Astronomers have found a planet around a red giant star that should have been destroyed, yet it still exists, leaving astronomers to wonder why the planet is still there. So what’s the puzzle?
Well, stars change as they age. Once a star has used up all its hydrogen fuel in the core, it begins to shrink, while the outer envelope expands as the star cools.
When our Sun enters this stage in about 5 billion years, it will grow so large that it will engulf Mercury, Venus, and possibly Earth. The Sun will become a Red Giant.
The planet in question is 8 Ursa Minoris b and was discovered in 2015 by Korean astronomers using the “Doppler wobble” technique, which measures the gravitational pull of the planet on the star. It is a red giant star 530 light years away in the constellation of Ursa Minor, the Little Bear. The star was named Baekdu and the planet was named Halla after the tallest mountains on the Korean peninsula.
Halla is very close, twice as close as the Earth is to the Sun and orbits Baekdu once every 93 days and is about 1.3 times the mass of Jupiter.
So what is going on here? How can a planet survive the expansion of its star into a Red Giant? How did it not get swallowed up being so close to it?
Astronomers think that one of two things is going on. Either Baekdu was one star in a binary system and the merger of these two stars may have prevented the expansion of either star to a size large enough to engulf planet Halla. Or Halla may be a relatively newborn planet. The violent collision between the two stars may have produced a cloud of gas and dust from which the planet could have formed. In other words, the planet Halla may be a recently born so-called “second generation” planet.
Nature always seems to have a way of surprising us, throwing us curve balls just when we think we know what’s going on. The discovery of a planet orbiting a helium-burning red giant star shows that exoplanets can appear in places where we didn’t think they could exist. When it comes to the 8 Ursa Minoris system, whether it’s due to the merger of two stars or the formation of a cloud of gas and dust from which the planet could have formed, or something else altogether, we can always count on nature to find a way to surprise us.