[BRIEF NOTE] On the HIP 11952 planetary system

[rfmcdonald]

HIP 11952 has made the news recently for its planets. This star 375 light-years away is noteworthy since, although it's broadly similar to the Sol mass-wise--it's only one-sixth less massive than our Sol, perhaps comparing to nearby famous Tau Ceti--it is a remnant of the very early universe. Our sun is 4.57 billion years old, our universe 13.75 billion (plus or minus 110 million years), but HIP 11952 formed in the youth of the universe, and is 12.8 billion years old. Because of its extreme age, when HIP 11952 was formed in the infant Milky Way it did so in an environment where there just hadn't been enough nuclear fusion in stars to make the elements heavier and helium that dominate our terrestrial worlds; spectrograms indicate that this Population II star has 1% of the "metals" of our own sun. There was a consensus in astronomy that stars with so few metals shouldn't have been able to support the formation of planets.

But HIP 11952 has two Jovian-mass planets regardless.

It is widely accepted that planets are formed in disks of gas and dust that swirl around young stars. But look into the details, and many open questions remain – including the question of what it actually takes to make a planet. With a sample of, by now, more than 750 confirmed planets orbiting stars other than the Sun, astronomers have some idea of the diversity among planetary systems. But also, certain trends have emerged: Statistically, a star that contains more “metals” – in astronomical parlance, the term includes all chemical elements other than hydrogen and helium – is more likely to have planets.

This suggests a key question: Originally, the universe contained almost no chemical elements other than hydrogen and helium. Almost all heavier elements have been produced, over time inside stars, and then flung into space as massive stars end their lives in giant explosions (supernovae). So what about planet formation under conditions like those of the very early universe, say: 13 billion years ago? If metal-rich stars are more likely to form planets, are there, conversely, stars with a metal content so low that they cannot form planets at all? And if the answer is yes, then when, throughout cosmic history, should we expect the very first planets to form?

Now a group of astronomers, including researchers from the Max-Planck-Institute for Astronomy in Heidelberg, Germany, has discovered a planetary system that could help provide answers to those questions. As part of a survey targeting especially metal-poor stars, they identified two giant planets around a star known by its catalogue number as HIP 11952, a star in the constellation Cetus (“the whale” or “the sea monster”) at a distance of about 375 light-years from Earth. By themselves, these planets, HIP 11952b and HIP 11952c, are not unusual. What is unusual is the fact that they orbit such an extremely metal-poor and, in particular, such a very old star!

For classical models of planet formation, which favor metal-rich stars when it comes to forming planets, planets around such a star should be extremely rare. Veronica Roccatagliata (University Observatory Munich), the principal investigator of the planet survey around metal-poor stars that led to the discovery, explains: “In 2010 we found the first example of such a metal-poor system, HIP 13044. Back then, we thought it might be a unique case; now, it seems as if there might be more planets around metal-poor stars than expected.”

What does this mean for planetary formation? What are the minimum requirements for planetary formation? When did the first planets in our galaxy form?

Centauri Dreams' Paul Gilster notes that this has implications from the perspectives of galactic history and the possibilities of extraterrestrial life. If planets formed so early in the universe's history, well, where is everybody?

The idea of ‘deep time’ exerts an abiding fascination. H.G. Wells took us forward to a remote futurity when his time traveler looked out on a beach dominated by a red and swollen Sun. But of course deep time goes in the other direction as well. I can remember wanting to become a paleontologist when I discovered books about the world of the dinosaurs, my mind reeling from the idea that the world these creatures lived in was as remote as any distant star. Paleontology was a grade-school ambition I never followed up on, but the Triassic and Jurassic eras still have a hold on my imagination.

In a SETI context, deep time presents challenges galore. Charles Lineweaver’s work offers up the prospect that the average Earth-like planet in our galactic neighborhood may well be far older than our own — Lineweaver calculates something like an average of 1.8 billion years older. Would a civilization around such a star, if one could survive without destroying itself for so long, have anything it wanted to say to us? Would it have evolved to a level where it had merged so completely with its environment that we might not be able to recognize its artifacts even if we saw them?

[. . .]

Are there, then, more planets around metal-poor stars than we have previously thought? We need to find more planetary systems in this age bracket to learn more, but Anna Pasquali (Heidelberg University), a co-author of the paper on this work, says “The discovery of the planets of HIP 11952 shows that planets have been forming throughout the life of our Universe,” a thought that reminds us to be careful about drawing hasty conclusions about planet formation. Don’t be surprised, either, if the idea doesn’t once again bring Dr. Fermi knocking at the door.

A commenter at Centauri Dreams suggests that gas giants, which are made substantially of the non-metals that were in exclusive abundance early in the universe's history, would likely form more easily than rocky planets like the Earth. Against this, a commenter at the first news link wonders--jokingly--if HIP 11952 is the home system of the Vorlons.