2021 PH27: Closest Sun Asteroid

2021 PH27, the asteroid closest to the Sun in the entire Solar System, has been discovered, with a revolution period of only 113 days

Our star has a new neighbor that was hiding within its brilliance. Asteroid 2021 PH27 is the closest object to the Sun in the entire Solar System and orbits our star in just 113 days. Its orbital period is the shortest known for an asteroid and the second shortest after Mercury. This discovery was made by Dr. Scott S. Sheppard of Carnegie in the images of the evening twilight.

2021 PH27 has a size of about 1 kilometer and is on an unstable orbit that intersects with those of Mercury and Venus. This means that inevitably, within a few million years, it will either be destroyed in a collision with one of these planets or with the Sun, or it will be ejected from its current position.

Asteroid 2021 PH27

Studying these objects can help understand the origins of asteroids and the forces shaping our Solar System. “Probably, 2021 PH27 was moved from the main asteroid belt between Jupiter and Mars. Gravity of the inner planets then shaped its orbit into its current configuration” Sheppard said. “Based on its inclination angle of 32 degrees, it’s possible that 2021 PH27 is an extinct comet that ventured too close to one of the planets, as the path of its journey brought it close to the inner solar system”.

2021 PH27 is near the massive gravitational field of the Sun, thus experiencing the general relativistic effects more than any other known object in the Solar System. This effect is evident from a slight angular deviation in its elliptical orbit. The movement called precession occurs at about one arcminute per century. The observation of Mercury’s precession puzzled scientists for years. Only Einstein’s General Theory of Relativity explained the orbital adjustments over time. The precession of 2021 PH27 is even faster than that of Mercury. “2021 PH27 gets so close to the Sun that its surface temperature reaches about 500°C at perihelion, hot enough to melt lead” Sheppard said.

Future observations of this object will shed more light on its origins. Comparing 2021 PH27 with objects orbiting beyond Earth will enhance knowledge of its composition and the materials that allow it to survive in these extreme conditions. The asteroid 2021 PH27 undergoes significant thermal and internal stresses due to its proximity to the Sun.

A GIF showing the discovery of the asteroid 2021 PH27, which Scott S. Sheppard of Carnegie found in the evening twilight images taken by Ian Dell'Antonio and Shenming Fu of Brown University.
A GIF showing the discovery of the asteroid 2021 PH27, which Scott S. Sheppard of Carnegie found in the evening twilight images taken by Ian Dell’Antonio and Shenming Fu of Brown University.

Twilight Photos

The only method to detect asteroids in the inner Solar System is to capture images during sunrise or sunset, in other words, during twilight. Researchers Ian Dell’Antonio and Shenming Fu used the Dark Energy Camera on the Blanco telescope of the National Science Foundation (NSF) in Chile. Their primary research involves observing the majority of massive galaxy clusters in the nearby universe. In collaboration with Sheppard, they transitioned from focusing on some of the most distant objects in the universe to the closest ones. The collected images allowed researchers to discover 2021 PH27, the asteroid closest to the Sun, just a few hours later.

“Since the object was in the glare of the Sun and moving toward it, it was imperative to determine the orbit before it was lost behind our star” explained Dave Tholen of the University of Hawaii. “I hypothesized that for an asteroid of this size to remain hidden for so long, it must have an orbit that keeps it so close to the Sun that it’s difficult to detect”.

Additional images were obtained the following night using the Magellan telescopes at the Las Campanas Observatory of Carnegie in Chile. However, a third night of observations was necessary to determine the orbit of the new asteroid before it was lost. Adverse atmospheric conditions in Chile suggested the use of the Las Cumbres Observatory in South Africa. The asteroid is now in solar conjunction, behind the Sun. It will become visible again in early 2023, when new observations will determine its orbit, allowing the asteroid to receive an official name

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