This artist’’s conception depicts an Earth-like planet. (David A. Aguilar/Harvard-Smithsonian Center for Astrophysics via AP)

In the universe of space news, Tuesday was a pretty big day. Scientists announced the discovery of two exoplanets that they claim are the “most similar to Earth of any known exoplanets to date.” Exciting stuff to be sure. But how certain are such announcements? And why do some of them end up being bogus?

In late September of 2010, there was a day a lot like Tuesday. NASA had just proclaimed the discovery of a new exoplanet. It even released one of those fancy artist’s conceptions, portraying a planet that looked just like Earth. News of this new planet, Gliese 581g, raised a slew of exciting interstellar questions that seemed plucked from an episode of “Star Trek.” Did Gliese 581g, one of four planets orbiting its sun, have air? What about water? Could we build houses on Gliese 581g? Scientists were positive Gliese 581g could answer the biggest question of all: Are we alone?

“Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say that the chances for finding life on [Gliese 581g] are 100 percent,” Steven Vogt of the University of California at Santa Cruz told Discovery News then. “I have almost no doubt about it.”

Turns out, however, that Gliese 581g didn’t have any life because it didn’t even exist. According to an article published last summer in Science, the alleged planet was in fact stellar activity “masquerading as planets.”

Gliese 581g, keep in mind, was thought to be just 20 light years from Earth. Now, scientists have announced the discovery of two exoplanets that, while promising, are significantly farther away. One of them, Kepler 438b, is 470 light years away. And the other, Kepler 442b, is 1,100 light years away. “The newly found planets are distant enough to make additional observations challenging,” the release said. Even with our advanced telescopes, it’s difficult to see that far. So how do scientists know these planets are out there, let alone have the potential to sustain water? It’s complicated.

In 2009, NASA launched the Kepler observatory with the mission of discovering “potentially habitable Earth-like planets with liquid water.” So far, it has discovered 4,175 planets outside our solar system, but nearly all of them have turned out to be uninhabitable. There are numerous requisites for a planet to habitable, but perhaps the most crucial involves what’s known as the “Goldilocks zone.” It’s denotes the sweet spot, when a planet is just the right distance from its sun to keep water liquid. Hence: Goldilocks. As The Post’s Rachel Feltman put it: “Too close, and water heats up and boils off. Too far, and the planet is covered in permanent, solid ice.”

To discover these planets, Kepler uses a system known as the “transit method,” which involves the close study of a star’s light, explained Joseph Stromberg in Smithsonian Magazine. If the star’s light suddenly dims — as though eclipsed by some passing object — that object may be a planet passing between Earth and the star. As such, even when scientists can’t clearly see the planet, they can infer its presence. The subsequent data graphed out looks a lot like a big “U,” showing when light suddenly dims. With this data, astrophysicists can study the size of the planet, as well as its distance from the star.

The method produces lots of mistakes. One study in Earth and Planetary Astrophysics suggested as many as 35 percent of all large, closely orbiting planets Kepler had identified could be false positives. Another study in a French academic journal called “false positives that mimic planetary transit” one of the transit method’s main limitations.

Still, some scientists despaired at the underwhelming findings. In June 2011, after several promising planets either proved to be figments or balls of heat and radiation, Harvard astrophysicist Howard Smith said we’re alone in the universe.

“We have found that most other planets and solar systems are wildly different from our own,” he said. “They are very hostile to life as we know it…. Extrasolar systems are far more diverse than we expected, and that means very few are likely to support life.”

Indeed, of all the thousands of planets Kepler discovered, only eight landed in the Goldilocks zone.