Extract from ABC News
A small planet that is as dense as pure iron has been discovered hurtling around a nearby star like a cannonball.
Key points:
- Astronomers are searching for small rocky planets that may resemble Earth, but these are hard to find
- Now a team has discovered planet smaller than Earth that zips around its star in less than eight hours
- The planet is so dense it is likely to be mainly made up of pure iron ,like Mercury
The rocky planet, which lies about 30 light-years away in the constellation of Vela, is about three quarters of the size of Earth and half as massive.
Dubbed GJ 367 b, the little world takes less than eight hours to orbit a red dwarf star half the size of our Sun.
It's the smallest fast-moving planet that we've got the precise measurements of, said Kristine Lam of the German Aerospace Center, who led the international team that made the discovery.
"This is a unique object with a short orbital period and a high density," Dr Lam said.
While it may not be the densest exoplanet ever discovered, her team's measurements, reported in the journal Science, indicate it is denser than any rocky planet in our Solar System.
Its high density suggests the planet is dominated by an iron-rich core, like Mercury.
Winks and wobbles give planet away
Small worlds that zip around their suns in less than 24 hours — known as ultra-short period planets — are very hard to detect.
"This discovery paves the way for future exoplanet scientists to find smaller and smaller planets, hopefully like something in our Solar System, or something completely different," Dr Lam said.
The first hint of this extraordinary planet came as dips in light around a star detected by NASA's Transiting Exoplanet Survey Satellite (TESS) in 2019.
Fleeting winks of light created by the planet passing in front of the star gave the scientists an indication of the planet's radius.
"To confirm this signal was a planet, we needed to go through a lot of rigorous ground-based follow up observations in order to check whether this was real," Dr Lam said.
The team used the High Accuracy Radial Velocity Planet Searcher (HARPS) in Chile to measure how much the star wobbled back and forth to work out the planet's mass.
The ability to measure the mass of such a small planet with such precision was an "incredible achievement," said Jonti Horner, an exoplanet hunter at the University of Southern Queensland who was not involved in the study.
"That we can measure this star that is 290 trillion kilometres away, and we can measure the speed with the precision of 80 centimetres per second to detect this planet, is astonishing," Professor Horner said.
"[The star's wobble] is almost slower than walking pace."
The team calculated the planet's density by combining the radius with the mass.
A bigger version of Mercury?
Professor Horner said the planet's discovery was a reminder of the cataclysmic forces that created our Solar System.
"You're probably looking at a bigger version of Mercury," he said.
About 80 per cent of the Solar System's smallest planet is taken up by a metallic core, making it the second densest planet after Earth.
The core of the newly discovered planet is estimated to take up a similar proportion to Mercury.
But with a density of about 8.106 grams per cubic centimetres, the planet is much denser than either Mercury or Earth.
It's unclear how such a dense planet formed, but like Mercury, it may have been hit by something else when it was young, which stripped off most of its outer layers.
"Mercury was initially twice as big it is today, and that collision stripped away the mantle and the crust, leaving just a core and a little bit of rubble on top," Professor Horner said.
Temperatures on Mercury can reach up to 178 degrees Celsius, but that's a walk in the park compared with the newly found planet.
Ripping around at speeds of about 240 kilometres per second, it is much closer to its red dwarf star than Mercury, which takes 88 days to go around our Sun.
"It's so close to the star that any atmosphere that it's got would have been stripped away," Professor Horner said.
The lack of atmosphere would contribute to GJ 367b's apparent density.
It also means temperatures on the planet are predicted to reach up to 1,500 degrees Celsius — hot enough to melt iron.
And it's probably locked in position with one side permanently facing its star, while the other is exposed to the vacuum of space and has no light coming in.
"You can imagine this little rocky world that is baked on one side and frozen on the other," Professor Horner said.
"Obviously, it's not somewhere you'd want to go on your holidays."
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