View Full Version : Exo-Oceans Not Detectable ?

2012-May-09, 09:44 PM
A recent study discusses the various problems of using specular reflectometry as a method for remotely detecting liquid water 'oceanworlds' ...

A False Positive for Ocean Glint on Exoplanets: The Latitude Albedo Effect. (http://arxiv.org/pdf/1205.1058.pdf)
(N. Cowan, D. Abbot, A. Voigt, May 2012).

The main issues are summarised in the initial Abstract section of the document.

The conclusions point out the limitations of the technology:

The latitude-albedo effect is a particularly convincing glint false positive for zero-obliquity planets, and such worlds are not amenable to latitudinal mapping. This effect severely limits the utility of specular reflection for detecting oceans on exoplanets.


Zero-obliquity planets are the worst case for three reasons:

1) variations in dominant latitude are large and symmetrical,
2) poles receive the least orbit-averaged flux, and are therefore most likely to harbor year-round snow, and;
3) these planets are not amenable to latitudinal mapping, because the sub-stellar point is always equatorial.

Although planets with negligible obliquities will likely be a minority of directly-imaged worlds, they are the norm in the HZ of low-mass stars. The latitude-albedo effect will therefore be an important glint false positive for missions characterizing the reflected phase variations of temperate planets orbiting nearby M- Dwarfs.Whilst this issue is a function of the exo-planet orientation from Earth's frame of reference, it might also be in part, a technology limitation. ('Twill be interesting to see how the JWST goes in performing such measurements).

Even so, even if an 'ocean-world' is detected, what then ?

More exo-life inferences and more speculation ?

How reliable would that speculation then be ?


2012-May-09, 11:50 PM
I think 'not detectable with current technology' might be a more accurate statement. 50 - 100 years from now we could very well have the means to detect and measure exoplanet oceans; reliably and with no speculation necessary. Although I'm not sure that speculation and inference given the current state of exoplanet discovery and research is a bad thing. How many good ideas come out of speculation and inference? Heck, bad ones too, but that's where the science comes in.

2012-May-10, 01:20 AM
Although I can't speak to the nitty-gritty details of this paper, it appears to be a well reasoned exploration of the inferences that can be drawn from a limited dataset and a particular scenario. For projects that require limited, expensive observing time, all of the possibilities for false positives need to be fully explored. Such studies will be combined with knowledge of the detector's capabilities and limitations. I would expect that any proposal for observing time would require extensive analysis such as this. One of the most interesting talks that I have watched was one given at SETI before Kepler started observing. A majority of the lecture was devoted to "How can we be fooled?" Such analysis helps prevent excitement and confirmation bias from taking too strong a foothold.

Given that we wont have an image beyond a pale blue dot any time soon, we need our conclusions based on several different kinds of data and lines of reasoning. We have to remember that we are at the beginning of our exploration of exoplanets. As someone whose childhood book on the planets had a map of Mars with all the the canals named, just getting data from an exoplanet is exciting.

With regard to speculation, as long as it is labeled as such, I see no harm in it. Imagination and curiosity fuel our desire to explore, and careful analysis keeps us grounded. "Imagination in a straitjacket" as Feynman would say.

2012-May-12, 08:49 AM
Waterworlds, ocean planets, or panthalassic worlds, as they are variously called, are quite likely to be covered in a layer of water droplet clouds, like the Earth is. These clouds would interfere with the detection of any specular glint, so I would expect glints to be an unreliable indicator of the presence of a water ocean. In most cases I'd expect the presence of a deep ocean to be determined by density calculations; the presence of a shallow or partial ocean would be more difficult to detect.

But as I've suggested elsewhere, ocean planets may not be good places to look for life-the conditions in which abiogenesis may occur are not well known, and may not be found in such dilute environments as an all-ocean planet.

2012-May-12, 12:07 PM
what then?
well, we will have detected an exo planet ocean world with reflectometry ... how cool is that!
im just a lay person, but come on, surely the point here is that we are at the very beginning of trying to study other planetry systems. Everything learnt id assume will go towards modeling sytem types relative to star types, eventually maybe one day seeing something that dosnt fit the expected model which may mean something interesting.