Not two weeks ago, astronomers in Chile used a powerful, high-resolution camera to discover an asteroid that circles around the sun faster than any other. The orbit of the approximately 0.6-mile-wide (1 km) object, dubbed 2021 PH27, brings it a searing 12 million miles (20 million km) from our central star every 113 days.
Only Mercury has a shorter orbital period (88 days) and average distance. But it comes no closer than 29 million miles (47 million km) to the sun, whereas 2021 PH27 better than halves that distance. The surface sizzles around 930° F (500° C) — hot enough to melt lead and zinc. And it's so deeply embedded in the sun's intense gravitational field, warped space twirls its orbital path. More on that in a moment.
Scott S. Sheppard of the Carnegie Institution of Science discovered 2021 PH27 in data collected by the Dark Energy Camera (DECam) on the 4-meter (157-inch) Blanco Telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile. The DECam is normally used to study massive galaxy clusters, but Sheppard teamed up with his colleagues to take a break from the big stuff millions of light-years away to search for small fry closer to home.
Mercury and Venus, the two innermost planets, never appear very far from the sun because they orbit it closely. That's why they're only visible during morning or evening twilight. Similarly, if you're going to hunt for asteroids interior to Earth's orbit, twilight's the time to do it. Following 2021 PH27’s discovery at dusk on August 13, another astronomer at the University of Hawaii measured the asteroid's position and predicted where to look for it the following evening. On the 14th, astronomers used the DECam to find it again. More observatories pitched in with additional observations on August 15.
While 2021 PH27 crosses the orbits of both Mercury and Venus, scientists think that it originated from the main asteroid belt between Mars and Jupiter. Gravitational interactions with the inner planets may have altered its orbit and clawed it closer to the sun. The asteroid may also be an extinct comet. Like many comets its orbit is steeply tilted to the relatively "level" plane of the planets.
If that's the case it would have started life in the outer solar system as an icy object until it passed near one of the planets during one of its runs around the sun. Their mutual gravitational interaction could have reshaped 2021 PH27 's original orbit into the tight loop of the present day. Astronomers hope that further observations will reveal more about its origin and true nature.
Either ways, astronomers have reason to believe its orbit is unstable. If the object doesn't collide with Mercury, Venus or the sun in the next few million years, the combined gravitational influence of the inner planets may eject it from the solar system altogether. Hey, wait a minute. Are we all just trying to make a living here?
There may be asteroids even closer to the sun, but the solar glare makes them difficult to find. When asteroids get so close to our nearest star, they experience stress both from solar heating and the physical kneading of gravitational forces. A loosely-bound "rock pile" asteroid would be expected to break up over time under these conditions. Solid objects should hold together longer. What will be the fate of 2021 PH27 I wonder?
Because 2021 PH27 is so close to the sun’s powerful gravity its orbit, like Mercury's, pivots over time, an effect called precession.. The closer you get to the sun the more warped the space becomes around it. You can't see this with your eyes, but planets and asteroids "feel" this curvature and respond to it by orbiting the sun more rapidly.
Like the depression made by standing in the center of a trampoline, the sun's gravity creates a deep dimple in the fabric of 4-dimensional spacetime. The severe warping causes 2021 PH27 's orbit to corkscrew around the sun at the rate of 1/30th the apparent diameter of the full moon per century — even more than Mercury.
Right now, 2021 PH27 is difficult to observe, but when it returns to view in early 2022, astronomers will determine a better orbit and gather more data, which will allow it to get an official name.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.