The number of confirmed planets outside our Solar System, known as exoplanets, has reached a new, impressive milestone of 6,000, NASA has announced. This figure, which is constantly updated in NASA's Exoplanet Archive, represents the culmination of decades of tireless research and technological advancement, completely changing our understanding of the universe and our place in it. It is important to note that there is no single, specific planet that holds the title of "the six-thousandth," but rather it is a continuous process of confirming discoveries by scientists around the world. This archive, which meticulously tracks every new discovery, is managed by the NASA Exoplanet Science Institute (NExScI), located within Caltech's IPAC center in Pasadena, California.

In addition to the 6,000 confirmed worlds, there are more than 8,000 potential candidates waiting for verification, indicating that we have only scratched the surface in cataloging the planetary wealth of our galaxy. These achievements highlight the accelerated pace of discovery, especially in the last decade, and lay the foundation for answering one of humanity's most profound questions: are we alone in the universe?

A milestone in space exploration

Shawn Domagal-Goldman, acting director of the Astrophysics Division at NASA Headquarters in Washington, points out that this milestone symbolizes decades of cosmic exploration led by NASA's space telescopes. "It is research that has fundamentally changed the way humanity looks at the night sky," he said. "Step by step, from the very discovery to detailed characterization, NASA's missions have built the foundation for solving a fundamental question. Now, with upcoming missions like the Nancy Grace Roman Space Telescope and the future Habitable Worlds Observatory, America will lead the next giant leap – studying worlds similar to our own around stars similar to our Sun. This is the promise of discovery that unites us all."

This success comes exactly 30 years after the discovery of the first exoplanet around a Sun-like star in 1995. Although planets around pulsars – the remnants of stars that have burned out their fuel – were identified before that, in the early nineties, the 1995 discovery launched a true revolution. Although scientists estimate that there are billions of planets in our galaxy, the Milky Way, alone, finding and confirming them remains an extremely challenging task.

A diversity of worlds: More than we could have imagined

As the number of known exoplanets grows, scientists are gaining a better insight into the general population of planets in the galaxy and can compare it to our "cosmic neighborhood." For example, while our Solar System has a roughly equal number of rocky and gas giants, it seems that rocky planets are statistically much more common in the universe. But what is most fascinating is the incredible diversity of planetary types that have no equivalent in our system.

Worlds have been discovered that defy our intuition. There are so-called "hot Jupiters," gas giants the size of Jupiter that orbit their stars at distances smaller than Mercury's from the Sun, with years lasting only a few Earth days. Planets have been found that orbit two stars simultaneously, like Tatooine from "Star Wars," as well as planets that orbit no star at all – so-called rogue planets – that wander alone through the cold darkness of interstellar space. Worlds covered in lava oceans have been discovered, planets with the density of styrofoam, and those whose clouds are made of gems like rubies and sapphires.

"Each of the different types of planets we discover provides us with information about the conditions under which planets can form and, ultimately, how common planets like Earth might be and where we should look for them," explains Dawn Gelino, head of NASA's Exoplanet Exploration Program (ExEP). "If we want to find out if we are alone in the universe, all this knowledge is of crucial importance."

Planet-hunting methods: How scientists "see" the invisible

Finding these distant worlds requires exceptional ingenuity, as exoplanets are incredibly small and faint compared to the brightness of their parent stars. Fewer than 100 exoplanets have been directly imaged so far, as most are lost in the glare of starlight. Therefore, scientists rely on several ingenious indirect detection methods.

• The transit method: This is the most productive method to date, responsible for thousands of discoveries. Astronomers monitor a star's brightness and look for tiny, periodic dips in light. The dimming occurs when a planet, from our perspective, passes in front of its star, blocking a fraction of its light. Missions like the legendary Kepler telescope and its successor TESS specialize in this method.


• The radial velocity method (Doppler spectroscopy): This method does not observe the planet itself, but its effect on the star. As a planet orbits, its gravity causes the star to "wobble" slightly. This wobble causes changes in the color of the star's light (the Doppler effect) that sensitive instruments on Earth can detect.


• Gravitational microlensing: This technique uses a phenomenon predicted by Einstein's general theory of relativity. When one star passes in front of another, more distant star, its gravitational field acts as a lens, bending and amplifying the light of the background star. If the lens-star has a planet, its own gravity will create an additional, shorter flash in the light amplification.


• Astrometry: This is the precise measurement of a star's position in the sky. If a star has a planet, it will cause a tiny, but measurable, shift in the star's movement across the sky.

Since the signals can also be caused by other phenomena, each exoplanet candidate must undergo a rigorous confirmation process, often using additional telescopes and methods, which can take time. This is why the NASA Exoplanet Archive has a long list of candidates. "We really need the collaboration of the entire community if we want to maximize our investments in missions that produce exoplanet candidates," says Aurora Kesseli from NExScI.

The future of exploration: In search of another Earth

The rate of exoplanet discovery has grown exponentially (the archive reached 5,000 confirmed planets just three years ago), and this trend is expected to continue. Scientists are eagerly awaiting thousands of new candidates from the European Space Agency's (ESA) Gaia mission, as well as from the upcoming Nancy Grace Roman Space Telescope. Roman will discover thousands of new exoplanets primarily using the gravitational microlensing technique.

At NASA, the future of exoplanet science is focused on finding rocky, Earth-like planets and studying their atmospheres in search of biosignatures – any sign, element, molecule, or feature that could serve as evidence of past or present life. NASA's James Webb Space Telescope has already analyzed the chemical composition of the atmospheres of more than 100 exoplanets, providing incredible insights. However, studying the atmospheres of planets the size and temperature of Earth will require a new generation of technology.

The main challenge is to separate the faint light of the planet from the blinding glare of its star. For example, the Sun is about 10 billion times brighter than Earth, which would be more than enough to completely obscure our planet's light to a distant observer.

The Roman telescope will carry a demonstration instrument called the Roman Coronagraph, which will test new technologies for blocking starlight to enable the imaging of faint planets. Along with advances in coronagraph technology, NASA is developing a concept for a future mission, currently called the Habitable Worlds Observatory (HWO), whose primary goal will be just that: to directly image Earth-like planets and analyze their atmospheres for signs of life.