Ocean worlds?

All of Earth's water, liquid fresh water, and water in lakes and rivers  

"How inappropriate to call this planet Earth when it is clearly ocean" - Arthur C. Clarke

But is it?

While the surface of our pale blue dot is almost 71% ocean, accounting for over 96% of the total water of our planet, it only goes so deep. According to the USGS (United States Geological Survey) the total volume of the oceans if contained in a sphere would be around 1,385km in diameter, or 1/10 the size of the Earth. Not very big.

The remaining 4% of the Earths water is locked up in the ground, glaciers, and in fresh water lakes and rivers. The main image is from the USGS website showing the Earths water in a series of spheres relative to the waterless sphere of the Earth. The tiny sphere hovering over Georgia, US that you can just make out is all of the water that we use everyday in surface water lakes and rivers.

But considering the distinctive voids where ocean once was perhaps we can forgive Arthur C. Clarke for his outside perspective of it all. 

It is only once we consider the ~6400km of land beneath our feet that we see how meager the contribution of water is to our planet. It is in fact just 0.023% of the total mass.

So ocean it may have, but ocean world it is not.

Pacific ocean from space -
you could be forgiven for thinking it was a ocean world

What about the rest of our solar system? Water is an incredibly abundant substance throughout the galaxy and probably the universe so if it is not on the Earth where is it.

The gas giants in our solar system are as much as 40% water most of this is in the form of ices. In fact a huge portion of the solar systems water is locked up as ice be that in the shadowy depths of craters on Mercury, the polar caps of mars, or the hurricane force winds of Neptune.

The search for liquid water in our solar system takes us 628 million km away to Europa, a moon of Jupiter. Although no direct evidence has been found of liquid oceans beneath the moons frozen exterior it is likely that internal heating and radioactive decay could maintain a body of water beneath the shell of ice.

But is water is so abundant what about other solar systems, could exoplanets be covered in liquid oceans?

There have been a number of reports over the past few years, of potential ocean covered exoplanets Super Earths with no land mass, but water as far as the eye can see.

The first of these was GJ 1214b a planet 2.6 times the radius of the Earth with a mass 6.55 times that of our own planet, giving it a bulk density of 1.87g/cm³, 33% of the Earths.

GJ 1214b orbits a small dwarf star 20% the size of our Sun this means that although small the planet makes a large difference in the amount of light we receive as it transits its star. Thus we are able to look at its atmosphere as it transits through transmission spectroscopy – looking for the atmospheric imprint on the planets radius as it transits its star. A number of studies have been done on GJ 1214b with little luck as to defining characteristic absorbers in its atmosphere. Rogers et al (2010) makes an important distinction for this potential ocean world – “an important conclusion from this investigation is that, under most of the conditions we considered, GJ 1214b would not have liquid water.” With an investigation by Berta et al. 2011 showing a flat transmission spectrum and concluding “The high temperatures and high pressures would form exotic materials like ‘hot ice’ or ‘superfluid water’ – substances that are completely alien to our everyday experience.” So don’t pack your dive gear just yet.

More recently the NASA Kepler team announced a number of planets Kepler-62e and Kepler -62f as well as Kepler-69b and Kepler-69c as potential ocean worlds. Although there were a number of press releases regarding these discoveries there is little to no evidence to support any strong conclusions as we do not even know the mass of these two planets just the radius relative to the star and the orbital period and distance. Relatively little information when we consider the hidden nature of our own solar system planets when they are right at out fingertips.

Follow up observations of exoplanets and our own solar system, with missions like Juno which will reach Jupiter in 2015, will continue to reveal the nature of waters role in our universe as well as opening up many new discoveries for the future.

This is a fantastic info graphic by Stephanie Fox showing the places in our solar system where water is present

What's next?

USGS - How much water is there on Earth - http://ga.water.usgs.gov/edu/earthhowmuch.html

Cfa press release for Kepler water worlds - http://www.cfa.harvard.edu/news/2013/pr201311.html

Snow line - where'd all the water go? - http://astrobites.org/2012/07/25/snow-lines-and-protoplanetary-disks-or-whered-all-the-water-go/

IMAGES: Main image credit: Howard Perlman, USGS; globe illustration by Jack Cook, Woods Hole Oceanographic Institution (©); Adam Nieman.

NASA

Our watery solar system info graphic by Stephanie Fox

Paper one - a PhD's journey

Each journey to the publication of your first peer-reviewed paper for a PhD student can be wildly different.

So in an act of relief or perhaps frustration I thought I would share with you the journey mine took.

I think it would be fair to say that the language I use is a reflection on the emotions I went through at the time but as we all know hindsight is a wonderful thing, so hopefully I manage to round it off with a more level view of the ordeal. See there I go already.

Over a year ago on July 9^th I received my first set of data from the large Hubble Space Telescope program headed by P.I David Sing, my supervisor. The data is near-infrared low-resolution grism spectra of the hot Jupiter host star HAT-P-1 from Hubble’s Wide Field Camera 3.

My job is to see if we could identify any chemical signatures in the atmosphere by producing a transmission spectrum, a measurement of the planets radius at a number of different wavelengths.

After reading a lot of papers in great detail and dissecting a few codes so I could write my own, I had my preliminary results in place. Just in time for my 2^nd year assessment presentation at the beginning of October to the whole college of Physics. As I am still here I am going to assume it went well.

Next is when the delusions start. I remember thinking great I have results. I had the models from the theory fairies and it all looked great. We had water!

The hard part was over; I could now go on my holiday leaving my first draft with my supervisor to look over.

I know what you are thinking, poor sad naive/delusional student, don’t worry so am I now.

I remember arrogantly wondering how other people had such trouble and that it all was very easy - I had my transmission spectrum, I had a pipeline to reduces the data quickly, and the paper is written – it will be out in no time.

You will be glad to know my internal monologue and ego have been taken down a few pegs since then.

Over the Christmas break I received similar results from a few of our collaborators, which nicely backed up my own. So 6 months after the data was received I sent off the paper to the CO-I’s, people whose names would appear on the paper who either directly contributed to the content of the work or were part of the group who acquired the data used.

Now as a lowly PhD student you are on the lowest of the unevenly distributed rungs of the academic ladder and with an increasing number of CO-I’s the percentage of responses you will receive reduces considerably. So deploy your patience and give it some time it will more than likely be worth it. By the middle of February I had naively named my LaTeX document ‘final draft’ and vowed to never have to run my seizure inducing code again - in hindsight perhaps removing all of the plotting commands would have been the best idea but seeing exactly what is going on in your code is very useful.

Of course this bout of confidence would not last long and just 3 weeks later I was on ‘final draft 7’. Which turned out to be the lucky one that once again got sent around to the CO-I’s; this time with a two-week deadline, which of course they frustratingly defied.

I think it is safe to assume that this was when my housemate learnt to not respond to my external monologue as I paced around the apartment with pen and paper in hand endlessly checking for mistakes or searching for clarity. Only one problem, now I can never get him to respond as he assumes everything I say is my brain externally questioning itself and the world around it – to be honest it is scarily a more accurate assumption than it once would have been.

The Trumps card for our observations

Finally on April 4^th I uploaded the manuscript to MNRAS and satisfyingly clicked submit.

I wish I could tell you that were the end, I truly do, but peer review rightly requires a peer response. Now when I say peer I mean someone else on the academic ladder, and as I am a PhD student and this is my first paper, that peer will be several seemingly unattainable rungs above me on that ladder.

Once you have submitted your paper and a reviewer has been selected you just have to sit and wait. We were lucky to get a response within a month and quickly got back to work. The reviewer recommended moderate revisions before it could be reconsidered for publication, which is the middle ground between major and minor revisions. I was really happy with some of the comments, I remember printing them out and highlighting them according to how much work needed to be done. There were a few that would require a lot of work and it took over a month with over 100 program re-runs before the responses were diplomatic and constructive enough to send back with the revised version of the manuscript.

The comments and the revised manuscript then are sent back to the reviewer for them to look over your changes and are given an additional 3 weeks to respond. They can then either then recommend more revisions or suggest to the editor that the manuscript is ready for publication. Just before the 3-week deadline we received a second report again with moderate revisions, something I would have been again happy with if only the response was logical and correct which, as I am sure you can guess by the tone it was not. I was having my first lesson in scientific politics.

After taking the evening to calm down in the pub with a glass of wine and some friends I sat down to write the response. Luckily my friend was on hand to moderate my responses before I could even show them to my supervisor. Let’s just say that the phrase ‘it appears the reviewer has misunderstood …’ was used in the response more than once. Which is a far more diplomatic statement than I could conceivably come up with myself. But the changes were quick and trivial so to appease the reviewer we made them and re-re-submitted it the next afternoon.

Four days later we got on a plane headed for St Andrew’s and this years RAS National Astronomy Meeting where my work would be presented as a poster and through a press release along with a colleagues work.

Three weeks later at the end of the reviewer’s deadline we received another set of comments and a re-re-re-revision was made. This time with some more trivial corrections, adding plots, tables, and references, I waved it goodbye and revision III was submitted.

But we have still not quite reached the end; I did warn you it was an ordeal.

Another three weeks passed and we received our 4^th set of comments from the reviewer, thankfully this time with only ‘minor’ revisions suggested before consideration for publication was suggested. Those minor suggestions, ‘Please add in reference XX and reference YY to the manuscript’ and guess what, those papers had an author in common with all previous papers suggested by the reviewer in previous comments – it is safe then to assume whom it is at that point.

Water, water, everywhere - well at least in HAT-P-1b's atmosphere

And it is here that you find me over a year since I first received the data to the day that it is up on Astro-ph/ArXiv. Although it has been an incredibly frustrating journey, for which I am forever marked with my first grey hairs, I am truly grateful because although patience is most certainly still not in my nature some aspects of caution have been added, and the paper is now far better than it was before.

See I told you I could end on a happier more leveled view of it all.

Thank you for joining me on the journey let’s hope the next one is not so stressful as I would very much like to keep my brown hair just a little longer.

Hubble Space Telescope hot Jupiter Transmission Spectral Survey: detection of water in HAT-P-1b from Wide Field Camera 3 near-infrared spatial scan observations à H. R. Wakeford, et al. 2013

An outside perspective

A picture of our pale blue dot taken by Cassini in orbit around Saturn

Could we detect changes in the Earths atmosphere if we were looking at it as an Exoplanet?

In all honesty, right now, no. 

Our best option would be to catch the Earth transiting the Sun. If we then had an instrument like Kepler at our disposal - and a tremendous amount of patience – our pale blue dot may reveal itself. This, however, would reveal a meager amount of information about our vibrant and teeming world reducing it down to an orbital period and radius.

When the Earth transits the Sun the amount of light received by the telescope is reduced by just 0.0084%. The Earth would take 13 hours to transit the Sun at its orbital distance (1 AU), and you would have to wait a year for it to transit again.

The NASA Kepler telescope has thus far discovered 32 planets with a radius smaller than twice that of the Earth. Bear in mind that our next biggest planet in the solar system is Neptune which has a radius of ~3.8 Earths. The closest Kepler has found to our look-a-like is Kepler-20f. Kepler-20f is in orbit around a star very similar to our own, and although the planet is Earth-sized in both radius and mass, it orbits its star in just 19 days giving it a temperature in the range of 700K, similar to that of Venus’ lower atmosphere. Kepler-20f has three other planetary siblings orbiting between it and its star with a fourth orbiting further out all however are still confined to a space just slightly larger than the orbit of Mercury around our Sun so perhaps there is another planet further out we have not been able to see just yet.

Only one of the confirmed Kepler planets is at an orbital distance of 1AU, the distance from the Sun to the Earth, with an orbital period sitting between that of Venus and the Earths. The planet, Kepler-34b, is however, 7.8 times the radius of the Earth with a mass nearly 70 times that of our own planet.

The NASA Kepler mission, although no longer running, has still given us more than enough data to sort through over the next few years that will no doubt yield more and more planets. With 3548 planetary candidates in their catalogue I am sure the list of the most Earth-like will become more and more refined.

If aliens like us are doing the same thing and looking in our neck of the woods then give them a few years and they might find us too.

So when you look up at the sky give it a smile you never know when they will be taking a look.

What's next? 

The NASA Kepler website is a great place to get information on the mission so check it out if you want to know more. 

http://kepler.nasa.gov/Mission/