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Episode 7: Eye Spy a Planet

17 Apr 2009, 04:00 UTC
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Up until the 1990s, we only knew of the planets in our own solar system. Since then, we have discovered over 300 planets orbiting other stars. However, most of these planets were found when scientists observed the effect of the planet's gravity upon their host stars. Astronomers could not show the world what we wanted most: a visible light picture of a planet around a star like the Sun. That situation changed in November 2008 with a discovery by the Hubble Space Telescope. Join us for the story that begins a new era in our knowledge of planetary systems.

Hubble press release:

Hubble Directly Observes Planet Orbiting Fomalhaut


Note that Hubble's discovery of Fomalhaut b is billed as the "first visible-light snapshot of a planet orbiting another star." It is important to note that the first direct detection of a planet will likely turn out to be the planet known as 2M1207 b. However, the host, 2M1207, is not a full-fledged star, but a brown dwarf (see below). In addition, pictures of three planets around HR 8799, released the same day as the Fomalhaut discovery, were taken in the infrared.

Let me clarify about 2M1207. It has less than 3% the mass of our Sun, roughly 25 times the mass of Jupiter. That mass places it in the brown dwarf category: large enough to ignite deuterium fusion in its core (thus not a planet), but not large enough for hydrogen fusion (thus not a star). Brown dwarfs glow faintly at formation and then spend the rest of their lives cooling and fading away. Brown dwarfs are generally thought to be those objects with between 15 and 70 times the mass of Jupiter.

I wanted to make a joke that what the Hubble image of Fomalhaut looked most like is the "Eye of Sauron" from the "Lord of the Rings" movies. However, New Line Cinema did not respond to my requests for permission, and my producer would not let me use the image in the podcast. That joke is one reason why the episode is called "Eye Spy."

A betting astronomer might have chosen Beta Pictoris as the first star around which a planet would have been seen. We have been getting intriguing evidence that planets should be there for more than a decade. However, since the disk in the Beta Pic system is roughly perpendicular to our line of sight, any planets will travel in front of and behind the star from our point of view. Hence, we could only observe them well during parts of their orbits. Face-on systems, like HR 8799, are much more favorable for direct images.

If confirmed, the Beta Pictoris planet would indicate that giant
planets can form quickly. Beta Pictoris is about 12 million years old. We believe that giant planets must form within the first 10 or so million years of a developing system, as winds and radiation from newborn stars should remove the gas from the system on that timescale. A giant planet needs to accrete some of that gas during its formation, and thus must form in millions of years. In contrast, it is thought that Earth may take as much as a couple hundred million years to form.

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