Dawn planet line

Explore the Night with Bob King

Solar System Gets its Ducks in a Row

We'll have not one but many opportunities to enjoy a rare lineup of the bright planets over the next few weeks. Amazingly, they'll be aligned in order of their distance from the Sun.

Outer Planets

People, Places, and Events

Piano Concerto on the Planets Premieres

A piano concerto based on the solar system’s planets just had its world premiere. And you can hear a sampling of the music right here!

Solar System

Misfit Meteorite Sheds Light on Solar System History

Scientists have discovered the first meteorite that doesn't fall into one of two fundamental groups. The meteorite provides a unique glimpse into the era of asteroid formation and migration.

Astronomy Questions & Answers

How Many Planets Are In Our Solar System?

There are eight planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Young solar system

Solar System

Radioactive Aluminum Sheds Light on Solar System History

Astronomers thought giant stars were reponsible for flooding the early system with a radioative isotope — but the Sun itself might have made it.

Illustration of New Horizons flying by Ultima Thule

Space Missions

Planning the New Horizons Exploration of Ultima Thule

Alan Stern, principal investigator of NASA's New Horizons mission, gives a detailed preview of what the spacecraft will be doing in the days up to, during, and after the impending flyby of the Kuiper Belt Object 2014 MU69.

Illustration of New Horizons flying past 2014 MU69

Space Missions

New Horizons On Approach to the First Exploration of a Kuiper Belt Object

Take a look behind the scenes as the New Horizons team gears up for the historic first flyby of a body in the remote Kuiper Belt, in this first of a four-part series from the mission's Principal Investigator Alan Stern.

Image of BepiColumbo launch

Space Missions

Mission to Mercury Launches

The BepiColombo spacecraft, Europe's first mission to Mercury, rocketed away from Earth to begin its seven-year trek to the innermost planet.

Scholz's star

Solar System

A Star’s Closest Flyby to the Sun

A red dwarf and its brown dwarf companion buzzed through the outer Oort Cloud some 70,000 years ago, around the time when modern humans began migrating from Africa into Eurasia.

Eight planets await

Astronomy Questions & Answers

What Is the Smallest Planet and Largest Planet in our Solar System?

When it comes to both mass and volume, Jupiter is the largest planet in the solar system, while Mercury is the smallest.

Jupiter with two moons and three shadows on its face

Astronomy Questions & Answers

Is it possible to detect Jupiter's satellites with the unaided eye?

I’ve heard it might be possible to detect Jupiter’s satellites with the unaided eye if Callisto and Ganymede appear together when Ganymede is at greatest elongation from Jupiter. Will this happen anytime soon? Jupiter is now setting soon after sunset. But three times in 2008, Texans (and North Americans generally)…

Tim Hunter and his telescope

Astronomy Questions & Answers

How Can Amateurs Find Asteroids?

In a 1996 article titled “Hunting Asteroids,” you said a dedicated amateur could discover an asteroid on almost any night using a CCD-equipped 8-inch telescope. Is that still true today? It was easier for backyard observers to discover asteroids a few years ago. Today, massive professional surveys such as LINEAR,…

Astronomy Questions & Answers

What defines a planet's north pole?

Uranus is often said to have a retrograde rotation with its axis tilted 98°. Why don’t we say it has a direct spin with the axis tilted 82°? Since 1982, the International Astronomical Union has defined the north pole of a planet to be the pole that lies north of…

Comet Halley

Astronomy Questions & Answers

Will it ever be possible to observe Halley's Comet around its entire orbit?

With larger and larger telescopes being built, will it ever be possible to observe Halley's Comet around its entire orbit? It's already being done! The most recent observations of Halley were made on March 6, 7, and 8, 2003, by Belgian astronomer Olivier Hainaut and his colleagues, who acquired simultaneous…

Full Moon

Hobby-based Q&A

Would a perfectly reflecting full Moon be just as bright as the Sun?

I've read that the full Moon has a magnitude of about -13 and an albedo of about 3%. That implies that if the lunar disk reflected 100% of sunlight it would appear more than 30 times as bright, or about magnitude -17. But since it has the same angular diameter…

Comet Holmes

Hobby-based Q&A

Does a comet's tail (or lack there-of) tell us what direction it's heading?

When Comet Holmes was heading away from Earth, we on Earth didn't see an angled view of the comet's tail. Was it tailless because of its motion? That is, would a comet also appear tailless if it were approaching, with Earth right in its path? It's impossible to tell where…

Hobby-based Q&A

Four Galilean moons shadows

Can all four Galilean moons cast their shadows on Jupiter ’s disk at the same time?No. A triple shadow transit is as good as it gets (S&T: July 2004, page 94). The motions of the three inner moons are gravitationally locked so that, as English astronomer George B. Airy wrote…

Mercury near the Sun's limb

Hobby-based Q&A

Will Mercury and Venus ever transit the Sun simultaneously?

Will Mercury and Venus ever transit the Sun simultaneously? Yes, but this will not happen until AD 69,163. A transit can take place only when Mercury or Venus is near an ascending or descending node, meaning a crossing point of its orbit on the plane of the ecliptic. For this…