A red-orange star with dark circles in front
The planetary system depicted here, Kepler-444, formed more than 6 billion years before our Sun was born. Credit: NASA, Tiago Campante/Peter Devine

The first stars were made of hydrogen and helium. That hasn’t really changed, but each subsequent generation of stars has a bigger fraction of heavy elements like carbon, oxygen, silicon, and iron—elements needed to make planets.

How low can a star’s metallicity go and still form planets?

Heavy elements make up only about 1.3% of the Sun’s mass. Astronomers call these elements metals and abbreviate their abundance with the atomic symbol for iron. Even at that low percentage, the Sun still had enough material to form eight planets, dozens of dwarf planets, and an uncounted number of smaller objects.

But how low can a star’s metallicity go and still form planets? To answer that question, Ji Wang and his team are turning to the oldest stars in the galaxy: galactic halo stars.

“Halo stars are the key to understanding planet formation in the very metal-poor regime,” said Wang, an astrophysicist at Ohio State University in Columbus. Wang discussed this project at Extreme Solar Systems IV in Reykjavík, Iceland, on 19 August.

Hiding in the Halo

Most stars in the Milky Way galaxy live in one of three places: a compact central bulge, a dense and thin spiral disk, or a diffuse cloudlike halo. Sometimes, halo stars plunge through the disk at high speeds and from random directions, like a comet streaking in from the cold reaches of the solar system before swooping outward again.

Those trajectories make halo stars stand out in surveys of stellar motion, like the European Space Agency’s Gaia mission. Halo stars also tend to be older and therefore more chemically primitive than disk stars.

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Wang’s team turned to halo stars to find out how often low-metallicity stars create planets. Of Gaia’s catalog of 1.7 billion stars, the researchers narrowed their search to stars with halolike trajectories that are within about 3,000 light years of us and have less than 10% the amount of metals as the Sun. They narrowed that list to stars bright enough for NASA’s Transiting Exoplanet Survey Satellite (TESS) to observe with high precision.

During the first half of its mission, TESS searched for planets around about 6,200 of the team’s chemically primitive target stars. The researchers focused on large, short-period objects called hot Jupiters, the type of planet most likely to transit.

“We didn’t find any planets,” Wang said. “This is okay because, even for the nondetection, we have put a very tight constraint on the occurrence rate around metal-poor stars.”

The team’s tests showed that TESS could have overlooked roughly half of potential hot Jupiters around these distant stars. On the basis of those statistics, the team calculated that hot Jupiters are born around metal-poor halo stars no more than 0.34% of the time.

Is It Age or Lack of Metals?

“It might be the case that old stars, regardless of their metallicity, are unlikely to have a hot Jupiter.”

“This is really cool work. I think it’s a great idea,” Kevin Schlaufman, an astronomer at Johns Hopkins University in Baltimore, Md., commented after the presentation. He pointed out, however, that some recent studies suggest that tidal interactions can make hot Jupiters crash into their stars. “If hot Jupiters are destroyed by tides, it might be the case that old stars, regardless of their metallicity, are unlikely to have a hot Jupiter.”

One way to resolve that issue, according to Wang, would be to look for metal-poor stars among the younger disk stars. But this would be like looking for a handful of needles in a haystack: Disk stars far outnumber halo stars, and they are mostly metal rich. Finding the few metal-poor stars would be a big task, he said.

The team estimates that TESS will observe about another 10,000 metal-poor halo stars by the end of next year, which will narrow down how often anemic stars create giant planets, Wang said.

“With the full sample, we could set a 0.14% upper limit if there are still zero detections,” he said.

In the meantime, “we can still look for small planets, although with a lower detection efficiency,” Wang said. “There are still a few planets we could detect around these halo stars.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer


Cartier, K. M. S. (2019), Hunting for planets around old, anemic stars, Eos, 100, https://doi.org/10.1029/2019EO131587. Published on 28 August 2019.

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