The first indication of an exoplanet was noted as early as 1917 but was not acknowledged as such until 2016. The very first scientific detection of exoplanets kicked off in 1988, although the first confirmed detection came in 1992 when astronomers spotted several terrestrial-mass planets orbiting the pulsar PSR B1257+12. A few years later, in 1995, astronomers discovered the first exoplanet orbiting a main-sequence star; a massive planet with a four-day orbit around 51 Pegasi.

But back in the sixteenth century, the Italian Philosopher and writer Giordano Bruno put forth the idea that there are other stars with planets out there.

This space we declare to be infinite… In it are an infinity of worlds of the same kind as our own.

— Giordano Bruno, 1584.

Bruno was right.

As of August 2019, NASA reports the existence of 4,043 confirmed exoplanets, orbiting stars in 3,004 planetary systems, with around 665 systems having more than one planet orbiting its host star.

But these are just a few planets compared to the total number of exoplanets astronomers think to exist in the Galaxy.

About 1 in 5 Sun-like stars have an “Earth-sized” planet in the habitable zone, astronomers believe. Discovering exoplanets has consequently increased our interest in alien life.

If there are so many planets in the Milky Way, could it really be that none of them is habitable? Not really. Astronomers have found many alien planets orbiting their star in the so-called habitable zone, the region around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.

The exoplanet nearest to Earth is Proxima Centauri b, orbiting the star Proxima Centauri located around 4.2 light-years from Earth. Proxima Centauri is also the closest star to our Sun.

But in addition to planets that orbit distant suns, there are also so-called rogue planets, which are basically exoplanets that do not orbit any stars. They are cosmic wanderers that travel the universe and astronomers say that, in the Milky Way, these types of worlds could number in the billions.

Confirmed vs Detected

Detected exoplanets and confirmed exoplanets are not the same. One of the most valuable tools in the detection of exoplanets was NASA’s Kepler mission which as of June 2017 identified more than 5,000 exoplanet candidates, several of which are Earth-like and located in the habitable zone of their respected stars. Furthermore, some of these Earth-like exoplanets orbit around Sun-like stars, which raises the possibility life may have developed on some of them.

As of August 2019, astronomers have confirmed the existence of 4,043 exoplanets. Most of these planets, around 97 % of them have been found by indirect detection techniques like radial velocity measurements of transit monitoring techniques.

NASA’s Kepler space telescope was retired in 2018. Replacing Kepler is the Transiting Exoplanet Survey Satellite (TESS), a space telescope par of NASA’s explorer’s program, specifically built and designed to search the galaxy for exoplanets using the transit method. TESS has the ability to cover an area 400 times larger than that which was covered by NASA’s Kepler mission.

The primary goal for TESS is to survey the brightest stars located in Earth’s vicinity for transiting planets for a period of two years. TESS allows astronomers to not only study the mass and size but the density and orbit of smaller exoplanets.

In addition to the 4,043 confirmed alien worlds, there are 3,983 potential exoplanets that are still awaiting confirmation. Of the 4,043 confirmed exoplanets, 1361 are Neptune-like planets, 1.267 are considered Super Earth’s, 1,248 are categorized as Gas Giant’s, 161 are terrestrial planets, and six are categorized by NASA as unknown.

There are four main methods scientist used to search for exoplanets: Transit method, radial velocity method, microlensing method, and imaging method.

The transit method is the most popular, resulting in 77% of exoplanets discovered through it. The transit method looks for alien planets that pass directly between their stars and the observer, dimming the star’s light by a measurable amount.

The second method is the Radial Velocity method. Planets orbiting their star can cause them to wobble, which produces an observable shift in the color of the star’s light.

The third method is Microlensing, and it consists of looking for light from a distant star that is bent and caused by gravity, as the planet transitions between the star and our planet.

The third method is Imaging and is when astronomers are able to take pictures of exoplanets using sophisticated techniques that remove the overwhelming glare of stats they orbit.

Exoplanets and Alien life

But just because an exoplanet exists, it does not necessarily mean that its inhabited. A potentially habitable planet suggests a world similar to Earth, located within the circumstellar habitable zone around its star, with conditions roughly or comparable to those than exist on Earth.

But all in all, the question to what makes a planet habitable is far more complex than we ever imagined. Our biggest mistake may be the fact that when searching for alien planets, we immediately assume that because life on Earth needs water, oxygen, and an atmosphere, life elsewhere must do so too.

For all we know, there could be alien life that feeds off of solar radiation. For all we know, there could be life on planets that lack oxygen and use hydrogen instead. Alien lifeforms could be so strange that it is possible that we wouldn’t even recognize them.

Another possibility is that alien lifeforms are so old and advanced, that they are not biological but mechanical in nature. So searching specifically for Earth-like planets hoping to be habitable may be a mistake.

But we have to start from somewhere, and that’s why astronomers searching the cosmos look to find planets similar to Earth. Since there is life on our planet, if we find an exoplanet similar to our world, it could mean there could be life there as well.

The numbers

Six years ago, in 2013, astronomers were doing the math and reported that, based on the data gathered by the Kepler Space Telescope, as many as 40 billion Earth-sized planets are most likely orbiting within the habitable zones of Sun-like stars and Red Dwarf stars in the Milky Way Alone. Eleven billion of them may be orbiting Sun-like stars.

If 50% of those Sun-like stars have one planet orbiting it in the right place, it means that there are 5.5 billion Earth-like planets orbiting Sun-like stars. If only one percent of those planets have life developed on its surface, it would mean there are 55,000,000 planets with life in the Milky Way Alone.

In 2016 astronomers re-calculated the number of galaxies in the observable universe from a previous estimate of 200 billion to a suggested 2 trillion or more, containing more stars than all the grains of sand on planet Earth.

Understanding those numbers is hard, let alone imagining all the stars and planets in the galaxy.

However, in June of 2019, when NASA had confirmed its 4,000th exoplanet, the Space Agency launched an amazing map of all the exoplanets that were confirmed to that date.

NASA even published a video showing when and where all the exoplanets are found in the night sky. This is the video:

In August of 2019, astronomers refined their calculations regarding the number of Earth-like planets around Sun-like Stars.

In the new study, scientists estimated that planets very close to Earth in terms of size, from three-quarters to one-and-a-half times the size of earth, with orbital periods ranging from 237 to 500 days, occur around approximately one in six stars.

The study, published in the Astronomical Journal suggests that there are around 10 billion Earth-like planets in our galaxy.

But these numbers are only expected to increase as new telescopes and technology become available. Future missions such as the European Characterising Exoplanets Satellite (CHEOPS), set to launch later this year and the James Webb Space Telescope set to launch in 2021 are expected to provide unprecedented data on the many alien worlds that are out there.

But even with the current technology, we have at our disposal, we are learning about new exoplanets on a nearly daily basis.

In fact, news about newly found exoplanets isn’t a surprise any longer. It has recently been reported that astronomers have found three Earth-sized exoplanets orbiting a star located just 12 light-years away.

One of those planets is believed to reside within the habitable zone of the star. All three exoplanets are believed to be from 1.4 to 1,8 times the mass of Earth, orbiting its star every three to thirteen days.

The exoplanet which orbits its star every thirteen days is the most interesting one to astronomers because it is located within the star’s habitable zone, making it possible for liquid water to exist on its surface.

The planets orbit a star called GJ 1061, a low-mass star that is the twentieth closest star to the sun.

Exoplanets with better conditions for life

A recent study has revealed that there could be exoplanets in the universe that are even better suited for life than our planet.

Most of what we believe we know about exoplanets tells us that a lot of the rocky planets we’ve found, orbiting their stars at just their right distance could be barren planets exposed to the dangers of stellar flares.

But a new study has challenged this view indicating that different exoplanets have different conditions, which means that there could be alien worlds out there where life can thrive, even better than on Earth.

And the key in their habitability lies within their oceans.

Geophysicist Stephanie Olson of the University of Chicago explained:

“Our work has been aimed at identifying the exoplanet oceans which have the greatest capacity to host globally abundant and active life. Life in Earth’s oceans depends on upwelling (upward flow) which returns nutrients from the dark depths of the ocean to the sunlit portions of the ocean where photosynthetic life lives. More upwelling means more nutrient resupply, which means more biological activity. These are the conditions we need to look for on exoplanets.”

Using software called ROCKE-3D which was developed by NASA’s Goddard Institute for Space Studies, the researchers modeled a wide range of different exoplanets in order to explore why of them are the most adequate to develop life, based on ocean circulation.

What they found surprised them. They discovered that thicker atmosphere combined with slower rotation rates and the presence of continents all generated higher upwelling rates.

“This is a surprising conclusion,” Olson revealed. “It shows us that conditions on some exoplanets with favorable ocean circulation patterns could be better suited to support life that is more abundant or more active than life on Earth.”

Conclusion

All of this means that there is simply too much space for Earth to be the only planet with life.

We’ve discovered thousands of exoplanets and many of them meet the requirements for life as we know it to develop on their surface. Many of these exoplanets are similar to Earth.

Of the 4,046 confirmed exoplanets to date, there are great possibilities that at least one percent of those worlds underwent the same processes as Earth, resulting in the emergence of life as we know it. That’s of course if we completely ignore the fact that there could be lifeforms in the universe that require the exact opposite for life than what humans do.

But say that we see exoplanets with water on their surface as planets that could potentially harbor life; according to astronomers, up to 35 percent of all known exoplanets that are bigger than Earth, most likely are water-rich planets.