If some stars that are lightyears away are dead, does that mean the exoplanets we find are also dead?

Question

I get really excited over exoplanets that are Earth-like so I’m wondering how this works.

In: Planetary Science

Answer 1

“Dead” has different meanings when talking about “dead stars” and “dead planets”. Dead planets are those planets which cannot sustain life – but I think you already understand this, as you mentioned Earth-like exoplanets.

A dead star, on the other hand, has nothing to do with sustaining life. It is a star that has burnt through its supply of fuel. Like our sun, all stars burn hydrogen – but when that runs out, the star swells and collapses in on itself, before exploding in one final, bright burst, called a supernova. This then collapses, leaving either a neutron star (the remnants of the dead star, compressed into a very small package), or – if the star was very big – a black hole.

So, why can we still see some dead stars today? Because the light from those stars has to travel vast distances across space to reach us. Light travels at just under 300,000 km per second – but those distances in space between stars and us are much, much greater. The distance light travels in one Earth year (a “light year”) is about 9.461 trillion km. The nearest star to us (apart from the sun) – Proxima centauri – is about 4.25 light years away. The light from our *nearest* star takes 4.25 years to reach us. Most of the stars we can see are tens or hundreds of light years away.

So, let’s say there’s a star in the sky that died 100 years ago, and let’s say that star is (was) 150 light years away from us. When we see that star in the sky, we are seeing that star 150 years ago. Even though that star died 100 years ago, we will still see it in the sky until the last of its light reaches us, in another 50 years.