As Sandhyanga Tarpana by certain Sampradayas, Deva Tarpana is done. And Navagraha Tarpana comes under Deva Tarpana.
The book "STOTRA KAVACHA ( SHIELD OF PRAYERS), hosted at Kamakoti.org, is comprised of various Stotras and Kavcahas dedicated to various deities, which are found in Puranas.
The book also have two Puranic methods of performing Sandhya. One from the Devi Bhagavatam and another one from the Narada Purana. The Narada Purana method have Navagraha Tarpana as a limb of the whole ritual (check on p.128 of the PDF).
Sayamkala Sandhya (Evening) Mantra states:
Agnischa ma manyuscha manyupatayascha manyu kritebhyah Paapebhyo
rakshantaam yadanha paapamakaarsham, Manasa vaachaa hastaabhyam
Padbyaam udarena sishnaa Ahastadalumpatu, yat kincha duritam mayi
idamaham maamamritayonau satye jyothishi juhomi swaha/
( Let all the Deities of Fire, fury and ferocity safeguard me from
their attack on me due to the unpardonable sins perpetrated by me by
the day by my mind, stomach, sex organ; may I be purified me of
despicable deeds and qualify me to proceed on the path of Moksha.
Achamana-Punarmarjana and Marjana be followed with the Mantra viz.
Dadhikravinno akaarisham jishnorasvasya vajinah, surabhi no mukha
karat prana Aagumshi tarishat/
(May the Lord who is the Sustainer, Administrator and Evaluator in the
form of Hayagriva the Victorious and the Seat of knowledge be
worshipped to facilitate our Life without any obstacle). This Mantra
may be followed by the afore-mentioned Mantra Aapo hi shtha while
sprinkling water on the head of the devotee and touching the feet and
thereafter recite Apo janayata cha nah while water is thrown round the
Arghya pradaanam: Recite the Gayatri Mantra pour water thrice to the
East in the morning, face north and pour water twice in standing
position and face west in sitting posture as follows:
Om Bhurbhuvassavah, Tat Saviturvarenyam bhargo devasya dhimahi, dhiyo
yo nah prachodayaat/
Praanaayama should be done there after.
Deva Tarpana:Adityam Tapayami, Somam Tarpayami, Angaarakam Tarpayami, Budham Tarpayami, Brihaspatim Tarpayami, Sukram
Tarpayami,Shanaischaram Tarpayami, Rahum Tarpayami, Ketu Tarpayami//
Keshavam Tarpayami, Narayanam Tarpayami, Govindam Tarpayami, Vishnum
Tarpayami, Madhusudanam Tarpayami, Trivikramam Tarpayami, Vaamanam
Tarpayami, Shridharam Tarpayami, Hrishikesham Tarpayami, Damodaram
Gayatri Avahaana: Omityekaaksaram Brahma, Agnirdevata Bhrahma
ityaarsham Gayatreem Chhandam Paramaatmam Swarupam, Sayujyam
Now, Tarpana, like Homa etc. is a way of worshipping the deities. And once worshipped these Navagrahas get appeased and they stop producing negative effects in the individual horoscopes.
This is mentioned in the Yajnvalkya Smriti:
CCCVII. —Whenever a planet has a bad aspect (in the horoscope of a person), that person should worship with great care specially that
particular good planet. For Brahma has given this boon to them "Being
honoured you will honour him"—307.
CCCVIII. —By lords of men should be worshipped with greatest care the
planets because the rise and fall of monarchs is dependent upon the
planets, so also the origin and destruction of the worlds.—308.
So, once these Grahas are duly appeased by Tarpanam, their negative astrological aspects are bound to get reduced.