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I have heard that the only planets able to be seen outside the Solar system are Jovian-sized planets with the occasional detection of planets three times the Earth's size. But, as far as I know, we haven't seen any Earth-sized (due to the range of our telescopes?) extrasolar planets. When will we be able to detect them?

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In 2013, the smallest detected exoplanet was Kepler-37-b, which is actually smaller in mass and size than Earth, so we already had a limited ability to detect these size exoplanets.

It is worth noting that there wasn't any new technology that allowed this advance, the paper linked to by NASA indicates that the same methods were used as they would usually use. Making it remarkable that they were even able to discover something this small.

Now, even more Earth-sized planets have been detected. One method used to aid in their detection is described as follows:

We account for Kepler's imperfect detectability of such planets by injecting synthetic planet-caused dimmings into the Kepler brightness measurements and recording the fraction detected.

Sources:

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    $\begingroup$ Maybe update this answer. There are now a number of confirmed Earth-sized planets and most of the small planetary candidates found by Kepler will be real planets. So the answer is indeed that we already can and have. What there isn't yet - is an Earth-like planet in an Earth-like orbit around a sun-like star. $\endgroup$
    – ProfRob
    May 3, 2015 at 14:16
  • $\begingroup$ June 2015, Kepler-138: For the First Time, a Mars-Sized Exoplanet Reveals All: slate.com/blogs/bad_astronomy/2015/06/18/… Mars weighs in at about 0.107 Earth mass. $\endgroup$
    – Wayfaring Stranger
    Jun 20, 2015 at 14:19
  • $\begingroup$ @WayfaringStranger Good find. Kepler-37b is still smaller since it is roughly the size of the Moon. $\endgroup$
    – called2voyage ♦
    Jun 21, 2015 at 1:39
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We are now able to find smaller exoplanets than the Earth.

Analysis of Kepler data has yielded the smallest known mass for an exoplanet orbiting a normal star. Its mass and size are similar to those of Mars, setting a benchmark for the properties of exoplanets smaller than Earth. See Letter p.321

Read the article published in Nature from here.

(Earth is twice bigger than Mars.)

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Perhaps after the launch of ESA's PLATO space observatory, currently slated to be launched in 2024.

Planetary Transits and Oscillations of stars (PLATO) is a planned European Space Agency space observatory that will use a group of photometers to discover and characterize rocky extrasolar planets of all sizes around red dwarf stars, yellow dwarf stars like our Sun, and subgiant stars where water can exist in liquid state. . . . The goal is to find planets like Earth, not just in terms of their size but in their potential for habitability.

According to this BBC article:

Critically, Plato would be tuned to seek out rocky worlds orbiting in the "habitable zone" - the region around a star where water can keep a liquid state.

See also Jeremy's answer to this question.

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    $\begingroup$ Earth-sized planets have already been detected. This answer (quote) addresses when we might find Earth-like planets. $\endgroup$
    – ProfRob
    Jun 18, 2015 at 15:36
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We can find Earth like Extra-Solar Planets, but they just might be too close to their star, and therefore, life might not be in the planet. We are right now living in a "Goldilocks Planet". Goldilocks planets are in the habitable zone, where there is life, and H2O that is at the right temperature to be a flowing liquid.

Anyway, a planet such as Gliese 876 d, is WAY too close to the star it orbits, (Gliese 876) Which it is highly unlikely that life exists, considering it is 377'C (710.6'F)

If you want to see a full table of exoplanets, then go to

http://en.wikipedia.org/wiki/List_of_nearest_terrestrial_exoplanet_candidates

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    $\begingroup$ You need to be much more detailed with your answers. The same goes for your others. $\endgroup$
    – HDE 226868 ♦
    Oct 27, 2014 at 22:51
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    $\begingroup$ Thanks HDE 226868, I will use this as a learning experience and I'll try doing that next time. $\endgroup$
    – Juka
    Oct 27, 2014 at 22:56
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    $\begingroup$ No problem. By the way, I can see a good answer in here. Try talking more about why it would be hard to find planets this small. $\endgroup$
    – HDE 226868 ♦
    Oct 27, 2014 at 22:58
  • $\begingroup$ I'll try doing that :) $\endgroup$
    – Juka
    Oct 27, 2014 at 23:00

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