# Calculating the Age of Planets without Radioactive Dating

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1. Mar 7, 2015

I understand that it's possible to calculate the age of terrestial planets through radioactive dating their soil. However, the gas planets present a different challenge since we cannot currently land on them.

Any ideas on how to calculate their ages in a different manner?

2. Mar 7, 2015

### Greg Bernhardt

3. Mar 7, 2015

Thank you Greg!

I was actually thinking of analyzing the rotation of the planets to date them (similar to your stellar age estimation article you shared), though I think we would have a problem with Uranus and its extreme tilt.

4. Mar 7, 2015

### SteamKing

Staff Emeritus
Technically, the radioactive dating is performed on rock formations. Soil is a mixture of bits of weathered rock and various types of organic material.

5. Mar 8, 2015

### |Glitch|

The rotation of the planets have been altered since their initial creation as a result of impacts. According to the latest ESA's Venus Express spacecraft, Venus's rotation has slowed down by 6.5 minutes per Venusian sidereal day since the Magellan spacecraft visited it 16 years ago. Earth's rotation before the Thea impact is unknown, but our rotation has definitely slowed down since that impact due to the gravitational pull of our moon. Rotation is not a good method for determining the age of a planet.

Without actually taking surface samples, the best method for determining the age of a planet is as Greg Bernhardt posted above, by determining the age of the star. Planets would have formed within a few million years after the protostar becomes a star. The only time I can think of where that would not be the case would be with captured planets, but they would be extremely rare.

6. Mar 8, 2015

Thank you so much for the thorough explanation! That is really incredible how much Venus's rotation is slowing down... Is it due to tidal locking with the Sun or some other factor? Thanks again for the informative response :)

7. Mar 8, 2015

### Garth

Actually the dating of the Earth is by radioactive dating performed on the rock formations of the Earth. The problem with this is the Earth's crust is by no means primordial. The earliest terrestrial surface rock formation is about 4.03 Gys old and is part of the Acasta Gneiss of the Slave craton in northwestern Canada. Older still is a tiny zircon crystal from Western Australia dated at 4.4 Gyrs.

The earliest lunar rocks date from 4.46 Gyrs ago (the Genesis rock) and typically from the great bombardment era of 3.8 Gyrs ago.

However the age of the Earth, and the Sun and its Solar System, is determined from dating the oldest meteorites, the carbonaceous chondrites dated at 4.6 Gyrs ago.

The dating of the other planets stems from this period of planetary formation from the solar protoplanetary disc.

Garth

8. Mar 9, 2015

### Chronos

Dating the age of the solar system is still a work in progress. I think we are pretty accurate at present. Radioactive dating is highly precise, which explains its continued popularity.