Systems in the early stages of formation experience the greatest number of impacts due to the presence of a huge number of embryos in unstable orbits. Will we be able to consider these processes and reveal the past of the Earth?
In the final stages of planet formation, young planetary embryos crash into other protoplanets, causing their surfaces and mantles to melt intensively. One such collision between the future Earth and Theia that struck it created the Earth-Moon system and led to the emergence of the magma ocean: a mixture of molten silicates and volatiles that extends throughout the mantle. Oceans of magma set the stage for the early surface and atmosphere on which life conditions eventually developed.
The collision of the newborn Earth and Theia (an object the size of Mars), which caused the formation of the Moon.
Unfortunately for geophysicists, but fortunately for life in general, several billion years of plate tectonics on Earth have destroyed the clear signs of an ocean of magma, and so scientists hardly understand how this hot and molten world turned into a habitable planet. However, it is believed that the general principles of the formation of rocky planets are similar in the systems of other stars, and, therefore, the most powerful impacts are not rare on the planets that are currently forming in the orbits of young stars.
This makes it possible to capture a snapshot of the afterglow from giant impacts in exoplanetary systems. Direct detection of a molten protoplanet will be the key to the early stages of planetary evolution.
The hunt for molten worlds
Young protoplanets are very hot and bright, as their surface temperatures can reach 3000 ° C. Thus, one might think that they are easy to spot in the night sky, but unfortunately this is not entirely true. In fact, as the molten mantle solidifies, the dissolved volatiles such as water and carbon dioxide are gradually released into the atmosphere. In the absence of strong stellar winds or high levels of ultraviolet radiation from the star, the planet's atmosphere will thicken, thereby obscuring the surface. In doing so, it will act like a blanket, prolonging the cooling period of the magma ocean.
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An artistic representation of an exoplanet covered in oceans of magma.
While the existence of magma oceans has been suggested by theoretical models of planetary formation, global melting of bodies as a result of collisions between protoplanets has not yet been observed. Since the number of such impacts is expected to decrease gradually over time, young planetary systems offer the best chances for detecting such objects.
However, to be visible, these molten bodies must satisfy two conditions. First, be not too close to their star, otherwise the telescope will not be able to separate the molten protoplanet from its bright host. Second, a sufficient amount of radiation from the magma ocean must penetrate the atmosphere.
In terms of emitted radiation, molten protoplanets are an attractive target for direct imaging because they are much brighter than older planets like Earth. So, if we ever want to start collecting immediate photographs of Earth-like extrasolar planets, then molten protoplanets is a good place to start!
What are the chances of detecting afterglow?
Unfortunately, even with the most advanced imaging tools, direct detection of molten planets remains out of reach. However, the 2020s will see the era of colossal ground-based telescopes: ESO's Extremely Large Telescope (ELT) in Chile, the Giant Magellanic Telescope (GMT) in Chile, and the Thirty Meter Telescope (TMT) in Hawaii. In addition to new ground-based observatories, future space missions are being considered for direct imaging of rocky planets in habitable zones of sun-like stars, in particular the LIFE (Large Interferometer for Exoplanet) interferometer, which promises unprecedented accuracy in characterizing extrasolar planets.
Artistic representation of ESO's Extremely Large Telescope.
The likelihood of seeing a molten planet depends on two main factors: the cumulative number of giant impacts experienced by objects in the planetary system and the time interval during which the molten body remains hot enough to be detected.
To determine the likelihood of observing molten protoplanets, you first need to establish the likelihood of giant impacts by simulating planetary formation. Computer simulations track the evolution of the orbit and the growth of planetary embryos as they merge into full-fledged planets during collisions.
Systems in the early stages of formation experience the greatest number of impacts due to the presence of a huge number of embryos in unstable orbits. That being said, those orbiting red dwarfs, the most common stars in the Milky Way, will be hit almost twice as many times as those around our Sun's counterparts. This is very promising regarding the likelihood of magma oceans occurring, but there is a caveat: the protoplanets in such systems will be located in close orbits and therefore cannot be separated from the star's radiation. In addition, the collisions will be less energetic and therefore the bodies will be dull. Thus, potential observability becomes a function of the star's age, number of impacts, and collision energy.
Given the frequency of occurrence of the magma ocean, scientists calculated the evolution and period of existence of magma oceans to determine changes in surface temperature depending on the size of the planet and the thickness of its atmosphere, which is expressed in the so-called emissivity: the lower it is, the more insulating the atmosphere is.
An artistic representation of a young exoplanet being bombarded constantly by embryos in unstable orbits.
Large protoplanets with a thick atmosphere will support oceans of magma longer, but they will also exhibit lower radiation and are more likely to be below the sensitivity level of telescopes. It is important to note that the probable composition of exoprotoplanets may differ significantly from the early planets of the solar system. Thus, the emissivity depends on an additional parameter: a variety of compositions and masses of exoplanetary atmospheres.
Naturally, the best place to start looking for molten planets with ELT or LIFE is determined by the proximity to the solar system. The most promising targets are young, nearby and massive stellar groups. Imagine that scientists already have a "suitable" telescope and must view all the individual stars in an association. Will a molten protoplanet be found? Neither yes nor no. The answer is statistical probability, depending on a number of physical parameters.
Panoramic shot of the Carina OB1 association, which contains several groups of young stars, such as the Trumpler 14 cluster, which is home to about 2,000 stars. The systems closest to us, like this one, are the main targets for detecting collisions of protoplanets.
For example, the association β Pictoris (Beta Pictoris), located 63 light years from the Sun, includes 31 stars with an average age of 23 million years. The probability of detecting at least one planet with an ocean of magma among their planetary systems will be negligible with an insensitive filter, but can reach 80% for observations with LIFE at 5.6 micrometers or with ELT at 2.2 micrometers.
What do these numbers mean and what to do next?
A number of questions remain. For example, it is still unclear whether planets are born around all stars and what types of planets should be expected depending on the class of the star.
Earlier studies, which discussed the potential observability of molten planets, wondered if the afterglow of a giant impact, similar to the one that created the Moon, could be recorded under proto-Earth conditions. Nevertheless, a survey of exoplanets in recent decades has shown that many of their characteristics (composition, mass, radius, orbit, and others) are wildly different from everything that was assumed as a result of studying the solar system. Therefore, scientists expect huge differences between the compositional properties of young protoplanets and their atmospheres, that is, the question of the potential observability of the forming proto-Earth is interesting, but not important due to the insignificant probability of the presence of such protoplanets in the foreseeable vicinity of the Sun.
Thousands of star systems living in the Milky Way.
To get closer to detecting a molten protoplanet in the next few years, several key questions need to be addressed: what are the typical variations in the atmospheres of rocky planets, how are volatiles distributed between the mantle and atmosphere?
Observational campaigns will enable scientists to improve their understanding of atmospheric properties and compositional distributions. In addition, it will be necessary to better limit the characteristics of individual member stars of the most promising associations: β Pictoris, Columba, TW Hydrae, and Tucana-Horologium. This requires the joint efforts of theorists and observers, astronomers, geophysicists and geochemists.
Eventually, sometime in the not too distant future, we may be able to see a glimpse of a glowing young world that may not be all that different from our own home in the universe.
Arina Vasilieva