In 2008, Ukraine's Space Agency Sent A Message To Planet Gliese 581c. It Will Arrive In 2029

In 2008, humans on Earth sent a message to exoplanet Gliese 581c. The message will arrive at the planet in 2029. If (an "if" of astronomical proportions) there were aliens there, the earliest we could expect a reply is 2050.

Discovered in 2007, Gliese 581c is the second planet discovered in the Gliese 581 system, and the third planet from its host star. The star is a red dwarf, the smallest type of hydrogen-burning star in the cosmos. While they have a lot less mass than the Sun and produce a lot less energy for any orbiting planets, these stars – also known as M-type stars – have been proposed as potential places to look for life.

Gliese 581c, with its mass of 5.5 Earths, is a class of planet called a "super Earth". The planet, like companion planet Gliese 581d, briefly drew attention as it was first thought to be near the habitable zone.

"Planets Gl [Gliese] 581c and Gl 581d are near to, but outside, what can be considered as the conservative habitable zone," a paper from 2007 explains.

"Planet ‘c’ receives 30 percent more energy from its star than Venus from the Sun, with an increased radiative forcing caused by the spectral energy distribution of Gl 581. This planet is thus unlikely to host liquid water, although its habitability cannot be positively ruled out by theoretical models due to uncertainties affecting cloud properties and cloud cover."

While not exactly the most promising candidate for extraterrestrial life, you work with what you've got, and in 2008 the "Message from Earth" project decided to send a message towards the planet on the (tiny ^-10) off-chance there is somebody home. 

On October 9, 2008, the State Space Agency of Ukraine sent a message towards Gliese 581c. Unlike the Voyager records, whose messages were chosen by a committee chaired by renowned physicist Carl Sagan, the messages sent to the exoplanet were submitted by (among others) the users of defunct social network site Bebo, and put to a public vote.

In the final 501 images and pieces of text included in the message, there were images of famous Earthlings including Hillary Clinton and Britain's inexplicable morning TV presenters Richard and Judy. Celebrities were involved too. X-Files star Gillian Anderson chose to send a picture of Barack Obama and George W. Bush Jr in order to "represent good and evil", while the band McFly sent an image of Cheryl Cole to represent the Earthling "perfect body". Other messages from the public, some on the theme of world peace and the meaning of life, were also sent to the planet.

Lying 21 light-years from Earth, the signal would take [counts on fingers] 21 years to get to the planet, arriving in 2029. Assuming that the aliens were evolved enough to have their own broadcast equipment, Bebo accounts, and extraterrestrial equivalent of McFly, the earliest we could expect a reply is in 2050.

More recent papers have been more optimistic about the potential habitability of the Gliese 581 planets.

"This study confirms that Gliese 581c is situated very close to the inner limit of Gliese 581’s [habitable zone], especially if the [habitable zone] limit for dry planets is considered. However, in that context, further studies including studies pertaining to the dry limit of super-Earths as well as planets in close proximity to M-dwarfs, would be needed to finally gauge the habitability prospects of Gliese 581c," a paper on the topic explains.

"Another relevant aspect is the principal possibility of extremophiles. For those life forms, notable augmentations of [habitable zones] are expected to manifest, based on results given by, e.g., Rothschild & Mancinelli (2001) and Des Marais et al. (2008)."

Nevertheless, we wouldn't hold our breath for a return message from aliens, even in 2050. In all likelihood, we have beamed a baffling selection of celebrities into the cosmos for absolutely nobody to see. Which, to be honest, is still pretty fun to think about.