Bad astronomy | HD 3167 has 2 planets in orbit perpendicular to a third


One of the most obvious things about our solar system is that all of the major planets orbit the Sun in roughly the same plane. If you looked at the solar system “from the side” it would appear almost flat.

This is a consequence of the way our planets were formed. Billions of years ago, a cloud of gas and dust began to collapse. Any rotation in this material would be magnified as it shrank (think of an ice skater who spins faster when bringing their arms back), and (due to centrifugal force) anything in the plane of that rotation has more difficult to fall towards the center than things near the poles. You end up with a huge disk of gas and dust several billion kilometers in diameter, with the Sun forming in the center and the planets orbiting the plane of the disk. Not only that, because of all this, the star spins in the same direction the planets spin, and the orbits of the planets align with the star’s equator (we say they are coplanar).

This is also true for other solar systems. We see these disks wherever stars and planetary systems form, and it’s a safe bet that the planets will all line up on this disk.

… But it’s not always the case. And it’s really great not the case for the neighboring star HD 3167. We know three planets in orbit, and two of them orbit in fact perpendicular to the other!

HD 3167 is a K star, just a little smaller, cooler, and darker than the Sun, located about 150 light years from us. In 2016, two planets were announced to orbit the star, found using this transit method.

The interior, called HD 3167b, is a super-Earth, about 1.7 times the diameter of our home planet and five times its mass. It is in an orbit very close to the star, taking only 0.96 days to circle once. Its surface temperature is probably around 1,500 ° C (2,700 ° F), so that is not really the case with Earth.

The exterior, called HD 3167c, is a mini-Neptune, about 3 times the size of Earth and 10 times its mass. It takes about a month to orbit the star once, and although it is further away, it is still close enough to the star to have a temperature high enough to boil water.

Soon after, it became clear that there must be a third planet in the system. (discovered by my friend Jessie Christiansen!). It does not pass through the star, but the planets have gravity, and when they revolve around the star, they shoot at it. This causes the star to reflexively make a small circle as the planets revolve around its orbit, and although the orbit is too small to be seen directly, the star’s speed as it moves is measurable as Doppler shift. This planet is called HD 3167d and it orbits between the other two, circling the star once every 8.5 days (note that the planets are named in order of discovery, not by distance from the star, so the order here from the star is b, d, c).

The article on the discovery shows that the orbit of planet d is strange, and it didn’t take long to show that this whole system is strange: while planet b orbits the star above its equator and in the same direction as the star turns, the planet is almost in a polar orbit, perpendicular to it. Not only that, but the planet d too!

It’s really weird. To better understand this, a new document came out analyzing the system very carefully. They use a sophisticated technique to confirm that planets c and d are in polar orbits, with inclinations quite close to each other (the angle between the two orbits is between 2 and 21 °).

How did they do that? Using a technique called the Rossiter-McLaughlin effect.

Imagine you are looking at a star above its equator, so the north pole of the star is at the top and the south pole is at the bottom. It turns from left to right. This means that the left side of the star is turning towards you and the right side is moving away from you. This induces a Doppler effect in the star’s light; the light coming from the left side has a very slightly shorter wavelength (we call this the blue shift) and the right side has a very slightly longer wavelength (red shift). A spectrograph can divide the light from the star into thousands of individual wavelengths (colors), and what it sees in a case like this is that the wavelengths are slightly spread out, because he sees all the movement of the star at the same time.

Now suppose that a planet transits through the star and orbits in the same direction that the star rotates (left to right). It first blocks the star on the left a bit, then moves to the center of the star, then blocks the light to the right. If you take spectra while in transit, you will first see some of the blue-shifted light coming from the star, and then the red-shifted light. This tells you that the planet orbits the star prograde, in the same direction as it turns.

In some cases, we see the opposite: the red-shifted light fades first, then the blue-shifted light. This means that the planet orbit retrograde, backward.

If the planet is in an orbit tilted to the star’s equator, you get a variation on it, with a slightly different gradation of the light shifted blue and red at different times. It is incredibly difficult and demanding work, but it is possible to obtain the orbital tilts of the planets this way.

And that’s what they did in the new newspaper. The orbits of planet b progress like a good little planet, but the other two orbit perpendicular to it, above the poles of the star: a polar orbit.

It’s very strange! How can this happen?

They believe that the three planets formed normally, like ours, and that all began to live in orbit in the same plane. But they postulate a fifth object in the system orbiting further, which could be a very low-mass star or a fourth planet. The details are complex, but objects in elliptical and slightly tilted orbits can change the shape and tilt of other objects in the system. Over time, this other object gravitationally interacted with planets c and d, causing them to switch to these polar orbits. Planet b was too close to the star to be affected.

It can also happen in other systems, where we see retrograde or polar planets. WASP-79b is an example, and this can also happen in binary star systems.

The trick now is to find this other object. This may not be possible with current technology; it may not be in transit and it may also be too weak and too far away to detect the opposite. At least for now. Astronomers are smart people and keep getting better at figuring out how to do things like this.

And oh how I love this news! Our solar system is amazing, wonderful and fascinating, and yet, compared to others, it is quite vanilla. Nature has a way of turning things upside down, of throwing a curve ball into what we might consider “normal”.

But normal is just an average, a point on a chart that you choose because a lot of things are lying around. But lots of things not, and the more we learn about them, the more we can understand what normal really means.

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