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!
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