What makes a star a star? A strange ‘in-between’ celestial object is testing astronomers’ boundaries

By Md Redyan Ahmed, PhD Candidate in Astrophysics, University of Sydney

Around 1,350 light years away from Earth is a star called TOI-2155. It’s a little bigger, heavier and hotter than the Sun, and it’s not particularly interesting or unusual in itself.

But orbiting around TOI-2155 is something very interesting indeed: a much smaller object called TOI-2155b, which we only know about by observing the tiny changes in light from the host star when the smaller object passes in front of it.

What is TOI-2155b? A mini-star? A giant planet? Or something in between? I’m glad you asked.

As my collaborators and I write in

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