Lava world

A lava world is a rocky planet so close to its star that the rock on its dayside is molten. Most known examples circle their stars in less than a day, a few times their star's radius away, and are tidally locked, so one hemisphere faces the star permanently. The dayside can hold a sea of magma hundreds or thousands of kilometres across, under a thin atmosphere of vaporised rock, while the night side may be frozen solid. Lava worlds are the easiest small planets to study, because short orbits mean frequent transits and hot daysides shine brightly in infrared light, and their magma oceans and rock-vapour atmospheres test ideas about how planets gain and lose their lighter elements.

Characteristics

How hot is hot enough

On a tidally locked planet with no atmosphere to move heat, the hottest point is directly beneath the star, where the ground reaches the substellar temperature

$$T_\mathrm{ss} = T_\star \sqrt{\frac{R_\star}{a}}\,(1 - A_B)^{1/4} = \sqrt{2}\,(1 - A_B)^{1/4}\,T_\star\sqrt{\frac{R_\star}{2a}},$$

about 1.4 times the planet's equilibrium temperature for a dark surface. Dry mantle rock starts to melt at about 1,390 K at low pressure ^[1], so a planet whose substellar point is hotter than that holds at least a pool of magma. Around a Sun-like star that happens inside about 0.08 AU, an orbit of about eight days, although only a small pool beneath the star melts at that distance. Most known lava worlds are much closer, with equilibrium temperatures of 1,300 to 2,700 K and molten daysides.

Magma oceans and rock-vapour atmospheres

At these temperatures the melt itself evaporates. Chemical models show that a magma ocean at 2,000 to 3,000 K is covered by a thin atmosphere made from the rock: sodium, potassium, silicon monoxide, atomic and molecular oxygen, iron and magnesium, with sodium dominant at the cool end and silicon monoxide growing at the hot end ^[2]. Such an atmosphere absorbs starlight high up and should be warmer at altitude than below, a temperature inversion that would show up in its spectrum ^[3]. Gas is continually exchanged between the magma and the atmosphere, which slowly changes the composition of both ^[4].

Because the atmosphere exists only where the rock is hot enough to evaporate, it is thickest beneath the star and thins towards the terminator. The pressure difference drives winds from day to night, which carry rock vapour to cooler regions where it condenses and falls as mineral grains: "rock rain" ^[5]. Models of K2-141 b, one of the hottest known, predict supersonic winds of more than 5,000 km/h ^[6].

Dayside and nightside

With only a thin atmosphere, a lava world's night side receives little heat, and on K2-141 b the night side is too faint to detect ^[7]. 55 Cancri e is different. Spitzer found its hottest point shifted away from the substellar point and a night side at about 1,400 K, too warm for bare rock, which implies that something, an atmosphere or flowing lava, carries heat around the planet ^[8].

Formation and evolution

Planets probably do not form this close to their stars: there is too little material and it is too hot. Ultra-short-period planets are more likely to have formed a little farther out and been pulled inward, their orbits shrinking through tidal interaction with the star and gravitational nudges from sibling planets ^[9]. Most are smaller than about twice Earth's radius ^[10]. At this distance any hydrogen envelope a planet started with is stripped away by the star's radiation, leaving a bare rocky core, which is one explanation for the super-Earths below the Radius valley.

What happens next depends on the planet's size. A large lava world can outgas carbon, hydrogen and other volatiles from its magma and hold them as a secondary atmosphere. A small one can lose its rock outright. BD+05 4868Ab, a planet thought to be no more massive than Mercury on a 30.5-hour orbit, is disintegrating: the star's heat vaporises its surface and the vapour condenses into dust that streams away in tails ahead of and behind the planet ^[11].

How we know

Lava worlds are found by transits and weighed by radial velocity. CoRoT-7 b, found by the French-led CoRoT satellite and announced in 2009, was the first rocky planet with a measured radius and one of the first recognised as a probable lava world ^{[12] [13]}. Kepler-10 b, Kepler's first confirmed rocky planet, followed in 2011 ^[14]. The Kepler survey showed that planets with periods shorter than a day orbit about 0.5 per cent of G-type stars and about 0.8 per cent of K-type stars ^[10].

Their atmospheres are tested by their heat. As a hot planet passes behind its star, the drop in infrared light measures the dayside's temperature; comparing it with the bare-rock prediction reveals whether an atmosphere is carrying heat away. JWST has now done this in detail:

• 55 Cancri e. The dayside measures about 1,800 K, well below the roughly 2,500 K expected for bare rock. The spectrum rules out a thin rock-vapour atmosphere and points to a genuine volatile atmosphere, likely rich in carbon dioxide or carbon monoxide, outgassed from and sustained by a magma ocean ^{[15] [16]}. It is strong evidence rather than a confirmation. The planet's brightness also changes markedly from one observation to the next, perhaps with volcanic activity or clouds ^[17].
• TOI-561 b. An ultra-hot super-Earth around an old, metal-poor star, its dayside is far cooler than the roughly 3,000 K expected for bare rock, which the authors read as a thick volatile envelope on a planet that should have lost one ^[18].

Notable examples

Kepler-78 b's two mass measurements, made independently by two teams with two telescopes and published together, agree within their uncertainties.

See also

• Barren rock world
• Volcanic world
• Super-Earth
• Radius valley
• Exotic worlds
• Atmospheric escape
• JWST and rocky exoplanet atmospheres
• Planet classification

References

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