Eyes in the sky – looking at exoplanets

Anyone who’s been paying attention this last year will know that it’s been a turbulent year for space telescopes. Turbulent, but not without some unexpected windfalls. One in particular is that earlier this year, the US National Reconnaissance Organisation (NRO) donated two space telescopes to NASA. These telescopes are both flight worthy, effectively brand new, and Hubble-class, each boasting a 2.4 metre mirror much like the world’s best known space telescope, with comparable field-of-view and aperture size. So the question is, how best to make use of these new trinkets?

Hunting for exomoons!NASA decided that the best thing they could do was to ask the general community, and so they’re accepting submissions from literally anyone willing to prepare one. I really do love the way NASA work sometimes. Now, apparently, one idea being investigated is to use these two to do a wide-field infrared survey. Yes, another one.* Quite frankly, I think that this would be boring, and an incredible waste of the potential of these two telescopes. You see, these two working together could quite possibly allow us our best view yet of an extrasolar planet. Allow me to explain…

There are two really important things with telescopes. Sensitivity and resolution. Generally, these are both defined by the telescope’s primary mirror – a larger mirror yields both higher sensitivity and greater resolution. But here, we have two telescopes. Two or more telescopes can be linked together to act as an interferometer, enabling a marked increase in resolution, albeit with no significant increase in sensitivity. Now, the resolution becomes defined by the baseline, or in other words, the separation between the telescopes being used together. Currently here on Earth, the most impressive example of an optical interferometer is the Very Large Telescope (VLT), whose individual telescopes can be used in conjunction to give some of the most impressive quality observations available. But from the ground, telescopes are limited by the atmosphere, which has an unfortunate habit of distorting light.

Seriously, this could give so much detail on exoplanets, it would be unbelievable!We are not talking about ground-based telescopes though. These two are spaceborne, and will have no such limitations! In other words, a space-based optical interferometer would be able to operate at the instrument’s theoretical limits. So consider this…

This diagram shows a scale model of the Earth-Moon system. With the Moon’s surprisingly distant orbit of ~384400 km, gravitational interactions with Earth form a series of 5 gravitationally stable points called Lagrange points. The two shown here, L4 and L5 could provide stable locations to park the two telescopes. They could them form an optical interferometer with a combined sensitivity twice that of Hubble, with a baseline of ~665800 km!

Theoretically, that’s exciting enough to make exoplanet scientists start dancing on tables. But exactly what does that mean in plain English? Let’s crunch a few numbers. The Sun’s near neighbour Tau Ceti has recently been found to potentially host 5 planets, 2 of which may be suitable for life. With a baseline of 6.658 x 108 m. At optical wavelengths, this gives an angular resolution of 2.032 x 10-10 arcseconds. Tau Ceti is 12 light years away, so at that distance, this interferometer would be able to resolve details as small as 6407 m. Details as small as 6.4 km on planets light years away could theoretically be seen.

Putting that into perspective, Greater London is about 40 km from side to side, meaning that if an interferometer like this was to look at Earth from Tau Ceti, assuming a clear day** and an appropriate viewing angle, it would theoretically be able to resolve the city of London on Earth’s surface. Larger surface features like continents and cloud patterns would be easy to pick out.

Of course, there are other issues to consider. Sensitivity may be an issue, as may the glare from any exoplanet’s host star***. But honestly, I can’t see any problem with this setup. If any stalwart survey astronomers really insisted, the telescopes could just as easily survey the sky from the Earth-Moon L4 and L5 points as they could from anywhere else!

So I ask you, dear readers. What do you think of this idea? Can you see anything I’ve missed? Any flaws? Any other comments or criticisms?

Time as we consider it really would cease to have much meaning for an interplanetary traveller.

NASA’s official release on their call for submissions is available to read here. ANyone wishing to submit any ideas (as I intend to do with this one) have until January 7th to do so!

* Yeah, I’ll freely admit it. I find surveys crushingly tedious, and I think the idea of doing YET ANOTHER ONE is entirely uninspiring.

** Perhaps the UK was a poor choice of target when hoping for a clear day…

*** There are a few fairly effective methods already in use to cut the glare from host stars. Nulling interferometry, for one.

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
This entry was posted in astronomy, physics, space, Tech and tagged , , . Bookmark the permalink.

2 Responses to Eyes in the sky – looking at exoplanets

  1. Harry R Ray says:

    I see a possible problem. Zodaical light. When NASA was planning their TPF interferometer mission, the planned to place it in an orbit BEYOND Jupiter’s to eliminate the problem.

    • invaderxan says:

      Thanks for your reply!

      Interesting… I didn’t realise zodiacal light posed a significant problem. I shall have to investigate further. Surely though, it would only be an issue when taking observations in the plane of the solar system? And also, I’d have expected it to be resolvable (at least spectroscopically) through the use of standard star observations, the same way ground-based observatories perform telluric corrections. Interesting if this is not the case – and I have no idea whether or not it would be.

      I’m unsure offhand what the intended resolution of the TPF was, nor for that matter which existing exoplanet missions (particularly Kepler) are looking in the solar system plane or not…

Comments are closed.