Since the beginning of astronomy.
Astronomers are ALWAYS looking for new things.
Mars has been in space for as long as any other planet, such as, earth.
The answer is long and complicated, and it is too long to place here. You need to read an entire encyclopedia articles on spectroscopy and the chemical composition of intersteller objects. D.A.W.
A revolution is how long a planet takes to orbit the sun.
Short
The Great Red Spot on Jupiter has been storming for at least 400 years, possibly even longer. It is a massive storm system that is larger than Earth and has been observed by astronomers dating back to the early 17th century.
Saturn has been viewed by the naked eye since prehistoric times. In the 1600's and 1700's astronomers began to look more closely at Saturn.
None. The planet had been observed by ancient astronomers by the 14th Century BC - so almost 3,500 years ago. Thus Mercury had been discovered a very long time before the first satellites were even dreamt of.
mars has been a planet for about 4.6 million years.
For two or three hundred years, astronomers have had a fair idea the the Sun is not the center of the Universe.
Astronomers had been searching for Pluto since the early 20th century, particularly after the discovery of Neptune in 1846, which led to speculation about an additional planet due to irregularities in Uranus's orbit. The search intensified in the 1920s, culminating in the discovery of Pluto by Clyde Tombaugh on February 18, 1930, after extensive observations and calculations. Thus, the search spanned several decades, driven by the quest to find the elusive ninth planet of our solar system.
Since we never seen Planet X, we don't know for sure. Astronomers are estimating its orbital period to somewhere between 10,000 to 20,000 years.
Astronomers can determine the expansion of the universe by observing the cosmic background radiation, and the red- or blue-shifting of distant objects.
Ceres was the first asteroid belt object to be discovered, and astronomers of the time had long speculated that a planet might exist between the orbits of Jupiter and Mars. Later, the discovery of other asteroid belt objects cast doubt on Ceres' status as a planet.
Since the planet was made.
Because she been on the planet for a long time
5000 years
Modern humans have been around for 200,000 years.