55 Cancri Ae

55 Cancri Ae, also known by its proper name Janssen, is a fascinating exoplanet. An exoplanet is a world that orbits a star outside our own solar system. Janssen orbits a Sun-like star called 55 Cancri A. This planet is classified as a super-Earth. This term describes planets that are larger than Earth but smaller than gas giants like Neptune. Janssen is about eight times the mass of Earth. Its diameter is roughly twice as wide as our home planet.

Scientists study this world to learn about different types of planets.

The planet moves through space at an incredible speed. It is the innermost planet in its system. Janssen orbits its host star in less than 18 hours. This very fast orbit happens because the planet is extremely close to its star. The average orbital distance is only about 0.01544 AU. Because of this proximity, the planet is incredibly hot. The day side can reach temperatures exceeding 3,000 Kelvin. This heat is intense enough to melt iron.

Due to its age and closeness to the star, the planet is likely tidally locked. This means one side permanently faces the star while the other side always faces away.

Astronomers discovered Janssen using a method called radial velocity. A team led by Barbara E. McArthur found the planet using data from 2003 to 2004. They used the Hobby–Eberly Telescope at the McDonald Observatory in Texas. This method works by measuring the Doppler shift in the star's spectrum. This shift shows how the star moves slightly due to the planet's gravity. At first, scientists thought the orbit took 2.8 days. However, later observations in 2010 showed the true orbital period is much shorter. The actual period is about 0.7365 days.

Determining what Janssen is made of has been a long scientific challenge. In 2011, scientists confirmed the planet's transit. A transit occurs when a planet passes in front of its star from our perspective. This allowed researchers to calculate the planet's density. Early studies suggested the atmosphere might be rich in hydrogen and helium. Other studies suggested it might be a world covered in a thin layer of vaporized rock. Some researchers even thought it might be a water planet. These different ideas show how much we are still learning about distant worlds.

Recent discoveries have changed our understanding of the planet's atmosphere. In 2016, the Hubble Space Telescope detected hydrogen cyanide. In 2017, the Spitzer Space Telescope suggested a global lava ocean might exist. It also thought the atmosphere had a pressure of about 1.4 bar. However, the James Webb Space Telescope provided new data in 2024. These observations ruled out the idea of a thin atmosphere made of vaporized rock. Instead, the data suggests a substantial atmosphere. This atmosphere is likely rich in carbon dioxide or carbon monoxide.

There is a unique theory that Janssen might be a carbon planet. In our solar system, terrestrial planets are made of oxygen-rich material. On Janssen, roughly one-third of the mass could be carbon. Because of the extreme temperatures and pressures, much of this carbon might be in the form of diamond. The planet also shows signs of possible volcanic activity. This volcanism might release large clouds of dust. These clouds could block thermal emissions and cause temperature variations. The planet's magma ocean might even be outgassing to sustain its atmosphere.

The name Janssen has a special history. In 2014, the International Astronomical Union started a contest called NameExoWorlds. This allowed the public to nominate and vote on names for exoplanets. In December 2015, the name Janssen was announced as the winner. The name was submitted by the Royal Netherlands Association for Meteorology and Astronomy. It honors Zacharias Janssen, a spectacle maker associated with the invention of the telescope. Studying Janssen helps astronomers connect many different ideas about how planetary systems form and change over time.