The European Space Agency is continuing the search for new Earths this week with the launch of Cheops, a new telescope whose name stands for CHaracterising ExOPlanet Satellite.

When will Cheops launch, and how can I watch it?

Cheops is scheduled to launch on a Soyuz-Fregat rocket from the European spaceport in Kourou, French Guiana, at 3:54 a.m. Eastern time on Wednesday, Dec. 18. It’s the second try after Tuesday’s first launch attempt was delayed. You can watch it live on the E.S.A. website.

The satellite will be launched into an unusual pole-to-pole orbit about 500 miles above Earth. The Hubble Space Telescope, by comparison, orbits about 350 miles above the surface, moving from west to east.

In a clever bit of celestial engineering, Cheops will circle Earth just along the terminator, the division between day and night down below, with its camera permanently pointed away from the sun, toward the dark.

Why was the launch postponed on Tuesday?

The delay was related to the rocket, not the Cheops telescope, which is only one of a number of payloads to be carried to orbit during this launch.

Less than 90 minutes before the launch, according to Arianespace, the company that manages the rocket, an unidentified prelaunch sequence in the countdown was interrupted. The company’s chief executive officer said on Twitter that equipment on the rocket was being replaced ahead of the rescheduled launch.

Is Cheops different from other exoplanet hunters?

Thousands of exoplanets have been discovered in the last three decades by ground-based astronomers like Michel Mayor and Didier Queloz, of the University of Geneva, who were awarded the Nobel Prize this year, and by planet-hunting satellites like NASA’s Kepler and TESS, as well as E.S.A.’s Corot. Dr. Queloz will be in attendance at the launch.

Rather than look for more planets, Cheops will study some that have already been discovered in order to understand them better.

What will the Cheops mission try to accomplish?

Many of the exoplanets already spotted by astronomers were discovered by observing the gravitational tug — or “wobble” — that they exert on their home stars as they go around. This method allows astronomers to calculate the masses of the planets, but nothing else about their nature or composition.

Cheops will make precise measurements of the sizes of these planets by observing small dips in the brightness of their home stars as the planets pass in front of them — the so-called blink method. Along with the wobble, this data will allow astronomers to calculate the densities of these planets and determine whether they are rocky, like Earth, or fluffy, like gas clouds.

Which stars are Cheops’s primary targets?

The goal is to find habitable planets. That means Cheops will focus on stars with exoplanets that range between Earth’s mass and Neptune’s.

Not all of those systems will be aligned so that the planets actually cross in front of their stars and produce a transit blip. But at least a dozen should meet this criterion, yielding information on the dividing line between so-called super-Earths — rocky planets that are much larger than ours — and worlds with large envelopes of gas, referred to as mini-Neptunes.

A new program looking for exoplanet transits with telescopes on Earth should also provide additional targets that will use Cheops to make precise follow-up observations.

Can we search for exoplanets from Earth?

The first known exoplanets were in fact discovered from Earth by the team of Dr. Mayor and Dr. Queloz, using the wobble method. And while spacecraft like Kepler and TESS make giant contributions to the search for distant worlds, ground-based observations continue to play an important role in following up on old discoveries and making new ones. That work goes on, providing more fodder for Cheops and its successors.

In Chile, special spectroscopes named HARPS and ESPRESSO were built to detect stellar wobbles around distant stars. And the Automated Planet Finder at the Lick Observatory in California performs similar work.

At the same time, the stellar blink method is used by ground-based telescopes such as the Next Generation Transit Survey, at Paranal Observatory in Chile.

What major space telescope missions are planned in the near future?

NASA’s James Webb Space Telescope, designed to record infrared radiation from the universe’s earliest days and heat from exoplanets, is on track after many mishaps, the space agency says, for a launch on March 30, 2021.

Another mission, the Wide-Field Infrared Space Telescope (WFIRST), designed to investigate dark energy and prospect for distant exoplanets, continues to survive the political vicissitudes of NASA’s budget process. It would launch in the mid-2020s.

The European Space Agency’s Euclid, which has the same mission as NASA’s WFIRST, is scheduled to be launched in 2022.