As we discover more worlds orbiting distant stars, we are finding that “conventional thinking” doesn’t seem to apply to the growing menagerie of exoplanets. And this most recent exoplanetary discovery is no different.
In fact, the two exoplanets found to be orbiting a star 375 light-years away shouldn’t exist at all.
The two gas giant planets were spotted during a survey of “metal poor” stars. When focusing on a star called HIP 11952, researchers from the Max-Planck Institute for Astronomy in Heidelberg, Germany, discovered a slight wobble in the star’s position.
The wobble is being caused by the gravitational tug of two exoplanets — one is nearly the size of Jupiter and orbits the star every seven days, the other is approximately three-times the size of Jupiter and has an orbital period of 290 days.
This may sound like a typical exoplanet discovery that uses the “radial velocity method” to detect the gravitational presence of planets around other stars, but this star isn’t the kind of star one would expect to find planets at all.
HIP 11952 is a “metal-poor” star, which, in astrophysicist-speak, means this stellar example contains a very low abundance of elements heavier than hydrogen and helium. It turns out that metals are very important in the construction of planets, so metal-poor stars aren’t exactly fertile places for planets to form.
“So far there are only very few planetary companions detected around stars with low stellar metallicity,” said Johny Setiawan, astronomer who led this research at the Max-Planck Institute for Astronomy.
In the case of HIP 11952, the logarithm of the ratio of iron and hydrogen — [Fe/H] — is less than one.
“That means, the abundance of heavy elements, e.g., iron, is less than 10 percent compared to that of the sun,” Setiawan told Discovery News.
This poses a very interesting question, and a conundrum.
So far, of the 750 confirmed exoplanet discoveries, there is a better statistical likelihood of a metal-rich star playing host to planets. And from our experience in the solar system, metals are obviously a very important component for planetary evolution. So, to find worlds orbiting a star with such a low metallicity seems to contradict this view.
But there’s another thing. Metal-poor stars formed when the Universe was very young. The heavy elements that are ubiquitous throughout the cosmos today were formed inside the cores of generations of stars and when massive stars popped-off as supernovae. But the presence of metal-poor stars in the modern universe suggest they are very old.
In fact, it is thought HIP 11952 may be a surviving artifact of the dawn of the Universe — it’s nearly 13 billion years old. In other words, this star was born when the Universe was less than a billion years old. It is also thought to be a star from another, older galaxy — the Milky Way probably acquired it during an ancient galactic collision.
As the star formed 13 billion years ago, it is likely that these two exoplanets (dubbed HIP 11952 b and HIP 11952 c) also formed around 13 billion years ago. These truly are cosmological oddities, they are the oldest exoplanets known.
OK, so these worlds are really old and they shouldn’t even exist, but as pointed out by Setiawan, it doesn’t mean we need to re-write planetary formation theories.
“Of course, there is a problem in the definition of ‘metal-rich’ and ‘metal-poor,’” he said. “HIP 11952 is from the point of view of the now universe ‘metal-poor.’ But in an epoch of the early universe (13 billion years ago) compared to other stars, HIP 11952 is extremely ‘metal-rich.’
“At this age, stars normally have much lower metallicities. Heavy elements were created much later.”
So when HIP 11952′s planets started to form, they were actually forming around a comparatively “metal-rich” star for the time, and although formation processes may have taken longer, HIP 11952 was actually the “richest kid on the block.” Back in the “old days,” before all the modern nouveau-riche stars moved into town, planets were still forming, albeit in a more anemic environment.
So, although exoplanet-hunting surveys generally hunt for exoplanets orbiting metal-rich stars, it is important not to neglect the metal-poor ones. They could be harboring exoplanets of special historical significance.
Future explorers may therefore want to visit these low-metallicity worlds not to search for alien life or “Earth 2.0.” They may be the Indiana Jones’s of the future, carrying out archaeological surveys of these ancient planets, unlocking the secrets of our Universe when it was just a baby.
Image credit: Timotheos Samartzides