There’s No Chemical Difference Between Stars With or Without Planets

Strange New Worlds

Imagine if a star could tell you it had planets. That would be really helpful because finding planets orbiting distant stars – exoplanets – is hard. We found Neptune, the most distant planet in our own solar system, in 1846. But we didn’t have direct evidence of a planet around ANOTHER star until….1995.…149 years later. Think about that. Any science fiction you watched or read that was written before 1995 which depicted travel to exoplanets assumed that other planets even existed. Star Trek: The Next Generation aired its last season in 1994. We didn’t even know if Vulcan was out there. (Now we do!…sortof)

Since 1995, with the advent of planet hunting telescopes like Kepler and TESS, we’ve found THOUSANDS of planets orbiting other stars. These missions find exoplanets literally by looking for their shadows. Sometimes an exoplanet’s orbit crosses our view of a distant star blocking out some of the star’s light. This “transit” of the planet creates a shadow in the observed light from the star which we can then use to determine the size of the planet, whether it’s a rocky  planet like Earth or a gas giant like Jupiter, and the length of the planet’s year around its parent star.

But planets are very small compared to their host stars. The amount of light they block is a fraction of the star’s overall light, so our equipment needs to be very sensitive. And if the planets are not orbiting in such a way that they cross our view of the star, say if we are looking at the distant solar system from the top down, we may have a harder time detecting their presence. So, scientists are looking for alternative means of discovering planets and one might be to study the parent stars themselves. Stars are big and bright and easy to spot. If stars that give birth to a solar system are somehow unique to stars that don’t, we might have a powerful new way of planet hunting. Specifically, astronomers are paying close attention to a star’s chemical composition – the right star stuff.

Building a Solar System

Planets and stars share the same stuff. Our solar system formed from one enormous rotating cloud of dust and gas called a protoplanetary disk. 99.8% of the stuff was concentrated in the centre  drawn together by gravity to form the Sun.

The remaining 0.2% of whatever didn’t end up within the Sun itself flattens out to form the disk – imagine like how a ball of dough flattens into a pizza as it is spun. This flattening is why all the planets orbit the Sun along a similar plane called the Plane of the Ecliptic. Within the spinning disk, material begins to accrete forming planetesimals which become the seeds of future planets. But what is this stuff? It’s important! It’s what the planets and you and I are made of. Astronomers refer to it as “metals.” In astronomy, “metals” are considered anything on the periodic table above atomic number 2 – so anything heavier than hydrogen and helium like the calcium in your bones or the iron in your blood. In fact, at the birth of the Universe, there was ONLY hydrogen, helium, and small amounts of lithium. None of the other elements existed. Those elements are themselves created by the stars, deep in their interior, as they convert hydrogen fuel through nuclear fusion into heavier and heaver elements – the metals. Once these stars explode at the end of their lives as supernova, they spill their guts into the interstellar void seeding it with the stuff that makes other stars as well as PLANETS. Likely the first generation of stars in the early universe had no planets at all. There wasn’t yet the raw material to build them. We call those Population III stars.

The next generation of stars, Population II, were the first to form in a universe that was enriched with heavier elements. We’re not entirely sure if this group of stars formed with enough metals to make planets. We want to pinpoint when exactly the first planets formed in the Universe to estimate how early life could have existed. But if planets did form around Population II stars, likely they were quite small and orbited very closely to their parent stars – far closer than Mercury does in our own solar system. Probably not ideal for life at a sweltering 1600K surface temperature. Even if life did form around these stars, it is likely extinct by now as these stars lived shorter lives than our Sun and have already burned out. (Unless of course that life left its solar system to explore the Universe and still exists somewhere as an ancient space-faring civilization from a long-dead star…one can imagine.)

Which brings us to Population I, the group of stars our Sun belongs to. Our Sun formed in a Universe where billions of years of star births and deaths had already occurred. The Universe had been fertilized with more metals. Not only do the metals in a protoplanetary disk create the raw material for planet formation, but also protect the disk itself from being blown away by the parent star’s radiation. More metals mean more time available for the planets to form before the star’s energy eventually evaporates the remaining material that hasn’t yet formed planets

“Where to Look”

Understanding how planets form give us our first clue as to where to look for them – stars with metals. Remember, the host star and their planets form from the same cloud of stuff, so some of those metals are mixed into the star. By looking at the light from a star using spectroscopy, we can tell how highly enriched it is with metals – the star’s “metallicity.” Studying these metal-rich stars, we know terrestrial rocky planets like Earth are 1.72 times more likely to form around them. Even gas giants are more likely to form around metal-rich stars. Although made from gases rather than metals, gas giants like Jupiter are theorized to form around an initial rocky seed or from the disruptions in the flows of hydrogen gas orbiting in the disk caused by the introduction of metals.

But while a star’s chemistry can tell us the likelihood that planets are there – can the chemistry tell us exoplanets ARE there!? Is there a key chemical fingerprint for a star to tell us in a booming stellar voice “Yes indeed, I host planets! Behold my children!”

The research so far SAAAYYYYYSS……no. I know. Kind of anticlimactic.

BUT there is still hope. Last week, the Monthly Notices of the Royal Astronomical Society posted a study by the National Centre of Competence in Research PlanetS. NCCR PlanetS researched 84 stars observed by the 10M Keck Telescope in Hawaii. The team of researchers were trying to determine if planet formation leaves a unique chemical tell on a star – a beacon for us to know that indeed the star had given rise to planets – but a unique indicator couldn’t be found. Comparing 16 stars with planets and 68 without, the team found that planets orbit chemically diverse stars. But the findings are still useful. The team issued a warning that given the preponderance of planet discoveries, most of the stars in the study “probably have planets” (pg 8/3698 of the study) that just haven’t been found yet. So, the study might not be entirely accurate. However, this research could yield future discoveries of what KIND of planets, in terms of size or composition, form around a star with a certain chemical signature especially if/when planets are discovered around more stars used in the study. So, while we may not be able to know IF planets exist because of a star’s chemistry, in the future we may be able to infer with more accuracy what types of exoplanets orbit a star given a certain metallicity. For example, we know that metal-rich stars on average give rise to more planets – perhaps the types and quantities of each metal result in a certain arrangement of the solar system, or the quantities of terrestrial vs gas giants, or whether the planets are habitable. More research is needed.

In the meantime, we continue searching for planets using transits. TESS completed its primary mission, imaging 75% of the sky in a two-year survey, just this past August 20^th. We don’t yet know what discoveries will be found in the data including perhaps new ways to understand the relationships between a host star and its planets. Whatever we find will certainly inform both future planet hunting missions as well as provide inspiration to the fictional stories boldly going to the strange new worlds our research has discovered. Engage!

Further Reading:

NCCR PlanetS Report http://nccr-planets.ch/blog/2020/08/17/stars-with-planets-show-no-special-fingerprint

“Revealing a Universal Planet-Metallicity Correlation for Planets of Different Sizes Around Solar-Type Star (Astronomical Journal) https://iopscience.iop.org/article/10.1088/0004-6256/149/1/14

“The First Planets: The Critical Metallicity for Planet Formation” – Johnson and Li (2012) https://arxiv.org/abs/1203.4817

“When Stellar Metallicity Sparks Planet Formation” – Astrobiology Magazine

“Do Metal-Rich Stars Make Metal-Rich Planets?” New Insights on Giant Planet Formation from Host Star Abundances – Johanna K. Teske, Daniel Thorngren, Jonathan J. Fortney, Natalie Hinkel, John M. Brewer 2019 https://arxiv.org/abs/1912.00255

“Metallicity and Planet Formation: Models” – Boss