Pale Blue Dots

Do you want to see an alien solar system?

Those roving dots are four real planets, orbiting a very young star called HR 8799 about 129 lightyears from Earth. The star itself is hiding under the black circle to block most of its light (i.e. it’s not added in post), so the telescope can actually pick out the much, much, much dimmer planets and debris disk (which is responsible for all that other visual activity).

This animation was created by astronomy grad student Jason Wang (UC Berkeley) based on a sequence of eight images taken over the span of seven years by the Keck Observatory atop Mauna Kea in Hawai’i – it came across my Twitter feed today courtesy of astronaut Chris Hadfield.

The HR 8799 system is special in a lot of ways – the star itself is relatively unique by having a lack of non-Hydrogen/Helium elements1 in its upper atmosphere, and a varying luminosity due to parts of its surface moving inwards while other parts move outwards at the same time (i.e. it pulsates) – but this was the first multi-planet system directly imaged by astronomers.2 The announcement of their discovery – or rather, that of the first three planets – came on November 13, 2008. The closest planet, designated HR 8799e and 15 times further from its star than Earth is from the Sun, was found a couple years later.

All four planets have masses larger than Jupiter, and generate light in the near-infrared (the kind of light thermal cameras detect) as they continue to undergo gravitational collapse in their young age. However, their compositions, from what astronomers have detected, do not directly correspond to any known astronomical objects. This study reports the (“tentative”) presence of methane, ammonia, acetylene (aka ethyne), and possibly carbon dioxide and hydrogen cyanide, though each planet has its own set of only some of these gasses.3 In short, you wouldn’t want to vacation on any of them.

I mention this, in part, because of how journalists jump onto the phrase “Earth-like” whenever astronomers release information pertaining to the discovery of a planet with the approximate mass of Earth, and especially when its orbit places it in the so-called ‘Habitable Zone’ (where it’s just the right temperature for water to exist as a liquid on the surface of the planet). It happened most recently, I think, when we heard about a possible planet orbiting the next star over, Proxima Centauri.

It also happened with Kepler-452b, located about 1,400 lightyears away. There was, perhaps, a bigger deal made with this exoplanet because it orbits a star very similar to our own (Proxima Centauri is a red dwarf). To date, it remains the most Sun/Earth-like duo we know of.

NASA released this concept art with their press release, which also made its way into most of the news outlet coverage:

452b_artistconcept_beautyshot
Credit: NASA Ames/JPL-Caltech/T. Pyle

Even if people know it’s an artist’s depiction, images like this can convey the idea that astronomers know way more about the planet in question than they really do.

News stories tend to ignore (or casually brush aside) that certain detection methods only give us some kinds of data. In the case of Proxima Centauri b, we only know its minimum mass (1.27 Earths worth) and equilibrium temperature.4 In the case of Kepler-452b – detected by a different method – we know its size (60% bigger than Earth) and equilibrium temperature. We don’t know Kepler-452b’s mass, so we can’t calculate its density to tell if it’s a so-called ‘superearth’ or a mini-Neptune. We also don’t know – if it is rocky – if it has an atmosphere, or what it’s made of. Venus is about the same size and mass as Earth, and technically is just barely inside the Sun’s Habitable Zone, but has a thick, 95% carbon dioxide atmosphere that traps enough heat to keep the surface a toasty 737 Kelvin (867 °F).

And finally, we definitely don’t know if there’s liquid water on the surface. Given the sheer number of planets out there in the cosmos, there definitely should be at least one planet that vaguely resembles Earth in size and composition – we just haven’t found it, yet. Or if we have found one, we don’t know we have.

Astronomers and engineers are working on developing new technology to collect more in-depth information about the couple dozen ‘Earth-cousins’ we’ve found so far. The JWST (James Webb Space Telescope), when it eventually launches5, will be looking at exoplanet atmospheres for part of its mission, but really only for gas giants, which we’re already doing. That’s why JPL might be proposing the Habitable Exoplanet Imaging Mission (HabEx) in 2020 – to directly image planetary systems around Sun-like stars.

I, for one, look forward to the day we can point to another pale, blue dot and call it Bob Earth 2.0, even if we’ll never be able to visit.

pale_blue_dot
The original “Pale Blue Dot”: Earth as seen from the Voyager 1 space probe, 6 billion km away from home. (Credit: NASA)            (If you’re having trouble seeing it, look right of center)

 


1. Astronomers, for reasons I will never fully understand even having a degree in astrophysics, call these elements “metals”. According to them, carbon is a metal. Oxygen is a metal. Neon is a metal… 
2. Instead of the more ‘usual’ methods of planet detection, like observing the Doppler effect in a star’s light as a planet tugs the star back and forth (called the “Wobble Method”), or measuring the drop in starlight as a planet crosses in front of it (called the “Transit Method”). No matter the method, it’s always easiest to find more massive/bigger planets. 
3. They will also, of course, have plenty of hydrogen gas – the most abundant element in the universe by far – and some helium. 
4. That’s the temperature the planet’s surface would be if it were only being heated by direct sunlight. It’s only based on the type of star and how far away the planet is from it. Earth’s equilibrium temperature is a chilly 255 Kelvin (-1°F); we are, of course, warmer because we have an atmosphere that traps some of that solar radiation. 
5. I remember when it was supposed to launch in 2011… 

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