Troy E. Meink’s comments were the first public acknowledgment of American weapons in orbit. The Pentagon has been preparing for potential conflict in space with Russia or China.

Thousands of planets have been discovered beyond our solar system, offering clues to where life might thrive and whether Earth is truly unique.

The Times’s Science section is introducing new columns, projects and more as part of its online relaunch.

New estimates using the planet’s surface wrinkles show that our smallest planet is growing even smaller, at a faster clip than expected.