Time was, there were only nine known planets in the entire universe—the gaggle of worlds that orbit our sun. The International Astronomical Union lowered that local count to eight on June 6, 2006 when Pluto was declared a dwarf planet. Even before Pluto’s pink slip, there was a planetary census that grew deeper into space. The first discovery of a Jupiter like planet orbiting an accelerated spinning pulsar in 1992 and then, later in 1995, the discovery of another planet with a Jupiter-like orbit around a sun-like star in 1995. Since then the planetary population has exploded, and, as NASA’s Jet Propulsion Laboratory reported this week, the official total of known worlds beyond our own has now topped 5,000.
The Kepler Space Telescope was responsible for most of the discovery. Launched in 2009, it hunts for planets using the so-called transit method—looking for the slight dimming in light that occurs when an orbiting planet briefly blocks the light from the star. The dimming is fantastically subtle—the equivalent of removing a single light bulb from a board of 10,000 of them, is how Natalie Batalha, former Kepler mission director, described it to TIME. Kepler only studied a small portion of the sky. It surveyed just 150,000 stars. It still confirmed 2,709 exoplanets in 11 years of operation and returned data about another 2,057.
Transiting exoplanet survey satellite (TESS), a newer model, uses transit but also has multiple telescope eyes that allow it to view the whole sky. In just the short time it’s been operating, it has confirmed the existence of 203 more exoplanets and has spotted another possible 5,459 that astronomers are now investigating.
You don’t have to use the transit method in order to search for exoplanets. Other telescopes—both space-based and Earth-based—use what’s known as the radial-velocity method. In this case, they study a star, looking for the slight wobble caused by the gravity of a planet—or multiple planets—tugging on it as they orbit. Only 39 lightyears from Earth is the Trappist-1 multi-planet cluster, which contains seven planets that orbit the red dwarf.
These planets are varied in their size and composition. There are so-called hot Jupiters—which, as their name suggests, are gaseous worlds that orbit close to the fires of their parent planet. Other smaller, gas-rich worlds are comparable in size to Neptune. Still others—the most promising ones—are compact, rocky planets like Earth, some orbiting in the habitable zone of their star, a place where temperatures are not too hot and not too cold for water, the sine qua nonLife as we know is to exist in liquid form.
The mere fact that astronomers find planets pretty much everywhere they look has led them to conclude that virtually every star in the universe is orbited by at least one planet—making for trillions upon trillions of potential worlds. “Each one of them is a brand-new planet,” said NASA astronomer Jessie Christiansen in a statement. “I get excited about every one because we don’t know anything about them.”
Original publication of this article was TIME Space. Register here.
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