Since 1995, more than 5000 exoplanets have been discovered by telescopes. Among them, there are worlds that until a few years ago, belonged exclusively to science fiction: the ocean planets.
The discovery of Kepler-138
In the constellation Lyra, 218 LY from Earth, lies the red dwarf Kepler-138. The star was first discovered during an astronomical survey that began in 1997 and ended four years later. In 2009, NASA launched the Kepler Space Telescope from Cape Canaveral, an ambitious project to monitor a small patch of the sky in search of Earth-like exoplanets. During its nine years of operation, the Kepler telescope analyzed 145,000 stars in the Swan, Dragon, and Lyra constellations.
The data collected by the telescope were then transmitted back to Earth for processing. The research team behind the Kepler mission then had to analyze this data by looking for significant dips in the brightness of the stars being monitored, using a technique called the “transit method“. If a star has several planets orbiting it, its brightness will periodically diminish. The greater the dip, the more massive the planet.
Throughout nine and a half years, 530506 stars were analyzed and 2662 exoplanets were detected. The telescope’s data also revealed that there are around 17 billion Earth-like planets ‘only’ in the Milky Way.
The Kepler-138 System
Kepler-138 is, as mentioned above, a red dwarf, a star with half the mass and size of the Sun. Its surface temperature is about 4073 K and its luminosity is 5.6% of that of the Sun.
In 2014, the first two planets were identified around the red dwarf Kepler-138, with orbital periods of 13.8 and 23 days. In the same year, a third planet with an orbital period of just 10.3 days was identified.
Kepler-138c is the second planet in the system and orbits its parent star in 13 days at a distance of 0.09 AU. The planet has 2.3 times the mass of the Earth and is 1.5 times the size of our planet. Kepler-138c appears to be similar in mass, diameter, and possibly chemical composition to the third planet in the system, Kepler-138d. Fifteen percent of the planet’s total mass is water, which is why people immediately thought of it as an ocean planet.
Given the proximity to the parent star, the water that makes up the planets might not be on the surface as an ocean, but in the upper atmosphere in the form of water vapor. A team of researchers from the University of Montreal recently disproved this hypothesis by observing the two exoplanets with the Hubble (the older brother of the JWST) and Spitzer space telescopes and confirming that the two planets are largely composed of surface water in a liquid state, although this has not been directly detected.
However, the certainty that they are ocean worlds comes from the measured values of the mass and size of the planets themselves, compared with model values that identify an ocean planet. The team warns that, given the proximity of the red dwarf, it is very likely that the atmospheric temperature on the planets is much higher than the boiling point of water, but under a thick and dense layer of water vapor there are oceans of liquid water at high pressure, where the water is in a “supercritical state“.
According to Nobel Prize winner Michel Mayor, the Milky Way could be full of rocky planets with liquid water on the surface. There are between 200 and 400 billion stars in our Galaxy alone; if each star is orbited by at least one or two planets, there should be hundreds of billions of planets. The best estimate is that a few billion planets should have liquid surface water, but beware: this does not mean that they are “habitable”.
What is an ‘exoplanet’?
An exoplanet is a planet orbiting a star other than the Sun, i.e. in another planetary system. However, every exoplanet discovered has been associated with an object of our Solar System: there are Earth-like planets, super-Earth-like planets (more extensive, and more massive than our planet), Jupiter-like planets, and Neptune-like planets, which share mass, density, and chemical composition with the solar planets.
A world made of water!
To introduce the concept of an ocean planet, we need to leap back in 2014, when the film that was about to become a masterpiece in every sense of the word was released: Interstellar. In Christopher Nolan’s famous film, there is an ocean planet called ‘Miller’. This planet has two peculiarities: it orbits a supermassive black hole 100 million times larger than the Sun and is completely covered in liquid water.
As of 2018, astronomers are convinced that there are ocean planets in the cosmos, thanks to the abundance of water in the universe. Some examples? TOI-1452b, Kepler-138c and Kepler-138d.
For a planet to be ocean-like, it has to meet certain requirements:
- be formed at the right distance from its star, neither too close to it nor beyond the snow line;
- have a mass similar to or greater than that of the Earth
- be less dense than the Earth;
- the structure of a planetary ocean should be common to all bodies belonging to this category;
- the metallic core should be 4,000 kilometers in diameter;
- have two mantles: a rocky one 3,500 kilometers high and an icy one 5,000 kilometers high;
- the planet should have an ocean more than 100 kilometers deep that completely covers the planet’s mantle;
- have an atmosphere dense enough to shield the planet from cosmic radiation and stellar winds, and to allow surface water to remain liquid;
A question might arise: can such a planet be considered ‘habitable’? While all this water absorbs the excess carbon dioxide in the atmosphere, a planet completely covered in water may prevent microorganisms from evolving. Extremely primitive forms of life could inhabit an ocean planet, but life as we know it is impossible.