Planet LHS 1140 b: Potential ocean world 48 light years away

Research on the discovery made using the James Webb Space Telescope has been published in the prestigious scientific journal "The Astrophysical Journal Letters". The discovered planet, which could be an ocean planet, is relatively close to Earth.

The planet in question is named LHS 1140 b and orbits a small star 48 light years away from us. This star, known as a red dwarf, has a radius that is only about 15 percent of the radius of our Sun and is significantly cooler, with its temperature around 2,760°C compared to 5,500°C of our Sun.

However, despite these differences, they do not prevent the existence of a so-called habitable zone (eco shell, habitable zone) around this star. This area is where the planet's surface conditions could allow for liquid water. For this to be possible, the planet must orbit much closer to the star than Earth does to the Sun. This is the case with LHS 1140 b, whose orbital period is only about 25 days.

This planet receives about 42 percent of the radiation level that reaches Earth from the Sun. Scientists' calculations suggest this should result in a surface temperature of about -50°C. However, considering atmospheric effects, there is a chance for liquid water if the planet has an atmosphere composed of nitrogen and carbon dioxide or an ocean world with an atmosphere rich in hydrogen.

LHS 1140 b was discovered in 2017 as part of the MEarth Project. This discovery was made using the transit method, which involves observing small dimmings of the star's brightness caused by the planet passing between us and the star in our line of sight. Confirmation of the existence of this planet came later through spectroscopic observations using the HARPS spectrograph. In 2020, water vapour was detected in the planet's atmosphere thanks to the Hubble Telescope, although this result has not yet been confirmed.

Initially, it was considered that LHS 1140 b might belong to the group of so-called mini-Neptunes, that is, gas planets smaller than Neptune. However, new observations conducted with the Webb Telescope confirmed that we are dealing with a rocky planet from the group of so-called super-Earths. This planet is 1.7 times the size of Earth and has 5.6 times the mass.

The question of what form water takes on LHS 1140 b depends on the composition of the planet's atmosphere. This requires further research, but based on currently available data, some modelling has already been done.

One possibility is that the planet is covered in ice. Still, on parts of the surface most exposed to the star's radiation, it has a liquid ocean that could be about 4,000 kilometres, roughly half the size of the Atlantic Ocean.

Another possibility is liquid water hidden beneath the ice surface, similar to some moons such as Ganymede, Enceladus, or Europa, which orbit near Jupiter and Saturn.

Scientists note that the planet's density suggests the presence of large amounts of water. This could be as much as 10 to 20 percent of the planet's mass. For comparison, all the water in Earth's oceans is only about 0.02 percent of Earth's mass. Signs of nitrogen have also been observed in the atmosphere of LHS 1140 b, and this gas is the dominant component of Earth's atmosphere (up to 78 percent). The study authors believe that the most likely atmosphere is one composed of nitrogen with an addition of water and carbon dioxide.

Another planet is also known in the system where LHS 1140 b is located. LHS 1140 c orbits closer to the star, with an orbital period of about 4 days. This is a globe with a size comparable to Earth (1.9 Earth masses and 1.3 Earth radii), but it is much hotter there (about 150°C).

The latest publications analysing the chances that LHS 1140 b is an ocean world were published in June and July 2024. The principal author of the first is Mario Damiano from the Jet Propulsion Laboratory in Pasadena (USA), and the second is Charles Cadieux from the University of Montreal in Canada.