In the last three decades, we went from not knowing if planets existed beyond the Solar System to finding so many that we now suspect almost every star has one. So far, over 5,000 planets have been confirmed. Astronomers have discovered some stuff that has truly challenged our understanding of what planets are like.
There’s a lot of diversity when it comes to the planets in the Solar System, but we can fit them into broad categories, linking them with similar formation histories and evolutions. Maybe we will come to realize that we are the odd one out, but some of the stuff out there is truly weird.
Earlier this year, the European Space Agency’s Cheops discovered Exoplanet LTT9779b. It is a very hot Neptune-sized planet that should have no clouds or atmosphere. However, unaware or uncaring about the expectations of the astronomers, the planet has glassy metallic clouds, despite a scorching temperature of 2,000 degrees Celsius (3,600 degrees Fahrenheit). And that blanket of clouds reflects 80 percent of the starlight it receives; this is the shiniest known planet yet, and has been referred to as the largest known "mirror" in the universe.
The mirror planet orbits its stars in 19 hours, but even far away from their stars, worlds can be scorching. There is a puzzling one called VHS 1256 b that orbits two stars further away than Pluto is from the Sun. It is not clear if it is a planet or a brown dwarf – a stellar object that was not massive enough to start nuclear fusion at its core – but it is hot and about 12 to 16 times the mass of Jupiter.
Its temperature, which reaches 830 °C (1,500 degrees Fahrenheit) on top of the atmosphere, its distance from us (40 light-years), and its distance from the stars made it an ideal candidate to be studied by JWST. The telescope saw carbon monoxide and dioxide, water, and methane in the world’s atmosphere. But the most peculiar thing was silica clouds; sand-like material is being churned as VHS 1256 b rotates every 22 hours.
Whether this world is a planet or a failed star is not just semantic. Planets form from the bottom up, and pebbles in a disk around a star accumulate, growing into a planet. Brown dwarfs instead form from the bottom down, gas clouds fragment, and gravity pulls them together into a planet-size object. There were arbitrary values for the biggest size of planets versus the smallest brown dwarf, but JWST is showing that the division is not just arbitrary, it does not work.
A brown dwarf has been discovered weighing just three to four times the mass of Jupiter. The smallest confirmed yet and roughly the limit considered for the smallest possible, that can form under the most ideal conditions. It was found in the Perseus star-forming region.
But some observations show free-floating planets in the Orion Nebula that are smaller than Jupiter, about 60 percent of its mass. They are in pairs, so they are called JuMBOs, Jupiter Mass Binary Objects. Researchers do not believe they formed like planets and they were later kicked out – they somehow formed like stars.
“If you have an interaction in a disk, maybe you can throw one of those planets out. So it's thought that in regions like Orion, there should be sort of free-floating planets, which originally formed in the disks,” Dr Mark McCaughrean, senior advisor for science & exploration at the European Space Agency, previously told IFLScience.
“The real problem is that about 40 of them are in binaries. They're in pairs. And so how do you kick two things out together that stay together as they go out in a violent event like that? I mean, I'm an observer: this is exactly what I aim to do, break things for the theorists!”
Things are breaking for theorists in so many different ways, showing the importance of improving observatories to get more observations to refine our models. JWST has shown that the building blocks of rocky planets can survive the extreme environments around the hottest stars, making the case for even more Earth-sized worlds out there.
Observations have shown that the inner portion of a star system doesn’t dry up completely, but retains some water. This was seen in the planetary system PDS 70, so rock worlds like ours are not just at the mercy of comets when bringing water and the possible building blocks of life. Earlier this year, there was the first tentative detection of water on an Earth-like planet, though researchers are not yet 100 percent sure whether or not the water emission might come from its star.
But let’s go back to PDS 70 for one second, because we need to appreciate how weird this system is, beyond the water where rocky planets might be forming. The system has two confirmed giant planets orbiting at a distance from the star further than Uranus is from the Sun, but the innermost of the two has something else forming on the same orbit: potentially, a third planet. Whilst there’s definitely something on the orbit, it is not clear if it’s a fully fledged planet yet, but it might be getting there, providing the possible first example of a co-planer planet.
And if we think about bizarre orbital configurations, the crown for 2023 is won by the six planets around star HD 110067. Discovered originally by NASA’s Transiting Exoplanet Survey Satellite (TESS), Cheops then revealed that these six planets are in resonance. This means that the planets are in a big dance, a pattern repeating in whole numbers. For example, the innermost planet does three rotations around its star for every two of the second innermost.
About 1 percent of all known systems are believed to be in resonance, but this is the brightest yet, making it a great candidate for future observations. But what is truly peculiar is the system's age. Resonance is believed to be the starting point of every planetary system, but it goes away quickly, due to a passing star, a collision, or a giant planet effect. And yet, this system has been like this for four billion years.
Such discoveries are expanding our definition and understanding of planetary systems, and telling us that "impossible" worlds might actually be quite common.