Second-generation planets

Betelgeuse, a red giant

Theories regarding planet formation have evolved over time. In the early decades of the 20th century, it was assumed that the planets of the solar system were formed when a passing star came close to the Sun and, through gravitational attraction, stripped away material that subsequently coalesced into planets. Given that such close encounters between stars must be extremely rare, this theory implied that the existence of planets around a star would be a highly uncommon phenomenon.

Current theory differs. It is now believed that when a cloud of gas and dust contracts to form a star, a portion of the cloud remains outside the protostar, forming a disk that flattens around the system's equator. Gradually, the gas and dust concentrate and condense around larger nuclei, eventually giving rise to planets. This theory explains why nearly all planets in the solar system orbit the Sun in roughly the same plane. It suggests that the existence of planets should be commonplace; indeed, the discovery of thousands of planets around nearby stars lends strong support to this theory.

In a 2010 article published on arXiv, Hagai B. Perets proposed a new theory regarding the formation of a specific type of planet—what might be termed second-generation planets. When a Sun-like star reaches the end of its life, it transforms into a red giant; upon subsequent collapse, it becomes a white dwarf—a star characterized by a very hot surface and extremely high density, ranging from several thousand to a million times the density of water.

When the Sun reaches this stage, its volume will be roughly equal to that of Earth. And when it passes through the red giant phase, it will swell so much that it will likely swallow Mercury, Venus, and Earth. These planets will become part of its mass, enriching the Sun with silicon and iron. The planets of the outer solar system—Jupiter, Saturn, Uranus, and Neptune—would remain unaffected due to their great distance, though their orbits might shift, bringing them closer to or moving them further away from the white dwarf star in which the Sun will eventually be converted.

Sirio A & B (a white dwarf)

Perets suggests that when one star in a binary system evolves into a red giant, some of its mass could be drawn toward its companion star. This material could form a new disk from which—alongside any pre-existing planets—a new planetary system might emerge. Some of these would be second-generation planets, or what some prefer to call phoenix planets, as they would be born from the ashes of their star.

It appears that a candidate for such a phoenix planet—or second-generation planet—has now been discovered. A recent Science News article reports the detection of an exoplanet candidate orbiting a white dwarf star named HS0209+0832. If confirmed, it would be the first exoplanet of this kind.

Analysis of the white dwarf's spectrum reveals an unusually high concentration of the element niobium; this is the most niobium-rich star known to date. Those who discovered the phenomenon link it to the possible presence of a second-generation planet. Furthermore, the TESS space telescope—designed to search for exoplanets—has detected fluctuations in the white dwarf's brightness with a 4.4-day period, which could indicate the presence of a giant planet located 6 million kilometers from the star. This could be a sort of niobium-coated Jupiter. Given the planet's proximity, its atmosphere, possibly containing niobium, could have been partially absorbed by the star.

The same post in Spanish

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Manuel Alfonseca