HR 8799

The planets of HR 8799 (labeled). The starlight has been suppressed and mostly removed (Credit: Project 1640, Oppenheimer et al.)

Using cutting edge techniques, a team of astronomers has directly imaged a distant system of four planets, and made history by obtaining simultaneous spectra of these worlds.

This first comparative look reveals that the objects each have distinct atmospheric compositions, none of which directly match any previously known class of astrophysical body.



Only a tiny handful of exoplanetary systems exist where we have been able to spot planets directly. Detecting their emitted, or reflected, light is awfully tough when it's millions to billions of times fainter than the radiation pouring off their parent suns.

One such system surrounds HR 8799 - a very young, variable, and moderately large star, weighing in at about 1.5 times the Sun's mass, and located about 128 light years away. Back in late 2008 it was announced that 3 planets had been imaged using advanced adaptive optics at the great Keck and Gemini telescopes in Hawaii. A couple of years later and a fourth planet was spotted.

These are not the kinds of planets we're used to. For one thing they are all hot - glowing still from their recent formation within the last 30 million years or so, with temperatures between about 800 and 1000 Kelvin (980F to 1300F). They are gas giants, but they weigh in at anywhere between about 5 or 7 times the mass of Jupiter to as much as 20-30 times. In fact, there has been some debate about whether they really are planets or more massive objects known as brown dwarfs.

Using older data from the Hubble Telescope the orbital periods have been estimated (NASA/Space Telescope)

This system also seems pretty alien if we look at where these planets orbit. Labeled b, c, d, and e in order of discovery, their present distances from HR 8799 are about 68 AU, 38 AU, 24 AU, and 15 AU - where an AU is the distance of the Earth from the Sun.

This means that the closest planet to the star, 'e', orbits about midway between where Saturn and Uranus orbit in our solar system, planet 'b' would be out where our Kuiper belt ends. It's a radically different architecture than what we're used to.

Getting a handle on the composition and atmospheric structure of these mysterious gassy worlds would be a tremendous advance. A spectrum of the light from such hot worlds would provide just such a fingerprint. But efforts to measure this have had limited success, until now.

Enter Project 1640, a multi-year effort using the 200-inch wide Palomar telescope in California. It applies state-of-the-art adaptive optics to minimize the blurring effects of Earth's atmosphere, a sophisticated coronagraph to block HR 8799's light in order to reveal the planets, and a spectrograph that turns the pixels of each image into 37, 146 spectra.

The Project 1640 instrument (left) about to be installed at the 200-inch Palomar (right) (Credit: Project 1640/AMNH)

If that sounds technical, well it is. The upshot is that not only can the planets be seen in a system like HR 8799, but, with some skill, a spectrum can be obtained for each simultaneously - a direct probe of their actual composition and nature.

The results of this exploration are reported In a new paper by Ben Oppenheimer and colleagues, to appear in The Astrophysical Journal. It's pretty jaw-dropping. Although the four planets are glowing similarly bright, they are each quite different from their siblings.

There are signatures of compounds like methane and ammonia, but also of things that might be acetylene and hydrogen cyanide - it's a real mix. To quote Oppenheimer et al. - their analyses suggest that the planets are like this:

• b: contains ammonia and/or acetylene as well as CO2 but little methane.

• c: contains ammonia, perhaps some acetylene but neither CO2 nor substantial methane.

• d: contains acetylene, methane and CO2 but ammonia is not definitively detected.

• e: contains methane and acetylene but no ammonia or CO2.

You might be glazing over with this, so what does it mean? First, it means that these objects look more like planets than they do brown dwarfs. They're also clearly, and remarkably, distinct from each other - despite (presumably) all being big, hot, gas giants. The only one that looks vaguely familiar is 'e' - whose spectrum is a bit like that of the night-side of Saturn.

Exactly how and why these worlds are so varied is a juicy puzzle. The researchers suggest that it might in part be a result of ultra-violet light flooding the system in bursts from the youthful star HR 8799. A thousand times brighter than the equivalent from our Sun, this radiation can drive all sorts of chemical and physical changes in planetary atmospheres.

In fact there is tentative evidence for the planetary spectra changing over a period of just a couple of months. This could be the effect of the changing stellar radiation. It could also perhaps be that most familiar of planetary properties, the phenomenon we call weather.

This is a gorgeous piece of astronomy, and it represents a new era of discovery, one where the diversity of other worlds is going to keep us very, very busy.

Not bad for a spot of early reconnaissance.

[Note: Full disclosure - it's only fair to say that I have been familiar with Project 1640 since its inception, and count several of the authors as good colleagues, so I am naturally biased in my excitement]