Who was here? How can you tell? This is called indirect evidence!

How does a planetary system form? The one we can study in the most detail is our solar system. If we want to know whether the solar system is common, whether “habitable” terrestrial planets are common, we have to understand this.

This is the general picture of planetary system formation, the nebular hypothesis**; it is reasonably well understood and accepted. But the final “product” varies widely. **The Nebular hypothesis: 1. Solar nebula of molecular gas 2. Contraction into rotating disk 3. Cooling causing condensing into tiny (dust sized) solid particles 4. Collisions between these form larger bodies 5. These accrete to form planets

But is a system like ours always the result? And what makes up a “system like ours”, so we can compare? + rocky planets + asteroid belt (failed planets + dwarf planets) + gas giant planets + Kuiper Belt (failed planets + dwarf planets) We need more data points -- more solar systems around other stars -- for comparison.

Fortunately, there are lots of other solar systems, or other stars with so-called exoplanets. Number is always growing -- see http://exoplanets.org/ for the latest.

Frequency of stars with planets: ~30% according to Kepler Current number of exoplanets (planets orbiting stars other than the sun): 729 confirmed, 3430 additional candidates from Kepler, as of 27 Aug. 2013

If all solar systems were like ours, there would be clusters of points corresponding to our set of Earth, Venus, Jupiter, etc. But there are not, because the exoplanets detected so far have a wide variety of locations and sizes.

Ideally, we would like to get a snapshot of all extrasolar systems and know them as well as we know our own. But these cases are rare and difficult to achieve. The one thing the systems all have in common is also their biggest problem -- the central star whose light washes out all the fainter bodies. In this image, a team has cleverly removed the starlight that previously overwhelmed the brightness of the planets. These challenging pursuits are called high-contrast imaging.

With the number of systems available for direct imaging being so few, we have a look at the indirect evidence instead.

Definition of a debris disk: Disk of dust and ices resupplied after the formation of the planetary system. This is not the original disk material -- it has been resupplied by already-formed planets, failed planets, comets, and asteroids.

Dust is produced through collisions and through the sublimation of icy and dusty bodies (evaporation of ice to gas).

We know this is freshly supplied material because the dust cannot survive long, compared to the age of the star. It cannot be the original material from the formation stages; this was blown out by stellar winds long ago, after the first 3-10 million years. Sun’s lifetime will be about 10 billion years and is currently 4.6 billion years old. Planetary system including the rocky terrestrial planets formed in roughly 100 million years, gas giants even faster, within only a few million (before all the gas supply was lost). Terrestrial formation via mergers continued long after the gas sweep-up by the most massive planets like Jupiter. Fresh dust production by collisions or sublimation/evaporation is most active within the first tens of millions of years, but does not fully end as long as there are parent bodies to generate dust. The dust itself is lost relatively quickly, though, in only tens of thousands of years.

Part of the solar system’s debris disk can be seen in the night sky, the zodiacal light. Dust generated by the collisions and evaporation of inner-solar-system bodies (likely Jupiter-family comets, to be specific) is illuminated by the sun.

Why is the dust easier to see than larger planets? Small particles scatter the starlight, but larger bodies absorb it. Much easier to see an equivalent amount of material if it’s in small particles.

But we don’t want to see the light source itself -- We’re interested only in the particles that scatter the light. So we block the central light source (the star) with a coronagraph. (Named that because it was crafted originally to look at the sun’s corona, at the edge of the main disk of light) Like using your hand or a visor in the sun to better see your surroundings.

Here are some examples of disk images taken using a coronagraph. Note the variety of structures, which can arise from a number of processes within them.

Disk began as roughly uniform in the very early stages, and then what did not become planets was blown away. Thus any “new” structure that appears since then suggests that parent bodies have supplied it recently. Otherwise we would expect to see the original primordial dust disk, and then nothing, if everything was dispersed.

But look, they are complex. Here we are completely outside observers. Such small dust particles in any other location would be (and have been) blown out by the sun, but here they are safe, and sculpted by the satellites of Saturn.

This diagram is a reconstructed image based on the locations of known near-earth objects in the solar system. Given a large satellite orbiting a star, there are stable locations for smaller bodies. Outside of these locations (Lagrange points), they are likely to be ejected by the larger satellite’s gravitational influence. In this case, the large satellite is Jupiter, and there are two families of asteroids shepherded by it. An outside observer might only detect the dust clumps associated with the asteroids collisional destruction, but from that the presence of a larger body can be inferred.

For an outside observer, Neptune might not be visible, but its effects on the dust disk are.

Planets can exert a dynamical influence on the small dust particles, variable depending on their orbit shape. In this system, it is believed that the non-circular (eccentric) shape of a planet’s orbit has systematically pulled the center of the dust ring away from the star. With the center of the ring not at the stellar position, this means one side of the ring is closer to the star, and is therefore heated more (as seen at left). A proposed planetary companion has been detected multiple times and may be one of the influences on the disk structure.

Disk structure is thought to be shaped by the presence of planets. The disk of Beta Pic was long suspected to harbor planets, in part because of the disk warp (the secondary inner disk inclined with respect to the main dust disk). A planet was recently directly detected; however, it is still not enough to explain disk structure. Must be careful in drawing quick conclusions. Picture is incomplete -- cases are complicated.

New discoveries from our group, resulting from reprocessing data in the HST archive. Images were taken with NICMOS, the Near-Infrared Camera and Multi-Object Spectrometer.