that block the overwhelming light from the star. Although these disks reflect only the star’s visible light, Hubble can still detect their infrared glow.

This limited sample of protoplanetary disks demonstrates significant diversity. As planets develop within these disks, their structural characteristics should be reflected in the disks’ forms. The findings from Hubble align with observations of exoplanets, revealing that planets often occupy orbits that differ significantly from those in our solar system.

These studies of protoplanetary disks also provide insights into the formation and evolution of our own solar system. For example, collisions between planet-sized objects could explain the formation of systems like Pluto-Charon or Earth-Moon, which occurred over 4 billion years ago. In some instances, debris from these collisions eventually coalesced into moons. With the launch of the James Webb Space Telescope, astronomers will be able to observe these disk systems in greater detail in the infrared, potentially uncovering evidence of newly formed planets.

List of Some Exoplanets that Resemble Earth the Most:

• KEPLER-452B

• KEPLER-442B

• KEPLER-186F

• KEPLER-69C

• KEPLER-62F

• KEPLER-22B

• KEPLER-1649C

• GLIESE 667CC

• PROXIMA CENTAURI B

• TRAPPIST-1E

KEPLER-452B

Discovered in 2015, Kepler-452b is the first near-Earth-size planet to orbit a star similar to our sun. This planet is approximately 1.6 times the size of Earth and orbits a parent star that is 10% larger than the sun. Located 1,400 light-years away, Kepler-452b resides in the habitable zone and has a “better than even chance” of being rocky. It completes an orbit around its star in just 20 days more than Earth.

KEPLER-442B

This exoplanet, discovered in 2015, is 33% larger than Earth and orbits its star every 112 days. Kepler-442 is located 1,194 light-years from Earth. A study published in 2021 suggested that this exoplanet receives enough light to support a substantial biosphere, indicating that multiple planets in this system could potentially perform photosynthesis.

KEPLER-186F

Kepler-186f was the first confirmed Earth-size planet to circle a distant star within the habitable zone, where liquid water may exist on its surface. Although its mass, structure, and density are unknown, its size is less than 10% greater than that of Earth. It is located about 500 light-years away in the constellation Cygnus and orbits its star once every 130 days, receiving one-third the energy from its star that Earth does. If you stood on its surface, the brightness of its star at noon would be similar to the brightness of the sun on Earth one hour before sunset.

KEPLER-69C

Kepler-69c is classified as either a “super-Venus” or a “super-Earth,” depending on its surface conditions. Discovered approximately 2,700 light-years away, it was initially hailed as the alien planet most similar to Earth. Since its discovery, there has been debate over whether it is more like Venus or Earth. For example, Kepler-69c’s orbit takes 242 days, similar to Venus’s. Scientists have speculated about the possibility of liquid water on its surface.

KEPLER-62F

Kepler-62f, located nearly 1,200 light-years from Earth, may resemble an Earth-like planet. It orbits a slightly dimmer and smaller star than our sun and is just 1.4 times the size of Earth. Discovered in 2013, it is part of a larger discovery that included seven exoplanets. Studies suggest that Kepler-62f may be covered in water, which could support life forms significantly different from those on Earth.

KEPLER-22B

Kepler-22b, the first Earth-like planet discovered by the Kepler Space Telescope in its star’s habitable zone, is considered a good candidate for the search for extraterrestrial life. Announced in 2011, it is believed to be 2.4 times larger than Earth and has an average surface temperature of 72 degrees Fahrenheit (22 degrees Celsius). Despite this, scientists have differing opinions on the planet’s actual habitability.

KEPLER-1649C

Kepler-1649c was identified through the analysis of data from NASA’s Kepler space telescope. It is located about 300 light-years away and is only 1.06 times larger than Earth. Scientists discovered that this exoplanet receives 75% of the light that Earth receives from the sun.

GLIESE 667CC

Located just 22 light-years away, Gliese 667Cc is at least 4.5 times larger than Earth. It orbits its host star in just 28 days. However, because the star is a much colder red dwarf, the exoplanet is believed to be within its habitable zone. This planet was discovered using the 3.6-meter telescope of the European Southern Observatory in Chile.

PROXIMA CENTAURI B

Proxima Centauri b is one of the most intriguing exoplanets discovered to date. It orbits Proxima Centauri, the nearest star to our sun. This planet has a mass of 1.17 times that of Earth and completes an orbit around its star in just 11.2 days. Despite being closer to its star than Earth is to the sun, Proxima Centauri b lies within the habitable zone because its star is a red dwarf, which is smaller and cooler than our sun. Remarkably, it receives about the same amount of solar radiation as Earth does from our sun.

TRAPPIST-1E

TRAPPIST-1e is a nearly Earth-sized exoplanet that orbits the ultra-cool dwarf star TRAPPIST-1, located around 39 light-years from Earth in the constellation Aquarius. It was one of seven new exoplanets discovered around TRAPPIST-1 by Spitzer Space Telescope observations. The planet is located in the habitable zone of its star and has physical properties remarkably similar to those of Earth, including mass, radius, density, gravity, temperature, and stellar flux. It also has a dense atmosphere, akin to the terrestrial planets in our solar system.

Is It Possible to Have Two Earths?

Some scientists speculate that since there is one Earth teeming with life, there is no reason why another planet, similar to Earth and under comparable conditions, should not exist in our galaxy. This second Earth could potentially harbor life, possibly even intelligent life. While no other planet identical to Earth has yet been discovered, researchers continue to explore the cosmos with optimism for what lies ahead.