Some links that are useful for the creation of new worlds or for information on real ones.

New:

Starmap! A very comprehensive stellar map, with easily searchable data. Hundreds of thousands of stars.

The Sky on Alien Worlds - written by Orion's Arm Contributors (SB)

Sky colors for ExoEarths - a good graphic representation of this data (SB)

Sky Colors by Robert A. Freitas Jr. (SB)

What is the largest possible inhabitable planet?- by Anders Sandberg (SB)

Toroidal Planets by Anders Sandberg (SB)

Greetings from Double Earth by Anders Sandberg - an analysis of planets with twice Earth's mass (SB)

For making cultures- (VT)

A list of Human Universals - Donald E. Brown - note that in OA we go way beyond this list (SB)

Axial tilt and its processions- (VIDEO)(VT)

List of Biomes- (VT)

Hypothetical types of biochemistry- (VT)

The Limits of Organic Life in Planetary Systems - ONLINE BOOK - well worth the read (SB)

A crash course of ecology and biology- (VIDEO)(VT)

If Earth had rings like Saturn- (VIDEO)(VT)

A cool tool that allows you to create a realistic terrain- (VT)

Space travel calculator-http://nathangeffen.webfactional.com/spacetravel/spacetravel.php - (VT)

A priceless resource for sci-fi worldbuilding- Project Rho (VT)

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Hard Science Fiction Tools - Exactly what it says on the tin, with this section particularly useful.

Plan a planet orbiting a Main Sequence Star, with a number of Input Options - Perhaps the most useful tool for making orbits from scratch around main sequence stars.

Planetary Classification List - a very useful and comprehensive by John M Dollan (OA Boardmember)

Planet Designer - An excellent online calculator that can make a range of realistic worlds.

SpinCalc; an Artificial gravity calculator for calculating 'gravity' inside rotating habitats and craft.

Natural gravity calculator Allows one to calculate the force of gravity acting on a variety of masses over a range of distances.

Space Engine Fantastic new procedural universe generator that allows you to explore billions of stars and planets, with very realistic physics (which is getting better with every update)

The Worldbuilding Hub - A useful and interesting set of essays and links including a project to map the Nearby Stars and a useful Excel spreadsheet which converts radial coordinates into galactic XYZ coordinates (something I've been looking for)

SciFi World Generator An interesting on-line world generator, together with a number of other useful tools

World Anvil - A site providing a set of worldbuilding tools that helps users create, organize and store their world setting.

Stellar Classification - Wikipedia page giving a really useful descriptions of main sequence stars, their luminosities and lifetimes

Star Tables Archived version of some really useful star tables (even better than the wikipedia version).

Orbitsimulator.com Calculators - Several really useful calculators by Tony Dunn, including one which will give orbital parameters for satellites and moons; also see his Gravity Simulator program, available for download here.

Gravity Calculator for Astronomical Bodies Based on Radius and Density - An online calculator that allows you to calculate gravity. Remember that materials become compressed in the centre of planets, so large planets are denser than small ones made of the same materials (see Valencia et. al. below).

Cloud Map generator this source creates a new texture daily from Earth satellite data.

Celestia - Not only a marvellous star display program, but also a good source of information on stars and planets out to 16000 light years (and galaxies too). I recommend this page as well Selden's Catalogs of Objects for Celestia.

Rassilon's ssc generator A generator that works with Celestia to create realistic systems; it uses John M Dollan's Planet Classification list, too.

Where is M13 Really simple downloadable software that displays the local galaxy in 3d

Stargen Updated Version -although there are still some problems, such as too many worlds in inner solar system regions, as well as being unable to generate a star system below 0.5 solar masses. To create a plausible system you might refer to remove every other planet; Stargen also does not produce enough hot-jupiter type planets, so such systems need to be generated by hand. (SB)

Stargen online - You can generate Stargen systems online here.

Astrosynthesis - A very cool generator, which also uses some good science behind it. However, only a demo is available for free, and is thus limited in functionality. Check out Fractal world explorer from this company also.

Planetgen - A usable fractal planet generator; can make some nice worlds, but the fractal look often has to be disguised or eliminated. Useful for inspiration if not for direct use.

Epona - A very good example of a created world available on the 'net. Very useful for ideas and inspiration.

Furaha: Life on Nu Phoenicis IV - A possibly even better fictional system on the 'net. This one is much more geared toward the average viewer, and has a very professional look. (SB)

Snaiad An excellent fictional ecology on a very alien world; and the artwork is phenomenal Also see the wiki, here.(SB)

ChView - The best 3d star mapping program out there. I really wish they would update it, but for right now the thing serves my purposes admirably. (JD)

Project Rho - The uber site for world building resources. Many, if not all of the links above are available through here, and a Galaxy of others are available as well.

Astronexus - A good and quick way to find stats on a few solar type stars. The Distant Worlds Mapper is excellent- it also shows proper motion, which is very nice indeed.

The Internet Stellar Database - The best source for information on stars that I can find, without needing to know scientific terminology, anyway.

Solstation - One of my favorite informational sources on stars and solar system planets. (JD)

Extrasolar Planets Encyclopedia - The best source for current extrasolar planet discoveries.

Extrasolar Visions - A visionary site from the late John Whatmough.

Extrasolar Planets - wiki page

Planetary Parameter Generator - This calculator is primarily intended to calculate the energy required to destroy planets; it does give the gravitational binding energy, which is useful if you simply want to disassemble them for peaceful purposes. (SB)

Instant Planet Generator - Requires Firefox.

Terragen - A landscape modelling program useful for making images of the surfaces of worlds; watch out for Terragen 2, a new development which allows planets and 3D objects to be imported into the scenes (preview stage).

The Five foundations of Worldbuilding - Things to ask yourself while creating worlds.

Worldbuilder Projects - Links to WB resources.

Selden's List of Planetbuilding Links - Contains some very interesting pages not found elsewhere.

Worldbuilding tips from the UK editor of The Martian.

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Some links to useful general papers on extrasolar planets

Observed Properties of Exoplanets(pdf) - Marcy et. al.

Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets orbiting M Dwarfs; Conditions for Atmospheric Collapse and the Implications for Habitability (pdf) - Joshi et. al.

Making other Earths: Dynamical Simulations of Terrestrial Planet Formation and Water Delivery - Raymond et. al.

Internal Structure of Massive Terrestrial Planets - Valencia et. al.

Detailed Models of super-Earths: How well can we infer bulk properties? - Valencia et. al.

A New Family of Planets ? "Ocean Planets" - Leger et. al.

Volatile rich Earth-Mass planets in the Habitable Zone (pdf) - Kuchner

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More useful Calculators

Hawking Radiation Calculator - Useful calculator by Jim Wisniewski (contributor to OA) giving the energy emitted by black holes of all masses.

SpinCalc - Calculates artificial "gravity" caused by rotation (for habitats and spacecraft).

Tranquility Calendar Converter Tool - Convert Gregorian dates to Tranquility dates and vice-versa.

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Content By John M Dollan, Thorbørn Steen and Steve Bowers (signed comments by Vaktus (VT), John Dollan (JT) and Steve Bowers(SB))