55 Cancri e

55 Cancri e, also known as Janssen, is an exoplanet located in the constellation Cancer. It is one of the most studied and intriguing “super-Earths” discovered to date, notable for its extreme surface conditions and close orbital relationship with its host star, 55 Cancri A. The planet is part of a multi-planet system, providing astronomers with valuable insights into planetary formation, atmospheric dynamics, and the diversity of planetary systems beyond our Solar System.

Discovery and Observation

The discovery of 55 Cancri e was announced in 2004 by a team of astronomers using the radial velocity method, which detects the gravitational influence of a planet on its parent star. It orbits the star 55 Cancri A, a G-type main-sequence star approximately 41 light years away from Earth. The system itself is part of a binary star configuration, with the secondary component, 55 Cancri B, being a red dwarf.
In 2011, the planet’s transit — when it passes directly in front of its star — was observed by NASA’s Spitzer Space Telescope, confirming its size and enabling precise measurements of its physical properties. This combination of radial velocity and transit data made 55 Cancri e one of the best-characterised super-Earths known in the early 21st century.

Physical Characteristics

55 Cancri e is classified as a super-Earth, a type of exoplanet with a mass higher than Earth’s but significantly lower than that of gas giants like Neptune or Jupiter. The planet has approximately eight times the mass of Earth and about twice its radius. This density suggests a predominantly rocky composition, though the exact internal structure remains uncertain.
The most remarkable feature of 55 Cancri e is its extremely close orbit to its host star — just 0.015 astronomical units (AU), or roughly 1/25th the distance between Mercury and the Sun. It completes one full orbit in less than 18 hours, making a single “year” on 55 Cancri e shorter than an Earth day. As a result of this proximity, the planet is likely tidally locked, meaning one hemisphere constantly faces the star while the other remains in perpetual darkness.
The surface temperature on the day side can exceed 2,400°C, hot enough to melt most metals and silicate rocks. This intense heat produces a harsh and volatile environment, with surface materials possibly existing in a molten state. Observations suggest that the night side is significantly cooler, potentially indicating atmospheric circulation or heat redistribution.

Composition and Structure

Early models of 55 Cancri e proposed that it could be a carbon-rich planet, sometimes referred to as a “diamond planet”. This hypothesis emerged from its host star’s chemical composition, which shows a high carbon-to-oxygen ratio. If similar proportions exist in the planet, it could contain a mantle rich in carbon compounds such as graphite and diamond rather than silicate minerals common on Earth.
However, subsequent spectroscopic analyses have cast doubt on this theory. Observations from Spitzer and Hubble have indicated the possible presence of volatile compounds, including silicates and possibly an atmosphere composed of hydrogen, helium, or even volcanic gases like carbon monoxide and carbon dioxide. The exact atmospheric composition remains debated, but the presence of heat variations between hemispheres implies some degree of gaseous envelope or surface lava flows capable of heat transport.

The 55 Cancri System

The 55 Cancri system is one of the most complex planetary systems known in the solar neighbourhood. It contains at least five confirmed planets, designated 55 Cancri b, c, d, e, and f. These planets vary widely in size, orbital distance, and composition. Planet 55 Cancri d, for instance, is a massive gas giant located far from the star, whereas 55 Cancri e orbits extremely close in.
This range of planetary types within a single system provides astronomers with a valuable case study for understanding planetary formation and migration. The existence of an ultra-short-period rocky planet like 55 Cancri e alongside gas giants supports the idea that planets can move significantly from their formation zones over time due to gravitational interactions.

Atmospheric Studies and Thermal Behaviour

Recent infrared observations from the James Webb Space Telescope (JWST) and earlier from Spitzer have revealed dynamic atmospheric and surface processes on 55 Cancri e. The planet exhibits striking temperature contrasts between its day and night sides, hinting at the possible presence of lava flows or volcanic activity. Some researchers suggest that the surface may be covered by a vast magma ocean, with the molten rock emitting intense infrared radiation detectable from Earth.
The temperature difference also raises questions about the existence of an atmosphere. If an atmosphere does exist, it is likely composed of metal vapours or silicate gases originating from evaporating surface materials. Alternatively, if the planet lacks a substantial atmosphere, the heat distribution could be explained by direct radiation from the molten surface itself. JWST’s future spectroscopic studies aim to distinguish between these models by analysing emitted light at various wavelengths.

Scientific Importance and Implications

The study of 55 Cancri e holds significant implications for exoplanet science. It challenges traditional notions of planetary habitability and composition, offering an extreme example of how planets can evolve in high-radiation environments. The planet’s short orbital period makes it ideal for repeated observation, allowing astronomers to refine models of planetary atmospheres, geology, and heat transfer mechanisms.
From a theoretical perspective, 55 Cancri e demonstrates the diversity of planetary systems beyond our own. Its characteristics have contributed to the broader understanding of:

• Super-Earth formation: how rocky planets of intermediate mass form and survive near their stars.
• Tidal locking and thermal balance: how close-in planets distribute stellar heat across their surfaces.
• Planetary composition diversity: exploring whether carbon-rich worlds exist in reality.

The planet also serves as a natural laboratory for studying extreme exoplanetary environments, which may help interpret future discoveries of similar ultra-short-period rocky worlds around other stars.

Ongoing Research and Future Missions

Ongoing research on 55 Cancri e continues to refine our understanding of its physical and atmospheric properties. Instruments such as JWST, CHEOPS (CHaracterising ExOPlanet Satellite), and ground-based observatories are conducting high-precision photometric and spectroscopic observations. These studies aim to:

• Determine whether the planet possesses a transient or stable atmosphere.
• Assess the composition of the potential magma ocean or volcanic plumes.
• Measure the chemical interaction between the planet’s surface and stellar radiation.

The results are expected to shed light on the evolution of rocky planets in extreme proximity to their stars, bridging the gap between terrestrial planets and gas giants in exoplanetary science.