in the Solar system, all planets share the same plane

Our observations, and those of other investigators, show that for exoplanets this is not the case; planets can occupy any orbital plane. Several explanations are given; most are linked to events happening during their formation, or shortly after. Studying orbital inclinations is to investigate those early conditions.

The Solar system is extremely flat. All planets approximately share the same orbital plane. This laid the foundations of the theory stipulating that planet form within a primordial disc of material. When hot Jupiters were first discovered, their surprising presence so close to their star (where they physically cannot form) was explained by interactions between the planet and this disc. Measurements of orbital inclinations, for hot Jupiters, have shown that many of them occupy orbital planes in a random orientation with respect to the equator of their host star. Although not the only explanation, this strongly points toward important dynamical processes happening after the planets have formed.
Current work now focuses on orbital periods longer than the hot Jupiters typically occupy, and towards planets of lower masses. In addition significant advances are being made by studying multi-planetary systems and measuring whether all planets share a same orbital inclination, or whether they are mutually inclined.
The study of the hot Jupiters' orbital inclination carries on, to complement what is already known, but also to study the impact of tides, raised between the planet and the star, which change as the star evolves. This becomes an unthought-for test for the physics of stellar evolution.

papers written on this topic

the Rossiter-McLaughlin effect

I am leading a team that measures the Rossiter-McLaughlin effect for transiting planets seen in the Southern Hemisphere, notably those found by WASP. We use the HARPS spectrograph, mounted on the 3.6m telescope, at ESO's observatory of La Silla, in Chile. I am also involved in a few other campaigns.

The Rossiter-McLaughlin effect is leading source of planetary orbital inclinations. This effect happens while a planet transits its host star. If that star rotates, then due to the Doppler effect, its forward advancing hemisphere will be blue-shifted, and its retreating hemisphere, red-shifted. This is graphically represented above. When the planet passes above the stellar surface, it will first cover the bluer hemisphere. Since less blue light reaches Earth, the star appears a little redder than usual. This translates into an anomalous, positive, velocity, a quantity measured by HARPS. As the planet scans the stellar surface, that velocity changes and gives a very characteristic shape. By estimating how long the planet stays on each hemisphere, and which hemisphere it starts covering, we can estimate the angle between the orbital plane of the plane and the equatorial plane of its star (projected onto the sky). Below, are presented two of our best cases: HD 189733b, a coplanar planet (like the solar system), and WASP-8Ab, the first retrograde planet that was measured.

Important papers to read on the topic

Written by competitors, who are also good friends and colleagues

J.N. Winn & Fabrycky 2015

Annual Review of Astronomy & Astrophysics
in press

A review of the field presenting exoplanets' orbital inclinations in the context of other observations.

G. Hébrard et al. 2008

Astronomy & Astrophysics, 488, 763

The first report of a non-coplanar inclination. It was later verified.

S. Albrecht et al. 2009

Nature, 461, 373

Exoplanets are not alone being on inclined orbits. Some binary stars too!