Documentation and Methodology
The Exoplanet Data Explorer (EDE) gives Web users access to the Exoplanet Orbit Database (EOD). Our full methodology is here and is available on astro-ph. Description of major updates from the Wright et al. (2011) documentation can be found below (labeled with '*' in the Summary of Methodology section). Questions and comments should be directed to email@example.com.
What is an exoplanet?
An exoplanet is a planet that orbits another star (i.e., not the Sun).
A "free-floating" exoplanet is not exactly an oxymoron, rather a planet that once orbited another star, but has been ejected from its birth system, and is now wandering "in the field."
The definition of "planet" is somewhat contentious. The IAU has defined the term "planet" for objects orbiting the Sun in terms of gravitational properties: massive enough for gravity to force it into a spheroidal shape, and to be the dominant gravitational object in its orbit. Asteroids share similar orbits, and so are not planets; Pluto shares part of its orbit with much larger Neptune. Not all astronomers agree with this definition, but it is "official".
Outside the Solar System, there is no formal definition.
Many astronomers feel that an object is a planet if it formed like the planets in our Solar System (i.e., from a bottom-up process in a disk of material around the young Sun). There may be other ways to form planets, though (from gravitational instabilities of said disks, or, "top down"), and we can rarely (if ever) tell how planets formed, so this definition is not that useful, in practice.
Ignoring formation mechanisms (and composition), one can also set up a mass spectrum from low to high: asteroid, dwarf planet, planet, brown dwarf, star. We cannot yet detect things much smaller than planets, so the relevant boundary to define is between planets and brown dwarfs. Brown dwarfs are basically low-mass stars, and so may have fundamentally different formation mechanisms than planets, but probably have signifacnt mass overlap with them (that is, the smallest brown dwarfs are less massive than the largest planets).
The IAU recommended a boundary at 13 Jupiter masses, that being roughly the mass at which a solar composition brown dwarf can just, briefly, fuse deuterium in its core. This recommendation also stated that "free-floating" exoplanets be called "sub-brown dwarfs".
So it's complicated. Basically, an exoplanet is a planet that orbits another star. We use a cutoff of 24 Jupiter masses to be on the safe (inclusive) side.
Why should I use the EOD instead of one of the other lists of exoplanets?
If you are interested in:
- The most accurate orbital ephemerides for known planets
- A carefully vetted list of confidently detected exoplanets with well measured orbital properties from the peer-reviewed literature
- Transit data, such as planet-star radius ratios and transit durations
- An elegant and powerful plotting tool for presentation or publication quality plots
then we believe you should use our site. Other sites include announced planets whose properties and existence have not been scrutinized by peer review, and lack many of the stellar and transit fields our site has. Our site has also corrected many typos appearing in the literature.
On the other hand, if you want a comprehensive list of all planet announcements that have not been disproven so you would like to decide for yourself which planets to include in your sample; or you would like comprehensive bibliographies or lists of many measurements for various stellar parameters; or if you are looking for the data underlying these parameters, you may want to use another site (we list our favorite ones here). We use these sites regularly to keep up-to-date on planet discoveries, and we share information with them about literature errors. We consider these sites complementary, serving slightly different purposes from exoplanets.org.
Why isn't my favorite exoplanet/datum listed in the EOD?
Why does your site list fewer planets than site X?
There are four primary reasons:
- The EOD contains only carefully vetted, peer-reviewed data. If your favorite planet has not been published (or accepted for publication) in a peer-reviewed journal, it will not appear in this Database, with rare exceptions. Also, in a small number of cases we have made a judgement call that a peer-reviewed datum (or planet) is not up to the Database's standards, or uses an unproven method whose results we are not comfortable including. Basically, we have done a lot of work to keep an updated list of planets with "good" orbits by our own published and peer-reviewed criteria, and this database is our way to share that list with the public.
- We are not all-seeing; sometimes we miss an ephemeris update, or even a whole planet! If you know of a better source of data than the reference in our table (or find an error!), please let us know.
- We are not fast; it can take time for us to fully digest a new planet announcement, decide that it warrants inclusion, enter the data, verify the transcription, and sync up the EOD with the EDE. Summer updates may be especially slow.
Why don't the numbers in the EDE agree with the numbers in the reference / exported file?
The EDE displays data with a conservative "significant figures" algorithm. The exported CSV files contain the data at the "full precision" stored in the database, which should be consistent with the literature values to machine precision. Thus, if the literature lists a datum as 1001 with a standard error of 100, that will appear in the CSV as such, but the EDE will display "1000 +/- 100" because it deems the trailing "1" not significant. This concession is necessary to make the online tables easily readable; the machine-readable exported files have full fidelity to the literature.
We have also made some minor modifications to data to make it conform to our database standards -- for instance we restrict the domain of little omega to [0,360).
Why doesn't FIRSTREF refer to this prior press release / poster abstract / conference proceeding?
The EOD contains only peer-reviewed references; see below. The FIRSTREF field is not a "credit" field.
How do I make the Table/Plotter do what I want?
Try the help pages. User the left-hand column to browse various help topics. These pages include videos made for the Kepler Data Explorer. Though they differ in the available planet properties, the behavior of the Exoplanets Data Explorer and the Kepler Data Explorer are identical.
Why doesn't the "Export" button export all of the data in the EOD?
The "Export" button in the EDE Table exports the table as you, the user, have currently displayed it. If you would like the entire database, it is available in a single click on the front page, or here.
Some of the content below is included in Wright et al. (2011) documentation of the EOD. Objects that are labeled with "*" are updates from Wright et al. (2011).
We have recorded literature uncertainties in stellar masses, but when estimating uncertainties in msini and a we conservatively assume a minimum uncertainty in stellar mass of 5%. We do this to account for likely systematic errors in model estimates of stellar masses (limits in their accuracy) for most planet-bearing stars.
We now report asymmetric error bars throughout the database in
all fields. For quantities with asymmetric uncertainties from
the literature, we record the uncertainty field as half of the
span between the upper and lower limits. We store the
asymmetry in an additional field, which ends in D, as the
value of the upper uncertainty. For instance, e =
0.5+0.1-0.2 would be stored as three
Kepler Planets and Kepler Objects of Interest (KOIs)*
The extraordinary precision of the Kepler instrument and detailed ground-based follow up allows planets to be detected purely photometrically, without the usual spectroscopic orbital parameter K. Since these planets' reality can be established as well as any in the EOD, and since a precise ephemeris is available, we have opted to include Kepler planets in the EOD even in those cases where the planets' radial velocity signature has not been measured. In these cases we leave the K field blank in the EDE (set to NaN in the EOD). In cases where planets' masses can be constrained dynamically, or have upper limits due to a lack of RV signature, we supply the MSINI field with the appropriate values and reference the source of those numbers, instead of calculating MSINI from K.
Kepler Objects of Interest (KOIs)*
The Kepler Objects of Interst (KOIs) are stars that are suspected to host one or more exoplanets, but yet to be confirmed or validated. Although they are not counted as part of the EOD due to their status of being 'candidates', we include them in our website so that these KOIs are put into the context of all confirmed exoplanets. To display the KOIs in the EDE table, check the 'Kepler' option on the upper left of the table (default selection only includes 'Orbit Database' and 'Other'). The KOIs listed in our table include two past Kepler data releases and the most up-to-date release from January 2013. The original data are from the NASA Exoplanet Archive.
The three fields for identifying a Kepler planet or a KOI are KEPID* (the unique Kepler star identifier), KOI* (KOI object number), and KDE* (if true, the planet/planet candidate appears in the Kepler archive).
Planets Discovered via Microlensing or Imaging*
EOD now includes planets discovered via microlensing or imaging (their corresponding PLANETDISCMETH fields are 'Microlensing' or 'Imaging'). They can be displayed in the EDE table by checking the 'Other' option on the upper left of the table.
The planets discovered via IMAGING must satisfy the criteria below in order to be included in the EOD (as approved by the QUAlity Control Board of the Exoplanet Database, QUACBED):
- Planet-star mass ratio q < 0.024 (< 24 MJup for a solar mass star); and in general we require (q+σq) < 0.024, where σq is the uncertainty in q.
- SEP < 100 AU × (Mstar/Msun), where SEP is the semi-major axis a if a is known, and the projected separation otherwise.
- Confidently detected: the detection is clearly of a real astrophysical source, and will unlikely later be found to have been spurious.
- Confidently bound: in opinion of the QUACBED the object will not be later found to be unbound or a chance alignment.
We also record whether a planet is detected via MICROLENSING*, TIMING*, IMAGING*, and ASTROMETRY*. Note that a planet can have any of the four fields being true but have a different discovery method. For instance, some of the Kepler planets have TIMING = true, but they were not discovered via TIMING initially but with TRANSIT (PLANETDISCMETH = TRANSIT).
We now record GAMMA*, defined as the systemic radial velocity of the star-planet system center of mass with respect to the Solar System barycenter. It is not measured for all systems, and there are systematic uncertainties in these numbers below the 1 km/s level.
We record the published fundamental observables of single-lined spectroscopic binaries: period, RV semiamplude, and eccentricity, argument of periastron, and time of periastron passage. In a few cases of multiple planet systems, the best orbital parameters come from dynamical fits, and in a small number of cases (e.g. GJ 876) planet-planet interactions cause these elements to detectably evolve with time. In those cases where osculating elements for the planets are reported, we have recorded those, and the epoch at which they are valid can be found in the reference cited in ORBREF.
Where uncertainties in orbital elements are not reported in the literature, they remain undefined in our table.
From the orbital parameters and the mass of the host star we calculate the "minimum mass" of the planet from the mass function, as described in Butler et al. 2006. Note that we calculate this quantity separately for each planet in a multiplanet system, as though it were a singleton planet. For simplicity and consistency, we always denote this quantity "m sin(i)", though strictly speaking it is simply the minimum value for the planet's mass as calculated from the mass function.
m sin(i) will occasionally differ from the values listed in the paper cited in ORBREF. The most common reason for this is that we have adopted a different stellar mass than that listed in ORBREF. In other cases, inclinations are known from transits, or constrained through astrometry or dynamical considerations. In a small number of cases m sin(i) was misreported in the original paper.
For planets in systems which do not have radial velocity measurements, we calculate their m sin(i) values if there are constraints on the planet mass (MASS) and the inclination I (e.g., via timing). In this case, the MSINIREF field is set to "Calculated from MASS and I".
We record in this field the planet mass, which is either directly measured (e.g., via microlensing) or calculated based on MSINI and a known inclination I. In the latter case, the MASSREF field is set to "Calculated from MSINI and I". For comparison purposes (especially useful when plotting), planets with known MSINI but not I have MASS = MSINI and MASSREF = "Set to MSINI; I unknown".
Separation between the planet and its host star. SEP is set to the semi-major axis a when a is known (SEPREFF = "Set to A"), and the projected separation otherwise.
Longitude of the ascending node.
We calculate orbital semi-major axes directly from Kepler's Third Law in all cases. In cases where it differs from literature values, the usual culprit is that we have adopted a different stellar mass. When the system does not have a measured orbital period or a stellar mass estimate, the semi-major axis value 'A' is unavailable and set to NaN.
Radial velocities and orbital fit
The orbital fit parameters refer to the fit in ORBREF and the listed orbital parameters. The corresponding RV curve is displayed in the upper-left of each planet's minipage, which can be accessed by clicking on a planet's name. The radial velocities in this plot are not necessarily those of the orbital reference, but are representative velocities collected from the Exoplanet Archive. This feature is not fully supported -- for many stars, including binaries, certain multiplanet systems, and those for which no velocities are published or otherwise available in The Exoplanet Archive, no plot is shown.
We have recorded the fundamental photometric transit parameters and some derived quantities as they appear in the literature. In some cases we calculate time of periastron passage and the argument of periastron, planetary density, planetary gravity, or the impact parameter where those elements are not listed explicitly.
Radius, density and gravity
Where available, we record the published radius, density, gravity for transiting planets from TRANSITREF, and do not attempt to recompute them from the transit parameters except where they are not otherwise available. Therefore, these values may be inconsistent with the mass derived from MSINI and the inclination because our MSINI values may have been computed from different spectroscopic orbital parameters or assuming a different stellar mass.
DR is the distance between the planet and the star at mid-transit in the unit of stellar radii. RR is the planet/star radius ratio.
SE (Secondary Eclipse) Depth*
SE, SEDEPTHJ, SEDEPTHH etc.
SE is true if the planet has at least one secondary eclipse detected in any of these bands: J, H, Ks, KP (Kepler photometry band), IRAC1 (3.6 μm), IRAC2 (4.5 μm), IRAC3 (5.8 μm), and IRAC4 (8.0 μm). The fields SEDEPTHJ, SEDEPTHH, ... , SEDEPTH80 are the secondary eclipse depths in these bands (in relative flux; unitless).
Fit & References
We have replaced the field DISCMETH with two new fields: PLANETDISCMETH and STARDISCMETH. PLANETDISCMETH documents the method of discovery for a specific planet, while STARDISCMETH documents the method of discovery for the first planet discovered in a system.
We have added references and URL fields for nearly every field in the EOD; all data in these fields are now uniquely referenced. The only fields without rigorous citations are those containing coordinates, magnitudes, and rotation/activity measurements.
The "first reference" field cites the first peer-reviewed journal article to contain a orbital solution and/or radial velocities documenting the existence of the planet. In many cases this does not correspond to the first public announcement of a planet's existence, which may have been by press release, in conference proceedings, or by reference in another paper. A list of planet discoveries, confirmations, and discovery claims is available at http://obswww.unige.ch/~naef/who_discovered_that_planet.html.
For stars with transiting planets, we record the radius with asymmetrical error bars when it is calculated in the literature. For other stars with masses less than 0.6 Solar mass, we apply the formula of Torres et al (2010) to our recorded spectroscopic parameters, log(g), Teff, and [Fe/H]. The Torres formula has systematic errors of no more than 3%. We propagate errors from our spectroscopic parameters assuming no covariance into the field URSTAR.
Density of star as measured from transit photometry and radial velocity information, and for some cases from asteroseismology.
Magnitude in the Kepler photometry band in the optical from 400 to 865 nm.
JHK photometry is from the 2MASS point source catalog (Skrutskie et al. 2006). In most cases coordinates come from the van Leeuwen (2008) rereduction of the Hipparcos data. B and V magnitudes are heterogeneous, coming variously from the Hipparcos catalog, SIMBAD, and other literature sources, and so are not strictly all on the same scale. Chromospheric activity measurements are from various sources, usually planet discovery papers plus references cited in Butler et al. (2006).
We have adopted the van Leeuwen (2008) parallaxes (from a rereduction of the Hipparcos data) for most stars.
This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France, NASA's Astrophysics Data System, the Exoplanets Encyclopedia maintained by Jean Schneider, and data products from 2MASS, which is a joint project of the University of Massachusetts and IPAC/Caltech. This research received generous funding from NASA and the NSF.