Chemical substance In organic chemistry, there can be more than one chemical compound with the same composition and molecular weight. Generally, these are called isomers. Isomers usually have substantially different chemical properties, and often may be isolated without spontaneously interconverting. A common example is glucose vs. fructose. The former is an aldehyde, the latter is a ketone. Their interconversion requires either enzymatic or acid-base catalysis. However, tautomers are an exception: the isomerization occurs spontaneously in ordinary conditions, such that a pure substance cannot be isolated into its tautomers, even if these can be identified spectroscopically or even isolated in special conditions. A
NASA Star and Exoplanet Database direct access to frequently accessed tables: NStED also serves photometric time-series data from surveys that aim to discover transiting exoplanets such as "Kepler" and the CoRoT. NStED provides access to over 500,000 light curves from space and ground-based exoplanet transit survey programs, including: The transit survey data are linked to an online periodogram service that can be used to search for periods in the transit survey data sets. A user can also upload their own time series data to search for periods in their own data. The planetary orbital parameters are linked to an online transit ephemeris prediction tool which