Illustration showing the rotation of WASP-79 star (left to right), and the orbital plane, motion and orbit of WASP-79b planet, which almost crosses the stars poles.
Illustration showing the rotation and motion of WASP-79b (left to right), and the orbital plane of WASP-79b. The planet almost crosses the poles of star. Photo credit: Brett Addison. Modified from ESO/L. Calcada)
The most striking thing about the solar system is the fact that the Sun and all of its planets orbit in the same plane. It would almost look flat if you looked at it from the side.
This is due to the way our planets were formed. A cloud of dust and gas began to fall billions of years ago. Any rotation in this material would be amplified by shrinkage (imagine an ice skater spinning faster when they pull their arms in). Also, due to centrifugal force, any stuff in the plane that rotates has a harder job falling to the center of the rotation than stuff closer to the poles. The result is a massive disk of dust and gas that spans billions of kilometers. There's the Sun, which is forming, in the middle and all the planets orbiting in the same plane as the disk. This means that the star rotates in exactly the same direction as the planets, and the orbits are aligned with the star's Equator (we call them coplanar).
This is true for all solar systems. These disks are everywhere stars and planetary system formation is taking place, so it's reasonable to expect that all planets will align with that disk.
But it is not always true. It's not always the case with the nearby star HD3167. It is home to three planets, two of which orbit it perpendicularly!
HD 3167, a K star that is just 150 light years away from us, is slightly smaller, cooler and more dimmable than the Sun. Two planets orbiting the star were discovered using this transit method in 2016.
The inner one, HD 3167b is a super-Earth with a diameter about 1.7 times that of our homeworld, and a mass five times as large. It orbits very close to the star and takes only 0.96 days to circle once. Its surface temperature is around 1500degC (2700degF), so it doesn't look Earth-like.
The outer one, HD 3167c is a mini-Neptune. It's about three times larger than Earth and ten times as large. It takes approximately a month to orbit the star one time. Although it is farther away, it is still close enough to the star to maintain a temperature high enough to boil water.
Illustration of a system containing planets around a red dwarf Star. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org) Photo: ESO/M. Kornmesser/N. Risinger (skysurvey.org)
It was discovered by Jessie Christiansen, my friend. Although it doesn't transit the star itself, planets have gravitation and tug on the star as they orbit it. The star does a little reflex circle when the planets move around in their orbit. Although the orbit is too small for direct observation, it is possible to measure the speed of the star's movement as a Doppler shift. This planet is known as HD 3167d. It orbits the star approximately once every 8.5 days. (Note that the names of the planets are based on their discovery order, not the distance from the star. The order from the star to this planet is b, d and c).
The discovery paper reveals that the orbit of planet D is strange. It didn't take long for this to become apparent. While planet B orbits the star over its Equator in the same direction as the star spins and while planet b orbits the star in the opposite direction, planet C is almost in a polar orbit. Planet d is also in a strange orbit.
This is truly amazing. A new paper that analyzes the system in detail has been published to help us understand it better. The team used a sophisticated technique to confirm that the planets c, and d were in polar orbits with tilts very close to each other (the angle between them is between 2 and 21deg).
How did they achieve this feat? They used a technique known as the Rossiter McLaughlin effect.
Illustration showing the rotation and motion of WASP-79b (left to right), and the orbital plane of WASP-79b. The planet almost crosses the star's poles. Photo credit: Brett Addison. Modified from ESO/L. Calcada)
Imagine that you are looking at a star above its equator. The north pole is at the top, and the south pole is at the bottom. It is spinning from left to right. This means that the left side is moving towards you and the right side is moving away. This causes a Doppler effect to the star's light. The light coming from the left has a shorter wavelength (we call it blueshift), while the light coming from right has a slightly longer wavelength (redshift). A spectrograph is able to break down the star's visible light into thousands of wavelengths (or colors). The wavelengths are slightly blurred because it simultaneously sees all the star's motion.
Let's say that a planet orbits a star. It will follow the star's path (left to right). First, it blocks some of the star's light on the left. Then, it moves to the star center and blocks the light on the right. If you look at spectra of the transit, you will see some blueshifted light coming from the star dimming first and then the redshifted light dimming later. This tells you that the planet orbits in the same direction as it spins, the star prograde.
Sometimes we see the reverse: In some cases, the redshifted light dims first and then the blueshifted. This means that the planet orbits backwards, retrograde.
You will get variations if the planet orbits tilted in relation to the star's Equator. This can result in slightly different blueshifted or redshifted light dimming at various times. This is a difficult and tedious task, but it is possible to obtain the orbital tilts for the planets in this manner.
An equatorial orbit is a satellite orbiting above a star or planet's equator. If it orbits in the same way as the primary object, it's prograde and retrograde if it's backwards, it's called an equatorial orbit. The orbit of a polar satellite is perpendicular to the primary object's poles. NASA Photo
That's exactly what they did in their new paper. The orbit of Planet b is like that of a small planet. However, the orbits of the two other planets orbit perpendicularly to it. This orbit is called a polar orbit.
This is so bizarre! This is so strange!
They believe that all three planets formed in a normal way, just like ours, and that they all began life on the same plane. They believe that a fifth object orbits further out in the system, which could be either a low-mass star (or a fourth planet). Although the details are complicated, objects orbiting in elliptical or slightly tilted orbits may alter the shape and tilt of other objects within the system. This other object gravitationally interacted over time with planets C and D, flipping them into these polar orbits. The star was too close for planet b to be affected.
This could also happen in other systems where there are polar or retrograde planets. This is what WASP-79b shows. It can also happen in binary star system.
Now, the trick is to find the other object. It may be impossible to find the object with current technology. It may not transit, or it might be too faint or far away to be detected otherwise. For now, at least. Astronomers are smart people and continue to improve their ability to figure out how things work.
This news is amazing! The solar system is incredible, wondrous, and fascinating. Yet it's quite vanilla compared to others. Nature can surprise us by throwing curveballs into our "normal" lives.
Normal is just an average. It's a place on a graph that you choose because many things live around it. Many things don't have normal, and we will learn more about them.
