A Sea of Blue Planets?

This week saw the second Kepler Science Conference, which took place at NASA's Ames Research Center in Moffett Field, California, where the latest analyses of data from the Kepler Space Telescope were discussed. The meeting, comprising of nearly 400 scientists from 30 different countries, produced some seriously exciting findings, including the prevalence of planets around Sun-like stars with the potential to host life.

Scientists from the University of California and the University of Hawaii have for the first time statistically determined that 22±8% of Sun-like stars possess Earth-sized planets within the habitable zone, from compiling data from Kepler and the W. M. Keck Observatory. The estimate that one in five stars similar to the Sun host temperate planets further suggests that the closest one could reside just 12 light-years from us, according to Erik Petigura, a UC Berkeley graduate student who led the analysis of Kepler and Keck data. This could mean there are billions of Earth-like worlds in the Milky Way, according to the report published on November 4th in the Journal Proceedings of the National Academy of Sciences.

The overall count of Kepler candidate planets from the first three years now stands at 3538, a number which has increased by 29% since January 2013. This rapid increase in planets has allowed a new comprehensive investigation into habitable zone bodies, which was a primary mission objective of Kepler.

Of the 833 new candidate planets announced this week by the Kepler team, ten are less than two Earth radii and reside in the habitable zone of their parent stars. This adds to the list of five Kepler planets already confirming this criteria, and brings the grand total to 12 confirmed and 38 candidate planets that may be potential habitable worlds.

Top of the list of possible Earth-like planets is Kepler-62 e, which was discovered earlier this year and has been issued with an Earth Similarity Index (ESI) value of 0.83. Residing 1200 light-years away in a system of five planets, it is believed to orbit the inner part of the habitable zone. Neighbouring body, Kepler-62 f, is ranked as #10 on the Planetary Habitability Laboratory's list of most habitable planets with as ESI of 0.67, and is thought to orbit the outer edge of the zone. With a radius of 1.6 RE and estimated surface temperature of 31 degrees Celsius, many researchers have postulated Kepler-62 e to possess a silicate-iron composition similar to the Earth's and possible oceans present on the surface. A recent paper submitted by Kaltenegger et al. discusses the potential environments of both bodies in detail: http://arxiv.org/abs/1304.5058.

Of course, just because a world lies within this zone it doesn't necessarily mean that it's habitable. The interior and atmospheric composition of the planet might not possess the key elements required to create or sustain life, or conversely could contain chemicals that prevent it from flourishing. There is also a long list of other factors that affect habitability, such as geochemistry, atmospheric structure and dynamics, orbital dynamics, evolution and variability of the host star, the evolutionary path of the planet itself, and also external factors such as bombardment. On the other hand, we also know that habitability cannot simply be restricted to the environment that applies to us, and that life may exist below the surface, or on moons orbiting large gas planets, far beyond the edges of the habitable zone.

Another exciting topic covered at the recent Kepler Science Conference was the future of the instrument itself. Without enough reactor wheels operating to continue detailed observations over a long time period, Kepler's future was thought to be bleak, but a recent evaluation suggests it may not have seen the last of its planet hunting days. The instrument is still able to point accurately at any patch of sky, but without its precision steering capabilities is unable to control its own rotation. The disturbance causing Kepler to spin is solar pressure, which falls unevenly on the craft and so gradually influences it to turn. The Kepler II team have found that when correctly positioned, the instrument can receive almost even amounts of sunlight on both sides and remain relatively stable for 2-3 months at a time, before the Sun obscures its view. As Kepler must detect at least three orbits of a planetary candidate, this means it will now only be searching for planets with much shorter orbital periods of 20-30 days. These periods correspond to bodies that reside much closer to a star than that of Mercury to the Sun; however, temperate planets can still be discovered within this constraint around dimmer red dwarf stars, where the habitable zone lies much nearer. This means that Kepler may continue its search for hospitable worlds but around a different target list of stars where we could discover an even more exotic array of planets. The proposal will be put to NASA soon for the funding evaluation process to begin, meanwhile tests have already begun on the new steering method. 

On top of this potential new endeavour, there is still over a year's worth of collected Kepler data yet to be analysed, from which hundreds more candidates are expected to arise. 

Happy One Thousand Exoplanets! ... (Probably)

The biggest news in exoplanet research this week is the discovery of the 1000th planet beyond our solar system, according to the comprehensive list at Exoplanet.eu. The Extrasolar Planet Encyclopedia, maintained by Jean Schneider of the Paris Observatory in France, has this week added the latest planet confirmations bringing the grand total into four digits. So what does this new milestone mean for exoplanet enthusiasts?

This was certainly an unforeseeable achievement for planet hunters back in 1992, following the first confirmation of a planet orbiting another star. The pace at which discoveries have ensued, depicted below, displays a promising future for prospective seekers.

The once short list, comprising of just our closest neighbouring planets, has rapidly evolved in little over 20 years to a plethora of exotic discoveries spanning the Milky Way. This week's figure marks the first time that we can collectively celebrate these many different worlds; however, there is a lot of ambiguity when it comes to placing such a grand title on the '1000th' planet.

Schneider's arguably optimistic list adds confirmed planets after their existence is presented at scientific conferences. NASA's database on the other hand, currently listing only 919 exoplanets, requires journal publication first. Other archives such as exoplanets.org list even fewer, and confirmed planets found in any of these catalogues are not necessarily set in stone. For a newly discovered planet to be listed as confirmed, its detection must be made with a five-sigma confidence level, meaning there is a one in a million chance that it is a false positive. General verification of planetary candidates result from multiple detections, in some cases via two separate techniques or instruments. Although rare, data errors have already seen several confirmed signals demoted back to noise.

Not only does this number depend on accurate signal retrieval, but also on the criteria we give to define a planet. A planetary body is not strictly required to orbit a star, but may instead be free-floating like the recently discovered body PSOJ318.5-22, the closest interstellar planet discovered to date. As well as location, there is also some controversy surrounding the upper mass limit of the largest planets. Currently, beyond 13 MJ the body is generally considered a brown dwarf; a failed star that is too cool to begin nuclear fusion. The uncertain nature of this boundary could later result in several so called 'super-Jupiters' to be removed from Schneider's catalogue. In contrast, low mass future discoveries may face the same problem that befell Pluto in 2006, and consequently be demoted to dwarf planets.

So where within this vast numbers of discovered exos is the 'Earth-twin' that many planet hunters are eagerly awaiting? Whilst currently we are still without the perfect match, the Planetary Habitability Laboratory at the University of Puerto Rico, maintains a rank of confirmed and candidate planets in order of their expected Earth-likeness. Top of the list are planets which may sound familiar: Kepler-62 e, Gliese 667C c and Gliese 581 g, all possessing an Earth Similarity Index (ESI) value of over 0.8, where 1 represents an identical body.

To truly assess a planet's capability to host or sustain life, we must look towards the next generation of instruments that will probe the atmospheres of these unknown bodies. The favoured method used to discern the main atmospheric constituents of a planet is transit spectroscopy, where starlight passing through the planet's atmosphere along our line of sight is absorbed by specific molecules. This technique has successfully examined the atmospheres of large gaseous bodies, and even the warm Neptune-like Gliese 1214 b, but we are yet to investigate the atmosphere of a cooler terrestrial exoplanet.

Future instruments aiming to accomplish this task are JWST, due to launch in 2018, and candidate missions FINESSE and EChO. This means that over the next decade we should not only witness further rapid increases in the number of confirmed exoplanets, but we might also begin to assess just how unique our blue planet really is.

For those of you solely concerned with planet counting, the good news is that with over 3500 candidates awaiting confirmation, we can safely say that known planets are currently within this figure and on the rise. So happy ~1000 exoplanets, and may the search continue!