Debris Disks

Giving us clues to our own Solar System's troubled past

Formation of the Moon

The Earth; our home. Today, we know it as a temperate, stable planet, filled with liquid water, and hosting a breathable atmosphere. We also have a large moon that helps raise tides and circulate energy around the globe. But the Earth wasn't always such a great place for life.

When Earth first formed, it was a barren, desolate planet. Its atmosphere was mostly made of carbon dioxide, and it did not have a moon at all. Suddenly, about 100 million years after the formation of the Sun, something the size of Mars smacked into the still-forming Earth. This planetesimal, called Theia, knocked material off of the baby Earth. The resulting debris fell into orbit around the Earth, eventually sticking together to form the Moon. This impact was a freak accident, which is why none of the other terrestrial planets have large moons. However the creation of the Moon turned out to be crucial for life on Earth. 

Making Earth Habitable

The Moon-forming event wasn't the only collision in the early Solar System that proved to be necessary for life as we know it. Just after the Moon was formed, the Earth was still toxic to life; there was very little water, and the atmosphere still wasn't breathable.

That all changed during the Late Heavy Bombardment. About 750 million years after the formation of the Sun (and 650 million years after the formation of our Moon), Jupiter and Saturn started causing trouble in the outer Solar System. When the Solar System was still young, the planets hadn't yet reached their current positions. Jupiter and Saturn, notably, were still finding their place. At some point, they ended up in what's known as an orbital resonance. What this meant was that every time Saturn orbited the Sun once, Jupiter orbited twice. Every so often, they would line up on the same side of the Sun, their joint gravitational influence wreaking havoc on the outer Solar System. The result was that the asteroids and comets residing in the Kuiper Belt were knocked free of their orbits and were flung into the inner Solar System. There, they collided with the Earth, Moon, and terrestrial planets.

While this may seem like a bad thing for the Earth, we wouldn't be here if this hadn't happened. When comets and asteroids stormed the inner Solar System, they brought with them water, nitrogen, and the ingredients for life itself; amino acids. 

So it was collisions that made Earth the planet it is today. What does that mean for our search for life in the Universe? If we want to find life on an "Earth-like" planet, what should we be looking for? What does "Earth-like" even mean? How often do we expect to find planets that have undergone a Moon-formation-like event, or a Late Heavy Bombardment? That's where my research comes in.

Cosmic Collisions

The question you should be asking yourself at this point is: do other planetary systems undergo these same sort of collisions? As it turns out, they do. But the evidence of these collisions is hard to find.

In 1983, IRAS (InfraRed Astronomical Satellite) was launched. IRAS is an instrument that images the night sky in the infrared. Infrared wavelengths are used to see objects that emit thermal radiation (night-vision goggles help you to see infrared radiation). When we pointed this telescope at Vega, a nearby star, we noticed something strange. Vega is a massive star - more massive than the Sun. It should look quite blue. But when we looked at it with IRAS, we saw that it was also emitting a ton of light at infrared wavelengths. As it turns out, that infrared light was coming from a giant belt of debris in orbit around the star, that was being heated up by the star itself. When we looked at other nearby stars, we noticed the same thing. We called it the "Vega Phenomenon."  

It is very difficult to distinguish the thermal light from the dust and the starlight itself. Most of the time, all we get as astronomers is a single point of light.But we can use spectroscopy to split the incoming light into its composite colors, in order to try and figure out how much of the light we're seeing is actually being emitted by the dust. To do this, we create something called a "spectral energy distribution", or SED. Typically, this is how new debris disks are found - by looking for the slight bump in the infrared that means that the star is emitting more light at those wavelengths than it should. 

In some special cases, however, the debris disk is bright enough and the star it close enough to Earth that we can actually take a picture of the disk itself. This is called a "resolved image." To see the dust, we need to be able to subtract the light from the star out of our picture. The problem is that a picture of a star taken with one camera won't be the same as a picture of a star taken with another camera. Each camera adds a characteristic "blurring" of the star to the picture called a "point spread function", or PSF. To remove it, we need to make a model of the PSF. Once we do that, we can finally see the light coming from the dust.

There's a lot we can tell about the dust just by looking at an SED, and even more that we can tell by looking at the image of the dust itself. We can tell how much dust is present, its temperature, and how close it is to its host star. These properties can help us to determine how that dust was formed. In some cases, the dust was created in a giant impact, much like the collision that formed our Moon. In other cases, the dust was formed through billions of small collisions between comets and asteroids, probably during a Late Heavy Bombardment type of event.