This episode is a double-feature of exploring new discoveries in exoplanet research.
The early universe was primarily composed of hydrogen and helium with short-lived supermassive stars. These stars fused heavier elements under their immense gravity and exploded, propagating planet-making materials. A new study suggests that planets could have formed earlier than previously thought from early development of water.
Planets around stars are typically found by observing their passing in front of or moving their host stars, but this becomes more challenging as the size of the star increases. New research uses asteroseismology to detect planets around stars roughly twice the size of our Sun.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In this episode, we're going to tackle two items, both related to exoplanets. So let's dive in.
The first article concerns how early in our universe's history planets could form. It's published in the Astrophysical Journal Letters by Eduard Verobyov of the University of Innsbruck and collaborators and it requires some background.
As we've talked about in other episodes, the beginning of our universe, the Big Bang, only produced hydrogen and a bit of helium — about 9% of the number of atoms. Not even stars existed at that point. But today, our solar system consists of about a tenth of a percent of heavier elements — the stuff of which planets can be made. This may still seem like a small amount, but obviously it's enough to make Earth and the other eight planets. Still, I would remind everyone that the four most massive planets in our solar system are made mostly of hydrogen, including the water mantles of Uranus and Neptune.
Somewhere between the end of the Big Bang and now, our universe became sufficiently enriched with planet-building materials and that's down to stars, as they drive the chemistry of our universe since the Big Bang.
In the most enriched era of planet-building materials — and our solar system is one of those — we find that at least 90% of all stars have planets. But this has some bias, because when we look for planets around other stars, typically astronomers don't look at stars that are too old, as they contain much less of the planet-building material. Or so we thought.
In their article, Dr. Verobyov and collaborators remind us that the first stars were most likely huge behemoths of 100 times the mass of our Sun or more, and they lasted only for a short time — perhaps a few million years — and then exploded violently, recycling their material into the universe. They then go on to write that the exploded gas was likely enriched in oxygen just by a tiny fraction, and that oxygen could have combined with the abundant hydrogen to make water. Thus, when the next generation of stars formed, that water vapor could have frozen to form ice planets — maybe as large as Mars or even Earth. Not rocky planets, but still planets much much earlier in the universe than otherwise expected.
The second item is one that's near and dear to my heart because it involves asteroseismology — that is, the seismology of stars.
Starlight only leaves the outer 1% of a star, and so to see deeper into it and determine its structure, we use seismology. Many stars vibrate, and we see those vibrations as periodic brightness variations, and that's what Logan Wilson of the Harvard-Smithsonian Center for Astrophysics and collaborators did. But they did not use those vibrations for structural purposes, but rather as a probe for planets.
Currently, the vast majority of exoplanets were discovered using the transit method, where if the planet goes in front of the star, it blocks a small fraction of that star's light. In our system, Jupiter would block about 1% of our Sun's light, and Earth just one one-hundredth of a percent.
Another method for finding planets is using Doppler shift, where as the planet orbits the star, it moves the star a small amount and that can be measured. Again, in our solar system, Jupiter moves our Sun at about 12.5 meters per second, or 28 miles per hour, while Earth moves the Sun at only 9 centimeters per second, or less than two-tenths of a mile per hour.
For stars larger and more massive than our Sun, planets block less light if they transit and move the star much less, making them far more difficult to detect using those common methods.
And the stars Dr. Wilson and collaborators were looking at are called spectral type A stars, which are about twice the size of our Sun and three times the mass. They were using a special type of A star, which vibrates and is a pulsation type called delta scuti stars. These stars go through a periodic brightening and dimming every few hours in a very regular way, and by using the phases, they can measure slight deviations in that cycle caused by planets moving the star.
They searched over 16,000 delta scuti stars and so far have found one with the periodic signal they think is caused by a planet. By their calculations, the planet would be 6.3 times the mass of Jupiter and orbits once every 2,200 days. That seems like a very long orbit, but as it's going around a star 40 times brighter than our Sun, that puts it in the habitable zone, where liquid water could exist. Of course this planet would be a gas giant, but perhaps like our Jupiter, it has sizable moons which could have water.
But that wasn't the main point of the work done by Dr. Wilson, rather they showed a method for finding planets around stars larger and hotter than our Sun, and that makes for two very interesting exoplanet papers.