An artist concept of the gas giant exoplanet WD 1856 b orbiting its tiny, compact white dwarf star. The planet's diameter is seven times greater than the white dwarf, and orbits it once every 34 hours. (NASA/Goddard Space Flight Center)
A NASA satellite known as TESS may have discovered a massive extrasolar planet twice as old as any planet in our solar system. While astronomers need to conduct more observations to confirm the planet exists, this gas giant world, named WD 1856 b and located 80 light years away in the constellation Draco, appears to be 14 times more massive than Jupiter, and around 10 billion years old.
But WD 1856 b’s size and age are not what make it most fascinating to scientists.
The planet orbits a type of object called a white dwarf, the tiny and dense remnant core of a dead star, and that orbit is so close in that WD 1856 b completes it in a mere 34 hours (compared to 365 days for Earth to orbit our sun).
Scientists want to know how the planet escaped the tumultuous death throes of its star, a process that destroys all nearby planets. They are also puzzled by how it has survived the powerful gravitational forces of a white dwarf star at close range, which should tear it to shreds.
How Does a White Dwarf Star Form?
During its lifetime, the star that became this white dwarf was probably not unlike our sun in size and mass. It may also have nurtured its own family of planets.
But when a sun-sized star begins to run out of fuel, it goes through some drastic changes. Its core, sputtering on fumes and losing the power of nuclear fusion that kept it stable for billions of years, begins to collapse and heats up intensely as gravity squeezes it into a small, dense, compact object not much larger than a planet like Earth.
Meanwhile, the escalating temperature of the white-hot core inflates the outer layers of the star like a balloon. The gases cool as they spread out, and the once sun-like star becomes a “red giant,” hundreds of times larger and engulfing any planets orbiting too close. When our sun goes through this phase, some 5 to 7 billion years in the future, its outer layers will expand beyond the orbits of Mercury and Venus, and possibly even Earth.
As the red giant phase of the star’s demise draws to a close, its outer layers of gas blow off into space as an ever-expanding cloud, becoming a beautiful object called a planetary nebula.
This Planet Might Have Migrated
Scientists believe that WD 1856 b must have been farther away from its star to begin with, only moving to its present position after the star’s red giant phase was over.
The idea that planets and smaller bodies can migrate from one place to another within a star system can help explain details of a planet's composition that are not consistent with its current location —indicators that it formed in a different location under different conditions. The theory of planetary migration may solve planetary puzzles in our solar system as well as in other star systems.
With several possibilities to choose from, scientists think WD 1856 b moved to its current location through gravitational interactions between it and one or more other gas giant planets, which have yet to be discovered.
“Tidal force” is the difference in gravitational pull that one body exerts on the near and the far sides of another body.
Earth experiences “tides” because the gravitational pull of the moon (and, to a lesser degree, the sun) is stronger on the side of Earth facing the moon than on the far side, exerting a gentle stretching action that pulls at Earth and the ocean’s waters along that direction. At a quarter of a million miles, the moon’s modest gravity draws the ocean’s waters several feet above the norm. We experience this as the rise and fall of the tides, as Earth’s surface rotates through the swell.
With the equivalent mass of a sun-sized star packed into a ball only a bit larger than Earth, the white dwarf’s gravity grows immensely powerful at close range, and the tidal forces on any nearby planet become enormous. In the tidal tearing action of a white dwarf star,if a planet is too close, it will literally be stretched apart and shredded to pieces.
Observations of other white dwarf systems have sometimes revealed encircling disks of dust, possible evidence of planets that have wandered too close and been stretched to smithereens. The rings of Saturn, it is believed, were formed when a small moon got too close to Saturn and was broken apart by the tidal forces of the planet’s gravity.
However WD 1856 b is managing to keep it together, it seems that this “lucky” world has dodged two epic obliterations.
Get the best of KQED’s science coverage in your inbox weekly.