How does the non-sticky dust in a protoplanetary disc stick together? S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)

It’s not easy to make Earth. Most of the explanations for how our planet formed have troubling problems. But if a new idea is right, we can thank a hyperactive young sun for Earth’s existence, plus solve a long-standing mystery about Mars.

According to standard lore, the planet-building process began when dust particles orbiting the newborn sun stuck together, forming rocks that built still larger objects.

But this story is in trouble. “I’ve been really, really disturbed by the problem of making terrestrial planets,” says Alexander Hubbard at the American Museum of Natural History in New York.


These planets are the first four from the sun: Mercury, Venus, Earth and Mars. They’re mostly made of rock and iron – whose particles don’t readily stick together.

They could have been sticky enough if they had a coating of snow and organic goo, Hubbard says. But despite all Earth’s oceans and carbon-based life, our planet has too little water or carbon to support this explanation.

Now Hubbard has suggested an intriguing solution to Earth’s difficult birth. In 1936, an infant star began to brighten, eventually shining over 100 times more brightly than it did originally. Now named FU Orionis, this star has stayed bright ever since. And several other stellar youngsters have done the same thing.

Here comes the sun

What if the newborn sun also did this? The outburst would have partially melted dust grains, making them sticky enough to become the seeds of Mercury, Venus, Earth and Mars.

“You naturally expect a terrestrial planet pattern that looks a lot like our own solar system if you have an FU Orionis-type event,” Hubbard says.

Meanwhile, in solar systems that didn’t experience such an eruption, dust grains would only be molten closer to the star, leading to compact systems like Kepler-11, as other astronomers suggested in 2014.

“It’s an interesting idea,” says Andrew Youdin at the University of Arizona, noting the difficulty of explaining terrestrial planet formation. “There’s clearly a major problem here, and so all ideas need to be looked at.”

Hubbard’s model also explains diminutive Mars, which is only half the diameter of Earth.

“Mars is pretty darn small,” he says. Most scientists blame gravitational interference from giant Jupiter, an idea dating back to Prussian philosopher Immanuel Kant in 1755.

But Jupiter may be innocent. An FU Orionis outburst wouldn’t melt dust grains much beyond Earth’s orbit. At the Red Planet’s distance, Hubbard says, “the peak temperature didn’t get high enough long enough for the molten grains to grow big enough”.

As a result, Mars had little material to form from and so ended up only 11 per cent as massive as Earth, he says.

Journal reference: The Astrophysical Journal Letters, DOI: 10.3847/2041-8213/aa6dae