Mercury Is No Longer The Closest World To The Sun

Astronomers in Chile used the powerful 570-megapixel Dark Energy Camera (DECam) to find an asteroid with the shortest orbital period of any known asteroid in the Solar System. They did this just ten days ago. Every 113 days, the asteroid, which is about 1 kilometer across, comes as close to the Sun as 20 million kilometers (12 million miles or 0.13 au).

Asteroid 2021 PH27 has the shortest period and the shortest semi-major axis of any known asteroid in our Solar System. Only Mercury has a shorter period and a shorter semi-major axis. Since the asteroid is so close to the Sun, it feels the strongest effects of general relativity of any known object in the Solar System.

Scott S. Sheppard of the Carnegie Institution of Science found the asteroid 2021 PH27 by looking at data from the Dark Energy Camera (DECam) on the Vctor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile. Ian Dell’antonio and Shenming Fu of Brown University took pictures of the asteroid in the evening sky on August 13, 2021, as it was getting dark.

Sheppard worked with Dell’antonio and Fu while using DECam to make observations for the Local Volume Complete Cluster Survey. This survey is looking at most of the large galaxy clusters in the nearby universe. They took a break from looking at some of the biggest objects millions of light-years away to look for asteroids much closer to Earth.

DECam is one of the best wide-field CCD imagers in the world. It was made for the DOE-funded Dark Energy Survey (DES), built and tested at the DOE’s Fermilab, and ran by the DOE and NSF from 2013 to 2019. At the moment, DECam is used for a wide range of science programs. The Community Science and Data Center is in charge of taking care of the DECam science archive (CSDC). The NOIRLab at NSF runs the CTIO and CSDC programs.

The best time to look for asteroids that are inside Earth’s orbit, in the direction of Mercury and Venus, is at twilight, just after sunset or just before sunrise. Any stargazer will tell you that Mercury and Venus never seem to get very far from the Sun in the sky and are always easiest to see near sunrise or sunset. Asteroids that orbit close to the Sun are the same.

After 2021 PH27 was found, David Tholen of the University of Hawai’i measured its position and figured out where it could be seen the next night. After that, on August 14, 2021, DECam and the Magellan Telescopes at the Las Campanas Observatory in Chile both looked at it again. Then, on the evening of the 15th, Marco Micheli of the European Space Agency used the Las Cumbres Observatory network of 1- to 2-meter telescopes to look at it from CTIO in Chile and from South Africa. Other astronomers used DECam and Magellan to look at the newly found asteroid as well.

Sheppard says that even though astronomers’ telescope time is very valuable, the international nature of their work and their love of the unknown make them very willing to put their own science and observations on hold to follow up on new, interesting discoveries like this one.

Planets and asteroids move in oval-shaped orbits around the Sun. The widest part of the oval is called the semi-major axis, and it has a radius. The semi-major axis of 2021 PH27 is 70 million kilometers (43 million miles or 0.46 au), which gives it a 113-day orbital period on an elongated orbit that crosses both Mercury and Venus’ orbits.

It may have started out in the main Asteroid Belt between Mars and Jupiter but moved closer to the Sun because of gravitational disturbances from the inner planets. Its high orbital inclination of 32 degrees, on the other hand, suggests that it might be an extinct comet from the outer Solar System that was pulled into a closer, shorter-period orbit by one of the terrestrial planets as it passed by. As more observations are made of the asteroid, we will learn more about where it came from.

Its orbit is probably also unstable over long periods of time. In a few million years, it will probably either crash into Mercury, Venus, or the Sun, or it will be pushed out of the inner Solar System by the gravity of the inner planets.

Astronomers have a hard time finding asteroids in the inner solar system because the Sun’s light often hides them. When asteroids get so close to the Sun, they are put under a lot of stress, including thermal stress from the Sun’s heat and physical stress from the Sun’s gravitational tides. Some of the asteroids that are not as strong could break up because of these stresses.

“The number of asteroids that are close to Earth and Venus compared to those that are far away will tell us about their strength and make-up,” says Sheppard. If the number of asteroids in the same orbit as 2021 PH27 seems to be decreasing, it could tell astronomers how many near-Earth asteroids are loosely held together piles of rubble instead of solid chunks of rock. This could affect how we deal with asteroids that might be headed for Earth and how we try to stop them.

“Understanding the population of asteroids inside Earth’s orbit is important to finish the census of asteroids close to Earth,” says Sheppard. “Some of the most likely Earth-impacting asteroids may come close to Earth during the day, but they are hard to find in most surveys because they are done at night, when the Sun isn’t out.” He also says that since 2021 PH27 is so close to the Sun, “its surface temperature gets to almost 500 degrees C (around 900 degrees F) at closest approach, hot enough to melt lead.”

Because 2021 PH27 is so close to the Sun, its effects from general relativity are the strongest of any known Solar System object. This reveals itself as a slight angular deviation in the asteroid’s elliptical orbit over time, a movement called precession, which amounts to about one arcminute per century.

The asteroid is now moving behind the Sun, which we can see from where we are. This is called solar conjunction. It is expected to return to visibility from Earth early in 2022, when new observations will be able to determine its orbit in more detail, allowing the asteroid to get an official name.


  1. It is wrong to say planets have an "oval" orbit (para.9). Planetary orbits are an ellipse. So, this is just wrong. It is also bad to refer to "worlds" - which technically doesn't mean anything. Planets and asteroids are not the same thing and the use of "worlds" will not equate them. Thus, the presence of an asteroid does NOT constitute a "world" closer to the Sun than Mercury. Asteroids come in all sizes and 1 Ceres is still the largest at about 600 miles across while Mercury is about 3000 miles across. I expect better science from The Space Academy.... instead of this nonsense.

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