What infrared trail stretches from the Sun to Venus in Project Hail Mary?

The infrared trail stretching from the Sun to Venus is called the Petrova line.

In Project Hail Mary, scientists notice the line as the Sun begins losing energy at an alarming rate. The phenomenon is associated with astrophage, an alien microorganism that feeds on stellar energy and travels along the gravitational and electromagnetic environment between a star and a nearby planet.

The name honors Dr. Irina Petrova, the scientist credited with first observing the feature. The Petrova line becomes one of the key clues in the global investigation into the astrophage crisis and ultimately helps point humanity toward the Hail Mary mission.

Readers sometimes confuse the Petrova line with the astrophage itself. The line is the visible or detectable infrared phenomenon; astrophage is the life-form producing the crisis. The story later reveals that Tau Ceti also has a Petrova line, but its ecology provides a crucial difference.