Unraveling the Mystery of White Dwarfs
A white dwarf is the vestige of a once grand main-sequence star, akin to our Sun. When a star of similar mass to our Sun concludes its main sequence phase, it inflates into a red giant. As the red giant matures and depletes its nuclear fuel, it sheds its outer layers, transforming into a planetary nebula – a radiant canopy of expanding ionized gas that graces many Hubble images.
After approximately 10,000 years, the planetary nebula disintegrates, leaving behind a solitary white dwarf – a symbol of its once vibrant glory. These white dwarfs, though dense and massive, are about the size of Earth. They radiate residual heat from past nuclear fusion events, and despite their proximity to the star, they may harbor habitable zones.
It's hypothesized that most of these stars possess planets. However, these planets face peril as they orbit a star transitioning from the main sequence to a red giant. This transformation can ravage the planets, consuming some and annihilating others due to the tidal lock that forces them to perpetually face the star.
Some white dwarfs are encircled by debris disks, believed to be remnants of the star's planets, disintegrated during the red dwarf phase. In 2020, scientists revealed the discovery of an intact planet nestled within the habitable zone of the debris disk around the white dwarf WD1054-226. The existence of one suggests the likelihood of others. But why haven't we discovered them yet? And does the first discovery being a gas giant like Jupiter imply that exoplanets orbiting white dwarfs are predominantly gas giants?
The Peculiarity of Surviving Exoplanets
A recent study scrutinized the exoplanets orbiting white dwarfs and questioned the apparent scarcity of rocky exoplanets. White dwarfs, despite their smaller habitable zones compared to stars like our Sun, are long-lived and stable, theoretically capable of supporting life on their planets.
The only known intact planet orbiting a white dwarf was detected by NASA's TESS space telescope and boasts a colossal mass of 13.8 Jovian masses. Given the relative paucity of giant planets compared to Earth, this discovery appears somewhat surprising.
A browse through NASA's catalog of exoplanets reveals 5,535 confirmed exoplanets. Among these, 1898 resemble Neptune, 1756 are gas giants, 1675 are Super-Earths, and a mere 199 are terrestrial. Research suggests an abundance of smaller radius planets, making the discovery of a larger radius planet around a white dwarf intriguing. The cosmic dance continues, and with it, our quest to understand the enigma of white dwarfs and their exoplanets.
We have not seen everything
But our measured numbers do not reflect what is really out there. We only know what has been observed but we still know precisely everything that can be found outside the Solar System. According to some theories, Jupiter-sized planets represent the minority of the planetary population. Therefore, the fact that the first transiting planet detected around a WD turned out to be a giant planet is incredible. WD 1856 b may be the only confirmed white dwarf planet, but there are other candidates, and most of them are also planets of Jupiter's mass or greater.
There is ample evidence for the existence of small terrestrial planets around white dwarfs. But the evidence is in the rocky debris disks of destroyed terrestrial planets. This indicates that these planets are out there, but the question then becomes: are there intact ones in habitable zones? Does the detection of WD 1856 b tell us anything about the existence of terrestrial WD planets?
A matter of radius
There are two ways to reconcile the evidence for the existence of small planets with the discovery of WD 1856 b. First, there is no absolute reason why small rocky or massive planets of mass equal to or greater than Jupiter should dominate the population of white dwarf exoplanets. Perhaps the distribution reverses at a certain radius, representing the most unlikely planetary radius, and then peaks. Or there could be an infinite number of distributions: we just don't know yet.
The other way to reconcile it is simple. A second possibility is that WD 1856 b was a fluke. Perhaps there really is a "bottom-heavy" distribution and it was really highly unlikely that an exoplanet the size of WD 1856 b would be the first to be revealed in transit. This is the challenge of working with only one dataset.
The scientists in the study calculated that the probability that the first white dwarf exoplanet was a massive planet was 0.37 percent. This is extremely rare, but this does not necessarily lead to reliable conclusions. Where does this take us? We have detected a single planet around a white dwarf and it is a massive gas giant, but we have multiple rocky debris disks around them that must be from terrestrial planets. So why conclude that small rocky planets around them are rare?
Don't give up the search
As is often the case, we need more data, and it would certainly be premature to stop ongoing and future efforts to search for terrestrial planets around white dwarfs. The science of exoplanets around white dwarfs is only in its infancy and holds out hope of finding them because these stars are stable and long-lived. And so are their habitable zones.
White dwarfs are unique among stars because their radius is the same as Earth's. They are smaller than other stars, which could facilitate the detection of Earth-sized planets. It could also facilitate atmospheric study, including the potential detection of biosignatures that may be more difficult around much larger stars.
The hypothesis that terrestrial planets are rare around white dwarfs is easily verified. A focused search will undoubtedly begin to reveal the true population of these planets. If we found more Earth-like worlds around white dwarfs, this would open another path to habitability and a greater potential for the survival of life in the Universe.
Source: Universe Today, Arxiv
Post a Comment