181 light years away, four planets pass in front of their star – K2-72. Kepler has just spotted a handful of the 104 confirmed planets NASA just announced.

An international team of astronomers used several ground-based telescopes including the North Gemini telescope and the W.M. Keck Observatory to confirm the planets seen by Kepler and its K2 mission.

Kepler uses the tried-and-true transit method to spot planets outside our solar system. The telescope looks for small dips in a star’s brightness caused as the planet moves in front of it as seen from Earth.

How did these 104 planets go from candidates to confirmed? Astronomers gather high-resolution images of the host star along with high-resolution optical spectroscopy. These spectrographs tell astronomers the physical properties of a star. Think mass, radius, temperature – that kind of stuff. From these measurements, the properties of any planets orbiting the host star can be inferred according to NASA.

Those four planets I mentioned at the beginning? They orbit a star that is less than half the size of the sun and less bright. And they’re packed in close. Orbits range from 5.5 days to 24 days. That’s closer than Mercury’s orbit around the sun. Two of them show promise.

Astronomers believe the pair may experience irradiation levels comparable to those on Earth. The smaller star offsets the potential radiation problems that would arise from orbiting so close. K2-72c is believed to be about 10% warmer than Earth. The other, K2-72e, is thought to be 6% colder than our world.

All four planets range in size from 20-50% larger than Earth by diameter.

Ian Crossfield, who led the international team of astronomers, says they can’t rule out the possibility that life could form on these two planets.

Today, the number of exoplanet candidates stands at more than 5,000. The Kepler mission discovered 4,696 exoplanets with 2,326 confirmed. The K2 mission (launched after a malfunction prevented the telescope from pointing at its original target area) has spotted 458 candidates with 127 confirmed.

K2 and second light

K2 wrapped up the special microlensing campaign called Campaign 9 on July 1. This observation campaign looked for planets in a new way using gravitational microlensing. It’s the same effect astronomers use to observe distant galaxies. Instead of looking for a dip in brightness, K2 was looking for a momentary surge. Here’s a video explaining how K2 would find free-floating planets.

The future is bright for Kepler. Last month, the mission received two more years of funding. That means it will be operational through the end of NASA’s 2019 fiscal year. By then, the Kepler team expects the spacecraft to completely run out of fuel. Until then, the K2 mission will continue looking for possible planets. And astronomers will be confirming them long after Kepler runs out of fuel.