After traveling ten years and hundreds of millions of kilometers, a small probe (探测器) called Philae has for the first time landed on the surface of a comet, a solar system object made of ice and rock.

Philae was sent from the European Space Agency’s main Rosetta spaceship early Wednesday, November 12, 2014. Originally, the mission of the spaceship was to target Comet 46P but when the launch was delayed due to problems with the Ariane 5 launch vehicle, the mission was redirected to Comet 67P, to study the appearance and materials that make up Comet 67P.

The Philae research probe left its mother ship Rosetta and then began a trip toward the comet and landed on its surface hours later. The effort to land on the comet was considered dangerous because just before the planned landing, scientists identified problems in Philae’s active descent system thruster (推进器) used to land the probe.

Rosetta has been orbiting Comet 67P since August 2014. However, it was sent up into space in March of 2004. The spaceship circled the sun until it was in position to meet with the comet.

Comets are the most primitive (原始的) objects in the solar system. Scientists think that they have kept a record of the physical and chemical processes that occurred during the early period when planets were formed. By studying Comet 67P’s make-up and history, scientists wish to unlock the secret of the solar system and even the origin of human lives.

The Philae probe did a series of scientific experiments. Philae examined the materials that make up the comet looking for organic, or carbon-based materials, and inorganic materials. It lasted until March 2015, when the comet was too close to the sun and the surface temperature was too high for Philae to operate.

A new study suggests Earth could have lost up to 60 percent of its atmosphere in the collision of space objects that created the moon. Most scientists believe this event happened about 4 billion years ago when Earth was still developing. They think a massive object, possibly the size of the planet Mars, hit the Earth, sending vaporized particles （颗粒）into space. Gravity is believed to have brought these particles together to form the moon.

The study presents evidence that this collision might have caused Earth to lose between 10 to 60 percent of the atmosphere it had. A report on the study appeared in the publication Astrophysical Journal Letters. The researchers‘ impact assumption is one of three leading theories of how the moon was formed. The others suggest the moon was either created at the same time as Earth, or that it was captured by Earth's gravitational field as it traveled through space.

The researchers decided to explore how massive collisions of objects affected rocky planets with thin atmospheres. To do this, they carried out more than 300 “supercomputer simulations （模拟）The experiment considered the positions at which the objects hit the planet, the speed of the impacts, as well as the size and mass of the colliding objects. Earlier research suggested that impact collisions during the latter stages of the formation of a planet can affect its atmosphere. The computer models in this study expanded on this idea. They showed that a collision like the one that may have formed the moon likely took a large part of Earth's atmosphere.

The Milky Way is full of habitable (宜居) places, with roughly half of all sunlike stars hosting Earth-size worlds that could be friendly to life.

Here’s a good sign for alien hunters: More than 300 million worlds with similar   conditions to Earth are spread throughout the Milky Way galaxy. A new analysis concludes that roughly half of the galaxy’s sunlike stars host rocky worlds in habitable zones where liquid water could pool or flow over the planets’ surfaces.

Astronomers estimated the number of these planets using data from NASA’s planet-hunting Kepler spacecraft. For nine years, Kepler stared at the stars and watched for the brief twinkles produced when orbiting planets block a portion of their star’s light. By the end of its mission in 2018, Kepler had spotted some 2,800 exoplanets (外星行星) — many of them nothing like the worlds orbiting our sun.

But Kepler’s primary goal was always to determine how common planets like Earth are. The calculation required help from the European Space Agency’s Gaia spacecraft, which monitors stars across the galaxy. With Gaia’s observations in hand, scientists were finally able to determine that the Milky Way is populated by hundreds of millions of Earth-size planets orbiting sunlike stars — and that the nearest one is probably within 20 light-years of the solar system.

It took more than half a century for scientists to start pinning down how many planets could possibly host life. In 1961, astronomers knew of no worlds orbiting stars other than the sun — and although planetary formation theories suggested exoplanets should be common, we had no observational evidence that they existed. But over the past decade, it’s become clear that planets are extremely common, outnumbering stars in the Milky Way. On average, nearly every star is home to at least one orbiting world.

Early estimates suggested that perhaps 20 percent of sunlike stars hosted a world that   met those criteria. We now know that the number is closer to 50 percent, if not more.

Of course, many factors determine whether a world in the habitable zone is truly friendly for life. Planetary characteristics such as magnetic fields, atmospheres, and plate tectonics all play a role, and those are difficult to observe on small, faraway worlds.