It could rain liquid metal on lava planets – just like on future Earth

Head of Tartu Observatory's Space Technology Department Mihkel Pajusalu and his colleagues from the Massachusetts Institute of Technology (MIT) have come to the conclusion that the clouds of lava planets are probably made of metal, and instead of rain or snow, there could be showers of liquid metal. The research helps to undestand both the formation and future of the Earth, because as the Sun turns into a red qiant, our planet could also be covered in lava ocens.

By now, more than 4000 exoplanets have been found, some of them lava planets – terrestial planets which orbit their stars so close that their surface temperature could rise to thousands of degrees. This means that the rocks on their surface have to be in molten state.

These planets are called lava planets because they are most likely covered in lava oceans. Examples of lava planets are 55 Cancri e, Kepler-10b and Kepler-78b. Similar planets can also be spotted in sci-fi movies, like Mustafar in Star Wars (mostly known from episodes II and III). However, the surface temperature was notably underestimated in the movie: in real life, no human could survive on a lava planet.

Metal showers

Currently, these planets are detected with quite indirect methods, which is why it is hard to get an exact overview of what is happening on their surface. Mainly, the so-called eclipse method is used, which is based on the change in the visible brightness of the star. Brightness changes when the planet is located between the star and the observer, or when light can't reach Earth because the planet is located behind the star.

It has been found that many lava-ocean planets reflect a lot of their stars' light (albedo higher than 40%), but the reason behind it has remained unknown. Up until now, one of the hypotheses has been that the brightness is caused by the reflectiveness of either the lava or the glass it turns into after solidifying. A team including Mihkel Pajusalu, Tartu Observatory's Senior Research Fellow and Head of the Department of Space Technology, also went on a journey to try and find an answer to that question. Pajusalu worked with Professor Sara Seager and Doctoral Student Zahra Essack from the MIT.

In the course of the research, they melted different rocks to evaluate the optical behaviour of lava, and investigated the reflective properties of the glass it formed after cooling. After measuring the reflectance values at different angles, they implemented the data to calculate the reflectance values of planets covered with certain materials.

They concluded that the surface of the planets cannot be reflective enough by itself (albedo lower than 10%). This means that the high reflectance value has to be caused by the clouds. With temperatures as high as on these planets (2000–3000 K, 1700–2700 °C), the clouds could even consist of mist or drops of metal, because metals start boiling in such temperatures. Further, it means that the atmospheres of these kind of planets must be made of substances with a very high boiling point, and there are probably showers of liquid metal instead of the rain or snow we are used to.
 

Earth could become a lava planet as well

The surface of a planet can be covered in lava even if it doesn't orbit its star extremely close. For example, the Earth had to be in a liquid state right after being formed, because a lot of warmth was released as the particles collided. The most popular formation theory of the Moon also implies a huge collision which would have left a major part of the Earth's surface in a molten state. Researcing current lava planets can therefore help us understand the formulation of planets in general as well as the past and future of our planet. According to what we know now, in billions of years the Sun will enter the red giant phase of its evolution, turning the surface of the Earth into lava oceans again.

Estonia is also participating in the ARIEL space mission with Tartu Observatory, University of Tartu as a partner. The mission goal is to create a space telescope which will enable to observe planets close to the stars they orbit. Lava planets will most likely be among the subjects.

More about the topic

• Research article "Low-albedo Surfaces of Lava Worlds" by Pajusalu and his colleagues; published in The Astronomical Journal
• News story by the MIT
• Podcast with Pajusalu as the guest, and the news story written based on it