The end of a world

Graphic: Bradly Wohlgemuth

We know that in about five billion years our sun will expand and destroy Earth. The question is, do we know what kind of changes Earth will go through in the time it takes for that to happen?

A team of 11 scientists, working in Spain at the German-Spanish Astronomical Center, collected data on the newly discovered Jupiter-sized planet, Kepler-91b, which may offer some insight into the process of planets being engulfed by their host stars. The planet is predicted to be engulfed by its star in less than 55 million years, a mere blink on astronomical timescales. Since the planet is so close to its demise, it provides a good basis of study to learn more about planets approaching their death.

Kepler-91b orbits the star Kepler-91, located 3,360 light years from Earth. This star is a red giant, and has a mass just under one and a half times that of our sun. The radius, however, is about six times larger. When Kepler-91 was first discovered, it was initially thought to be the second sun in a binary star system, as a second signature was detected in close proximity to the star. Upon further investigation, this second signature was determined to be the star’s closest planet, Kepler-91b.

So far, there have not been many planets discovered orbiting stars in the red giant category or higher. One of the reasons that this discovery holds great value and importance is because the “evolution of planetary systems is intimately linked to the evolution of their host star,” according to the paper discussing Kepler-91b, published in the journal Astronomy & Astrophysics.

Because of the dearth of statistical data for planets orbiting stars within the giant category (like Kepler-91), “the question of how planets die remains open,” the paper explains.

It is thought that the planet is in its final stages before being engulfed by its star, as its orbital radius is approximately 11 million km. By comparison, Mercury (the planet closest to our star) is located 58 million km away from the sun. Earth is 150 million km away from the sun.

Kepler-91b takes approximately 6.25 Earth days to orbit its star; Mercury takes 88 days. Since Kepler-91b is remarkably close to its star, it is believed that more than 36 per cent of the planet’s sky is covered by the star during the day (compared to 0.0005 per cent of the Earth sky being covered by the sun), and that the star illuminates approximately 70 per cent of the planet’s atmosphere.

Using Kepler-91b as an observational model, scientists can predict how the Earth may respond to the expansion of our sun in the future.