Comet Siding Spring Meets the Red Planet

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This composite Hubble Space Telescope image captures the positions of Comet Siding Spring and Mars in a never-before-seen close passage of a comet by the Red Planet, which happened at 2:28 p.m. EDT October 19, 2014. On that date the comet passed by Mars at approximately 87,000 miles (about one-third the distance between Earth and the Moon). At that time, the comet and Mars were approximately 149 million miles from Earth. Courtesy NASA, ESA, J.-Y. Li (PSI), C.M. Lisse (JHU/APL), and the Hubble Heritage Team (STScI/AURA)

One of the benefits of having an amazingly keen-eyed space telescope like Hubble orbiting Earth is that you get great scenes like this one — Comet C2013 A1 Siding Spring passing by the planet Mars on October 19th. Hubble Space Telescope is actually so sensitive that scientists had to adjust the exposure to get the image. In addition, the scene is a dynamic one; that is, the comet and planet are moving with respect to each other, and a single exposure of the view was impossible to get without some blurring. So, the telescope took a series of images which were then composited together to get this interplanetary selfie. The image was taken using the Wide Field Camera 3. For more information, check out the Hubble press release page for the image.

Comet Siding Spring is getting closer to the Sun and will reach perihelion (the closest point in its orbit) on October 25th. After that, it will head back to the outer solar system on its lengthy orbit. The comet originated in the Oort Cloud and has spent at least a million years on the inbound leg of its first orbit since leaving its birthplace. Its orbit could be changed through gravitational interactions with other planets in the future.

Beta Pictoris has Comets

An artist's concept of comets around the star Beta Pictoris. The HARPS instrument at ESO’s La Silla Observatory in Chile was used to make the most complete census of comets around another star ever created. A French team of astronomers has studied nearly 500 individual comets orbiting the star Beta Pictoris and has discovered that they belong to two distinct families of exocomets: old exocomets that have made multiple passages near the star, and younger exocomets that probably came from the recent breakup of one or more larger objects.  Courtesy ESO/L. Calçada

An artist’s concept of comets around the star Beta Pictoris. The HARPS instrument at ESO’s La Silla Observatory in Chile was used to make the most complete census of comets around another star ever created. A French team of astronomers has studied nearly 500 individual comets orbiting the star Beta Pictoris and has discovered that they belong to two distinct families of exocomets: old exocomets that have made multiple passages near the star, and younger exocomets that probably came from the recent breakup of one or more larger objects. Courtesy ESO/L. Calçada

The southern hemisphere star Beta Pictoris has been one of the great “go to” places for exoplanet stories in the years since it was discovered to have a ring of debris around it by the IRAS satellite in 1983. To be specific, IRAS found excess infrared emission around the star, which indicated the presence of the disk. The disk has been imaged from ground-based observatories, and studied by Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer (FUSE), among others. The Hubble studies showed that Beta Pictoris has no one, but two disks of material, and it’s very likely that the materials in this disk are in the early stages of planetary formation.

In a spectacular study just released by the European Southern Observatory, astronomers using the HARPS “planet hunter” instrument at the La Silla Observatory in Chile have made a census of comets orbit Beta Pictoris. They studied about 500 of these “exocomets” and grouped them into two “families”. One is a group of old comets that have been around the star many times and have been heated and lost some of their mass. The other is a collection of young comets that are the result of the breakup of larger icy objects orbiting the star. This is the most extensive survey of comets around another star ever made and may well help us understand the role of comets in the early stages of solar system formation.

Beta Pictoris is a very young star system — only about 20 million years, which makes it an infant, as stars go. By contrast, the Sun is 4.5 billion years old, and the entire solar sysem may have looked very much like Beta Pic back when it was a baby.

The disk surrounding Beta Pictoris is filled with gas and dust, plus larger particles and planetesimals. As astronomers watched this star over the years, they noticed that the star itself would ever-so-slightly dim as something passed in front of it. The star does have a planet, but many of these dimmings did not correspond with that planet’s orbit.

What were they? It turns out that they were comets. Yes, comets can dim a star’s light as they evaporate. Their gases and dust are perfect “light traps” to capture starlight as it passes through, thus causing the star to seem just a little bit less bright for a short time. The science team studied the light to determine the types of comets the instrument detected.

So, the age of exoplanet discovery has expanded into the age of exocomet studies. And, as I recall from another news story, it joins the age of asteroidal dicovery around other stars. All of these ages not only help us understand the formation of planets and objects around other stars, but give us a fascinating look back at how our own system must have looked nearly 5 billion years ago.