Astronomers today often find exoplanets using the transit method where a planet blocks part of its star's light as it passes in front of it. We expect most planetary systems to stabilize with planets along similar planes of orbit, much like our own, but TOI-201 seems to challenge this understanding. The system features a super-Earth, warm Jupiter, and brown dwarf in highly inclined and eccentric orbits, giving insights into planetary formation.
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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
As we discover more exoplanets and other solar systems, we continue to be surprised by the variety. But as a reminder, most of the planets and solar systems we've discovered are not at all like ours. We have a very ordered solar system with rocky planets close to our sun, where it is warmer, and gaseous planets beyond what is called the ice line, where it is cold enough for water ice to condense. The idea is that large planets can only form with the help of ice condensing out of the nebula.
We have been regularly finding exoplanets for over 30 years. Our methods are always improving, but especially early on they were biased toward finding large planets in short orbits. That bias still holds true today. Most planets are discovered using the transit method, where the planet goes in front of its star and blocks some of the light. Larger planets block more light than small ones, so they are easier to find. Also, since measurements always have some quantity of error associated with them, small transits require more observations to be confirmed. So for a large planet like Jupiter, two or three transits might be enough to establish the relative size of the planet and its orbital period. But for a small planet, it might take ten or more transits to build up sufficient signal to overcome the noise, and of course it is easier to accumulate ten or more transits if the orbital period is short.
With our current technology, it is still incredibly difficult to detect Earth‑sized exoplanets around sunlike stars with orbits of a year. Historically we have been biased to find solar systems unlike ours, and that is exactly what we have found. First, we found very large Jupiter‑like planets with orbits much shorter than Mercury's. The very first planet discovered around a sunlike star was 51 Pegasi b, which is a Jupiter‑like planet in a three and a half day orbit. This raised some very interesting questions. What would happen to Jupiter if you heated it up? Jupiter is made mostly of hydrogen, which is a transparent gas. We see it because of clouds of molecules. But if it were hot enough to not have clouds, what color would that much hydrogen be? It turns out blue, as measured from a warm Jupiter.
As methods improved, we found smaller and smaller planets until we were able to find Earth‑sized planets, but only in very short orbits. The planet 55 Cancri e is a super‑Earth that orbits its star in under one day. It is a lava planet with surface temperatures nearly 5000 degrees Fahrenheit. But it is not the hottest known exoplanet. That trophy goes to KELT‑9b, which is a Jupiter‑like planet but in a 1.5‑day orbit with an expected temperature of nearly 8000 degrees Fahrenheit.
We have also found planets around dead stars, both white dwarfs, which die peacefully, and neutron stars, which die in violent explosions. That either of these has planets is amazing. And that brings us to the TOI‑201 system, and an article published in Science Advances by Ishmael Moreles, a test support associate at MIT and collaborators. They find a system with three planets. The closest one is a super‑Earth, rocky planet that is 39% bigger in radius and nearly six times more massive than our Earth. It orbits its star, which is 30% larger than our Sun, in under six days.
The next planet is a Jupiter‑like planet that orbits in about 53 days, which is still a shorter orbit than Mercury's. The last planet is over 16 times more massive than Jupiter, with an orbital period of just under eight Earth years. This one's orbit is still a little closer than Jupiter's in our own solar system.
So on the exoplanet front, there is nothing particularly unusual about those planets. But what is unusual are their orbits. In our solar system, all eight planets orbit very close to the same plane, and that is what we find with most exoplanet systems. That makes sense as we expect solar systems to form out of flattened disks of gas circling their host stars.
But in this system, the planets are not orbiting in the same plane. The two inner planets differ by 28 degrees, and the outer planet is off alignment by a further 13.4 degrees, making it almost 42 degrees out of alignment with the innermost planet. Also, the two outer planets have highly eccentric orbits. The inner Jupiter has a 27% eccentricity, and the outer one has over 65% eccentricity, which means its orbit is more than twice as long as it is wide.
These very elongated, out‑of‑plane orbits are torquing on each other, and the whole system is evolving on human time scales. Dr. Moreles and collaborators observed the TOI‑201 system on and off for nearly seven years, and over that time the transit timings of the inner Jupiter changed by more than 30 minutes. They estimate that given another 200 years, none of the planets will even transit their star from our viewpoint.
We can learn a lot about how planets form from these weird systems.